chapter 20 emergent properties in ecological systems - mcconnell biology...

Integrating Concepts in Biology Chapter 20: Emergent Properties of Ecological Systems

Campbell, Heyer & Paradise

December 17, 2013 Chapter 20, page 1

Chapter 20 Emergent Properties in Ecological Systems

Opening photo UN20.1 Lamar Valley, in Yellowstone National Park, where emergent properties arise

from the re-introduction of the gray wolf, a top predator.

From Yellowstone National Park (US National Park Service) website

http://www.nps.gov/archive/yell/slidefile/scenics/mvnortheast/Images/07192.jpg.

Learning Objectives

Biology Learning Objectives

1. Describe the properties of ecological systems, including food webs, indirect effects, and

nutrient cycling.

2. Evaluate how indirect effects arise through biotic interactions in ecological systems.

3. Understand how a top predator affects organisms with which it does not interact.

4. Analyze how the non-living components of ecological systems (abiota) can alter the outcome

of a biotic interaction.

5. Analyze how properties emerge from biotic interactions within ecological systems.

6. Describe how a limiting resource often results in competition.

7. Evaluate the ecological conditions that allow for species coexistence.

8. Define primary production and trophic level.

9. Explain how carbon dioxide cycles and energy from sunlight flows through an ecological

system.

10. Explain why energy transfer in ecological systems is relatively inefficient.

Bio-Math Exploration Learning Objectives

1. Predict predator and prey population dynamics by quantifying the interaction between the

two populations.

2. Calculate the overlap in resource use of competing species.

http://www.nps.gov/archive/yell/slidefile/scenics/mvnortheast/Images/07192.jpg




Ethical Legal and Social Implications Learning Objectives

1. Evaluate the wider implications of re-introducing a species to an ecological system, beyond

the emergent properties and indirect effects that may arise.

2. Discuss the various ethical issues surrounding re-introduction of a predator to an area where

they may come into contact with humans or human economic interests.

3. Understand the legal issues of laws that attempt to protect endangered species, and the

implications of listing or de-listing a species.

Chapter 20 Outline

20.1 Why did more trees grow when wolves were reintroduced into Yellowstone National Park?

Bio-Math Exploration 20.1 Predicting predator and prey populations

ELSI 20.1 What are the arguments for and against species re-introductions?

20.2 What determines the outcomes of competition for a resource?

Bio-Math Exploration 20.2 How can you measure resource use overlap?

20.3 How efficient are ecological systems at transferring energy from the sun and carbon dioxide

from the air to predators?

Conclusions

You Are Here

Organizing, Big Ideas of Biology

Information Evolution Cells

Emergent

Properties Homeostasis

Levels of the

Biological

Hierarchy

Molecules 1 6 11 16 21

Cells 2 7 12 17 22

Organisms 3 8 13 18 23

Populations 4 9 14 19 24

Ecological

Systems 5 10 15 Chapter 20 25




A hawk swoops down and catches a bird eating seeds at a birdfeeder. This is an interaction

between two individuals, a predator and its prey, but it leads to interactions between the hawk

and seed-eater populations and can cause properties to emerge in ecological systems. The

predator-prey interaction leads to energy flow in the ecosystem and affects growth of both

populations. Population growth and energy flow are emergent properties that arise because of

the actions of individuals and the coexistence of many species in ecological systems, which often

compete for limited resources. Interactions between individuals often depend upon the location

of the individuals, which may vary randomly. You cannot predict exactly where an animal will

be within an ecosystem, although you can make a reasonable guess based on its habits. The

hawk may search near feeders for potential prey. The location of an organism determines its

interactions with other organisms, populations, or ecological systems. Location leads to

flexibility in the response of ecological systems because if all the individuals in a population

were in the same place at the same time they might all get wiped out by some natural disaster or

predator. In contrast, if individuals of a species are widely distributed, it is less likely that the

entire species will die. If individual birds do not group together, predators such as hawks are

unlikely to detect every bird, and some individuals will survive. In this chapter, you will

examine emergent properties of ecological systems.

20.1 Why did more trees grow when wolves were reintroduced into Yellowstone National Park?

Context: Addition of a species into an ecological system can have unpredictable effects.

Major Themes: Biological systems exceed the sum of their parts, randomness within a

biological system provides flexibility of response, and biological systems require resources,

which results in competition or cooperation.

Bottom Line: The indirect effects of a predator on an ecological system are emergent

properties.

You will start your investigation of the emergent properties of ecological systems by looking

closely at the impact of wolves in the Yellowstone National Park ecosystem. In ecological

systems, there are predators and prey. A predator is any organism that consumes another living

organism, such that the other organism, the prey, is killed. Predators can be classified by the type

of other organism they eat, and wolves are carnivores, animals that eat other animals. Wolves

hunt in packs and eat a variety of other animals. Wolves feed mostly on large mammals, such as

deer, elk, and moose, which are all plant-eating herbivores. If more trees grow in the presence of

wolves as the Section question suggests, the wolf has effects that go beyond its direct effect as a

predator. Ecologists often construct food webs (Figure 20.1) to depict how energy flows in an

ecological system. Arrows are drawn between prey and predator, with the arrow pointed in the

direction of energy flow. Energy and nutrients, in the form of carbohydrates, proteins, and fats,

flow upward toward the predators. Organisms that are prey contain energy and nutrient

resources for predators. {Definitions: Predators are organisms that obtain energy by consuming,

and usually killing, other organisms. Prey are organisms that are consumed by predators, either

in whole or part. Food webs are diagrams that show who eats whom and how energy flows in an




ecological system. Energy flow refers to the movement of energy-containing chemicals from

one organism to another.}

Figure 20.1 A food web. The arrows point in the

direction of the energy flow. Here plants are eaten by

grasshoppers and rodents, which are eaten by an

assortment of predators, ending in the owl and fox,

which have no predators. From

http://www.curwensville.org/146110371492860

/lib/146110371492860/f oodweb1.gif.

To understand why more trees grew when

wolves were introduced, you need to first examine

the Yellowstone National Park ecological system.

You should consider the population of wolves, the

populations of trees, and the entire ecological

system. Most of the park is more than 7,500 feet

above sea level, with forests filled with conifers,

trees with cones. There are also areas of mixed

deciduous forests, with trees that lose their leaves

each year, and grasslands. Grasslands and shrubby

plant communities predominate at lower elevations, which grade into coniferous forests at higher

elevations. Deciduous trees in this part of the world typically grow along floodplains near rivers,

but they also grow in higher elevations. Winters are very cold, and summers are mild. In

addition, wildfires are a natural part of the Yellowstone National Park ecosystems.

Integrating Questions

1. Search for information on the different abiotic conditions in forests vs. grasslands. What did

you discover? How do different conditions affect animals that live in different habitats?

Provide examples of animals that prefer to live in forests, and others that prefer to live in

open fields.

2. Search for Yellowstone National Park at the National Park Service websites (www.National

Park Service.gov and http://www.nps.gov/yell/). From what you’ve read and Yellowstone

National Park’s location (look on a map), determine the common deciduous trees that grow

in Yellowstone National Park.

3. Using the same websites, investigate the large mammals (larger than rabbits) that are

currently present in Yellowstone National Park. List the species you discovered.

4. What do the large mammals that you discovered in IQ #3 eat? Quickly sketch the species of

mammals in Yellowstone National Park along with the other species on which they feed, no

matter what type of organism they are. If you draw arrows representing the flow of energy

http://www.curwensville.org/146110371492860%20/lib/146110371492860/f%20oodweb1.gif

http://www.curwensville.org/146110371492860%20/lib/146110371492860/f%20oodweb1.gif

http://www.nps.gov/

http://www.nps.gov/

http://www.nps.gov/yell/




and nutrients from prey to predator, you will have constructed a food web like the one in

Figure 20.1.

As European settlers expanded across most of the United States in the late 18th

and early 19th

centuries, they hunted large predators to extinction or near extinction. Within Yellowstone

National Park, the gray wolf was hunted out of existence before the National Park Service was

established to protect the natural resources. In the early days of the park, poachers routinely

hunted and logged within the boundaries of the forest. From the late 1800’s through the mid-

1920’s, hunters eliminated the gray wolf from all of the lower 48 states, with the exception of

Minnesota. Minnesota, Alaska, and much of Canada became the only refuge for the remaining

gray wolves in North America.

After wolves were eliminated, their prey faced less predation pressure. In Yellowstone

National Park, the wolves primarily eat elk, large deer-like herbivores. While an individual elk

is much larger than a wolf, an emergent property of wolves is their social behavior of hunting in

packs, which allows them to work as a coordinated unit to kill animals much larger than

themselves. Predation is an example of our theme that biological systems require resources,

which results in competition or cooperation. In this case, wolves require food resources, and

they have evolved to work cooperatively in hunting packs to acquire those resources. The social

behavior of wolves is an emergent property because you cannot predict their social behavior by

studying solitary wolves.


5. Predict the response of prey populations when populations of predators are eliminated from

an ecological system.

An emergent property of interactions between individual predators and prey is that the

populations of these species are affected. The gain in resources by a predator may provide the

energy needed for reproduction, leading to population growth. The loss of an individual in a

population of prey could affect evolution or growth rate of the population. {Connections:

Examine the effects of loss of individuals to a population in Section 24.1.} After removal of

predators such as wolves from an area, prey populations often increase in abundance very

quickly, as you may have predicted. This population boom may require the need for humans to

control the rate of growth of the prey population. One of the tasks of the National Park Service

is to monitor the populations of many species within the parks, and this is especially important

for populations such as elk that lack major predators (Figure 20.2). Elk populations had been

monitored for decades within Yellowstone National Park, and until the late 1960’s, their

populations had been managed by human hunters inside the park. Without the wolves, humans

had to limit the elk population to prevent starvation and disease. After 1968, the National Park

Service adopted a “natural regulation” strategy and no longer allowed hunting within the park.




Figure 20.2 Elk and wolf population counts in

Yellowstone National Park. Park rangers count elk

from small planes. Years of missing data for elk

counts are due to counts attempted during times of

bad visibility, or from counts not being made at all in

1996 and 1997. From US Fish & Wildlife Service

and National Park Service, original art by CJP.

Natural regulation meant that park managers

would let natural factors, such as predation,

disease, or lack of food be the primary causes of

elk death. Note that other predators besides

wolves, such as grizzly bears and mountain

lions, are still in the park and they will eat elk. Hunting was allowed outside the park, and since

elk migrate, some of the Yellowstone National Park population encountered hunters when they

left the park. During the time when there was no hunting inside the park, and no wolves, the elk

population increased steadily to a maximum of about 19,000 individuals.

The gray wolf was re-introduced to Yellowstone National Park in two separate releases

during 1995 and 1996, which was done to help the wolf population of North America recover,

but could have an added benefit of naturally controlling elk populations. As of 2012, about 80-

90 wolves in several packs lived within the park (Figure 20.2), and over 200 were in the Greater

Yellowstone Ecosystem which is an area that includes the park and the surrounding National

Forests and other federal lands. Figure 20.2 compares changes in the wolf population with

changes in the elk herd. The amount of human hunting varied from year to year, but during the

20-year period in the graph, hunting pressure was fairly consistent – humans killed an average of

1,148 elk during 1987-1995, and an average of 1,297 elk during 1996-2004.


6. What do you conclude about the relationship between wolf and elk populations? That is, can

you determine the impact of wolves on the Yellowstone National Park elk population?

7. Wolves exhibit emergent properties in their social behavior. Do elk or other large

herbivorous mammals exhibit any emergent properties that you would consider an anti-

predator defense strategy? Search Encyclopedia of Earth (www.eoearth.org) or google for

information on anti-predator defense strategies of large mammals.

8. In 1992, before wolves were released, elk numbers were about the same as in 1998 and 1999,

after wolves were reintroduced. What biotic or abiotic factors could account for the similar

number of elk regardless of the wolves?

9. In order to assess the impact of wolves on elk populations, what else would you want to

know in addition to the data in Figure 20.2? For example, what might you want to know

about these populations or their behaviors?

http://www.eoearth.org/




Bio-Math Exploration 20.1 Predicting predator and prey populations

Concept: Discrete dynamical systems to model animal populations.

Objective: Predict future elk and wolf populations.

Required Skill: Understand and manipulate logistic growth model (BME x.x).

Before making a major wildlife management decision, such as whether to introduce a new

species into an ecosystem, or how much hunting to allow, scientists and policy-makers work

together to predict the impact of the decision. Mathematical models of predator-prey interactions

can be used to help them predict outcomes. A simple model of the interaction between the gray

wolf and elk populations in Yellowstone National Park can help you see why the wolf population

decreased between 2004 and 2006, and predict what will happen to both populations over the

next several years. This predator-prey model is based on the law of mass action, the same law

that describes interactions between molecules {Connections: See BME x.x} and individuals in an

epidemic. The law of mass action states that the rate of a reaction, in this case, the rate at which

wolves kill elk, is proportional to the product of the concentrations of the reactants, in this case

the number of individuals in each population. In symbols, if we let Wt represent the wolf

population in year t, and Et represent the elk population in year t, the number of elk killed by

wolves that year should be proportional to WtEt. The increase in the wolf population due to elk

kills also should be proportional to WtEt. To complete the model, we need to make two more

assumptions:

If there were no interactions between wolves and elk, the elk population would experience

logistic growth at a rate rE {Connections: See BME 15.1} up to some carrying capacity K.

Elk are the primary food source for wolves in YNP. Therefore, if there were no interactions

between wolves and elk, the wolf population would gradually die out at rate rW.

Putting these assumptions together, the following equations allow us to make predictions

about the wolf and elk populations in year t+1, based on the populations in year t:

Et+1 = Et + rEEt(1-Et/K) – cEWtEt

Wt+1 = Wt – rWWt + cWWtEt

The carrying capacity, K, and rate constants cE and cW, in these two equations can be estimated

from population data over the last several years.

Bio-Math Exploration Integrating Questions

20.1a: Suppose there were 12,000 elk and 82 wolves in 1998. Using K=20,000, rE = 1.6, cE =

0.006, rW = 0.8 and cW = 0.000085, how many elk and wolves does the above model predict

there would be in 1999? How closely do these predictions match the actual population data




in Figure 20.2? What might explain the discrepancy between predicted and actual

populations?

20.1b: Using the predicted 1999 elk and wolf populations from the previous question, repeat the

process to predict the populations in 2000. Again, compare the predictions to actual

populations.

The Excel file wolf-elk.xls automates the prediction process for years 1999-2020, and graphs

the predicted and actual populations on the same set of axes. Open the Excel file to check your

answers to the previous two questions, and look at the trends in the populations. How well do the

predictions track actual population sizes over this time period? How do the actual and predicted

elk population sizes differ from those predicted from logistic growth of elk in the absence of

wolves? Discrepancies between actual and predicted population sizes occur, and at least in this

instance, scientists attribute some of these changes to major environmental disturbances,

including the large fires that occurred in 1998 and a drought that occurred in the late 2000’s.

The hard part of creating a mathematical model is finding the right rate constants to match

the observed data. Once you have a model that predicts population numbers fairly accurately,

you can easily predict future population numbers. You can also estimate the potential impact of

elk hunting on both elk and wolf populations. Policy-makers and biologists need these

predictions to help them make wise wildlife management decisions.

---------------------------------------------------------------------------------------------------------------------

So far, you have considered two key species, but you have not examined the trees yet. When

wolves were absent from Yellowstone National Park, several species of trees (aspen, two species

of willows, and two species of cottonwood trees) failed to produce significant numbers of

offspring trees. The adult trees produced seeds, the seeds germinated and grew, but the seedlings

did not reach maturity.

Narrowleaf cottonwood and black cottonwood trees produce many thousands of seeds every

year. Regardless of the number of wolves, many of their seeds never become seedlings, and only

a few seedlings become mature trees. Every species evolves a reproductive strategy, which

may include such traits as age at maturity, number and survival probability of offspring, and life

span. Natural selection influences each of these traits. {Connections: Natural selection is

examined in the Big Idea of Evolution, Chapters 6-10.} A common reproductive strategy in

plants is to produce more seeds than the environment could support as mature individuals. It

might seem as if producing more seeds than a plant needs to ensure survival of one replacement

individual is a waste of resources, but there are many reasons that seeds fail to germinate and

seedlings die. Long-lived plants such as cottonwood trees that produce many seeds each year

continue to live even when a very small percentage of offspring in any one year live to maturity.

{Definition: A reproductive strategy is a suite of evolved life cycle-related traits that taken

together lead to successful existence of a species in the context of that species’ environment.}




Although cottonwoods produced many more seeds than could survive, ecologists had noted

that even fewer cottonwood seedlings and trees were surviving than expected in Yellowstone

National Park compared to historical trends. One researcher, Robert Beschta, examined the

cottonwood trees in the Lamar Valley, in the

northeastern quadrant of Yellowstone National

Park in the years following wolf reintroduction.

Cottonwoods grow primarily near water, so most

of the trees were found along the Lamar River

that runs through the Yellowstone valley.

Beschta measured the diameter of 700

cottonwood trees from two species that were >5

cm in diameter (Figure 20.3a).

Figure 20.3 a. Frequency distribution of diameters

of two species of mature cottonwood trees from the

Lamar Valley of Yellowstone National Park. The

black bar on the left of the graph represents the

estimated density of seedlings, and is not part of the

700 trees used to measure diameter. b. Frequency

distribution of estimated narrowleaf cottonwood year

of germination, in 20-year increments, using two

different diameter-to-age estimates. If new trees are

growing as expected from high density of seedlings,

the bars should consistently be in the shaded region.

From Beschta 2003 Figures 4 and 6.

Diameters of over 98% of these mature

cottonwood trees were between 30 and 110 cm

(685 of the 700 trees). Less than 2% (15 trees) had diameters between 5 and 29 cm. You can

verify these percentages by adding up the heights of all the bars in Figure 20.3a for trees under

30 cm in diameter, and dividing that total by the total number of trees found by summing the

heights of all the bars.

In addition to sizing all 700 mature trees, Beschta estimated the density and heights of

seedlings. He found most seedlings to be between 0.1 and 0.6 meters high, with some rare

individuals 1 – 2 m tall. Based on the sizes and calculations of density, Beschta estimated that

these seedlings were between 1 and 5 years old, with densities from 4,000 to 70,000 per hectare

(1 hectare = 10,000 square meters).

Beschta used tree ring data on narrowleaf cottonwoods to determine the relationship between

diameter of trees and tree age. Hollow cores extracted from the center of a subset of the trees

allowed determination of tree age. Beschta used the relationship between diameter and age for

the subset to determine age of all trees. He could also determine when a particular tree had

germinated from its seed (Figure 20.3b). The scientist developed an expected distribution of trees

of different ages if seedlings were maturing into adult trees at a rate normal for cottonwoods. He




then compared the actual and expected distributions by comparing the bars to the shaded region

in Figure 20.3b.


10. Go to Wikipedia and google to look up reproductive strategy of dandelions and rabbits

{http://en.wikipedia.org/wiki/Reproduction#Reproductive_strategies}. What aspect of their

reproductive strategies do these species have in common? For each species’ strategy,

describe the age at maturity, number of offspring (many vs. few), offspring survival

probability (high vs. low), number of reproductive events in a lifetime (many, several, or just

one), amount of parental care (if any), and life span. Look up information on reproductive

strategies of elk and humans. What characteristics do the latter species have in common that

differs from the first two species? For instance, do they exhibit more or less parental care,

many or fewer offspring per reproductive episode, or other differences?

11. Several diameter classes are missing from Figure 20.3a – trees of certain sizes are simply not

found. What could explain their absence?

12. Describe the actual and expected distribution of trees of different ages and any differences

you note between the two distributions (Figure 20.3b). What event corresponds to the time at

which differences between the two distributions arise?

Reproductive strategies vary among species, but you discovered that dandelions and rabbits

have some common aspects of their strategies. Both species mature early, have a short lifespan,

and have many offspring with low survival probability. You discovered that they have

differences, too, as dandelions produce many more offspring than rabbits. Elk and humans have

long lifespans and have fewer offspring with higher probability of individual survival. Both

species provide parental care to their offspring, although humans, as you know, provide much

more parental care for much longer than elk. Cottonwood trees have characteristics in common

with both groups of species – they are long-lived, mature later in life, but also produce many

offspring.

Before 1919, the number of cottonwood trees in each age category was within the expected

distribution because seedling maturation occurred normally – this means that new trees were

surviving as expected up until about 1919. However, the number of new trees decreased

dramatically by 1920, and the trees that remained were mostly large and old (Figures 20.3b and

20.4a).

What led to the missing cottonwood trees in the Lamar Valley? From his analysis of the

history of the area, Beschta concluded that: 1) fire was unlikely to have affected the particular

groves of trees he studied, 2) long-term changes in climate had been insufficient to affect woody

tree species, and 3) it was unlikely that insect attack, frost, or disease would have had the same

effect on both species in each and every grove in the Lamar Valley. Beschta concluded that,

despite changes in management strategies and sizes of elk populations in Yellowstone National

http://en.wikipedia.org/wiki/Reproduction#Reproductive_strategies




Park, the effect of the elk browsing on cottonwood seedlings was great enough to account for the

observed gap in seed production and seedling maturation.

Figure 20.4 a. Cottonwoods in winter, with

elk herd (photo credit: Yellowstone National

Park; circa 1970s). Note the lack of small

and intermediate sized trees. b. Cottonwood

grove at Devil’s Slide refuge site along the

Yellowstone River, with a wide distribution

of tree sizes. In this site, the road on the

upslope side and the river on the downslope

side prevented elk from foraging. From

Beschta 2005 Figure 1a and 3b.

Other studies have shown heavy plant feeding by

elk on aspen and willow trees, with a similar

pattern of gaps in the frequency distribution of

ages. Beschta also studied stands of cottonwoods

where seedlings did grow to maturity (Figures

20.4b and 20.5a-c) and he found many trees in the

size ranges that were missing from the Lamar

Valley study (Figure 20.5).

Figure 20.5 Frequency distributions of narrowleaf

cottonwood (and black cottonwood in panel d)

diameters for trees >5 cm in diameter at breast height

(dbh) at five study sites in the northern Yellowstone elk

winter range. dbh is a standard measure in ecology, and

refers to diameter 1.3 meters off the ground. Note that

panel d contains the same data shown in Figure 20.3a.

From Beschta 2005 Figure 5.





13. Summarize the major differences in the frequency distributions of cottonwood trees at the

five sites in Figure 20.5.

14. If you knew that both La Duke Spring and Devil’s Slide (Figures 20.5a and b) were bounded

on one side by a river and the other by a road, what could you conclude about factors

influencing the size distribution of cottonwood trees?

15. Beschta concluded that long-term climate change was not the reason that seedlings weren’t

maturing into trees. Compare the panels in Figure 20.5 and propose an explanation to

support his claim.

16. Beschta concluded that the loss of wolves from Yellowstone National Park probably caused

the lack of cottonwood maturation in some areas. Support his claim using data from Figures

20.2, 20.3 and 20.5.

The patterns of frequency distributions at the different sites illustrate the randomness and

variation of emergent properties, one of this Unit’s themes. Randomness and variation within

biological systems allows populations to be flexible in their response to changing environmental

conditions. Some locations of trees led to exclusion of elk, whereas others did not. The

topography of La Duke Spring and Devil’s Slide effectively excluded elk. Both sites are

bordered by a river and a road, with fairly steep slopes. While there were not many very large

trees, because the grove of trees was established after the new road was built, there were high

proportions of smaller trees present, which would not be expected if the site were frequented by

feeding elk.

The exclusion of elk allowed the narrowleaf cottonwoods to have a flexible response to the

elk-cottonwood interaction. Cottonwood populations survived in some locations where elk

cannot eat saplings, allowing long-term survival of cottonwood within the entire region, even if

some other populations may not survive. The concept of a flexible growth response by a

cottonwood population was used by Beschta to illustrate the pattern of tree growth in areas

where elk were excluded, which parallels the presence of wolves. Also, by demonstrating the

growth of young trees in elk-free areas, he was able to conclude that climate changes to the entire

ecosystem were not responsible for lack of seedling growth.

So far, you studied the elk-wolf interactions, and elk-tree interactions, but you have not

connected the wolves to the resurgence of trees within the park. You should consider all the data

to see how wolves can influence tree growth. From Figure 20.2, you determined that as wolf

numbers increased, the elk herd declined, although the correlation was far from perfect.

However, you also know that tree maturation was low even when humans restricted the size of

elk herds through hunting and relocation programs that occurred from the 1920’s through the

1960’s. It was not until after the “natural regulation” of 1968 began that elk populations reach

their highest levels. Yet narrowleaf cottonwood sapling growth is absent throughout much of the

Lamar Valley from 1920 to the mid-1990’s, which indicates the number of elk is not the only

determining factor for tree survival. The wolf was the only component of the ecological system




missing during that time period. What is the difference between when humans controlled elk

population (1926 – 1968) and when wolves controlled elk populations (mid-1990s and on)?

When a group of scientists tried to determine the difference between human and wolf

regulation of elks, they initially focused on the direct effects of wolf predation. However,

predation did not sufficiently answer the question about the re-emergence of aspen, willow, and

cottonwood trees. If the number of elk killed was the only factor that determined the success of

trees, then human control would have yielded similar results to the natural control. They

reasoned that indirect effects of predation might be the key to understanding the emergent

property of tree survival in the presence of wolves. Indirect effects are another emergent

property of ecological systems.

Once wolves were reintroduced to Yellowstone National Park, elk had to face a predator that

they had not encountered in many decades, and certainly no elk living in the park in 1995 had

ever confronted a pack of hungry wolves. The elk that survived this new predation threat would

be the individuals that were more vigilant for these predators. Are there any costs to the

surviving elk with increased vigilance? You will see how investigators collected some of the

data to answer this question. {Definition: Indirect effects in ecological systems are effects of

one species on other species mediated through shared interactions with a third species or group

of species.}

Scott Creel and his colleagues used radiotracking to study the locations of 14 elk that were

part of herds living in Gallatin Canyon, during 2002 and 2003. Radiotracking is a method in

which scientists fit individual members of a population with collars that transmit a radio signal.

The scientists pick up the radio signal with receivers, which they used to determine their

locations. Creel and his colleagues also tracked wolves in two out of three packs that used the

same geographic area. The scientists assessed the habitat use of elk during times when wolves

were present and when wolves were not detected in the area of the herd (Figures 20.6). When

these data, which are from just one area the scientists studied, are put into a computer model with

all their other data to predict habitat use when wolves are present or not detected, they were able

to estimate the probability of grassy

areas or forest occurring where elk

are located (Figure 20.7).

Figure 20.6 Radiotracked elk locations

and elk kills in Gallatin Canyon within

the Greater Yellowstone Ecosystem

(outside of Yellowstone National Park).

Locations were in conifer forests, the

edge between forest and open grass, and

open grass habitat. Wolves were either

detected nearby or not when each elk

observation was made. Data from Creel

et al. 2005, Figures 1a and b, original

art by CJP.




Figure 20.7 Effects of wolf presence on habitat

use by elk. The probabilities shown are the

probabilities that grassy areas or coniferous areas

were prevalent where elk were found. Bars show

means and 95% confidence intervals. From

Creel et al. 2005 Figure 3, original art by CJP.


17. What do the data in Figure 20.6 tell you

about the location of elk kills and the location

of elk when wolves were known to be nearby

or not detected?

18. What are your conclusions from this graph regarding habitat use of elk when wolves are

present or not detected?

19. What is the difference between wolves being “not detected” and being “absent?” The

scientists performing this study were clear in the use of the phrase “wolves were not

detected.” What are the possible implications of this distinction for elk behavior and

increasing the variability in the data set?

Creel and his colleagues concluded that the elk alter their habitat use when they detect the

presence of wolves. From Figures 20.6 and 20.7, you see that the elk behavior is not perfectly

correlated with the presence of wolves – there is variation in elk behavior. Elk are not perfect in

their detection of wolves, nor are they perfect in selecting habitats based on their assessment of

the risk of predation. Furthermore, the scientists had not placed a radio transmitter on every wolf,

and so some could be in the area without detection. Elk cannot know where every wolf is, nor

do they know the best place to avoid detection by their predators – if they did, wolves would

starve for lack of prey. In addition, prey animals often assess their risk not only on the presence

of predators, but also on their need for food. For instance, a hungry elk might take more chances

on feeding in the grassy areas, where their preferred food is, than an elk with a full belly that

may remain in the relative safety of the conifer forest.

The scientists concluded that elk move in response to the presence of predators within 1

kilometer of their location. Again, variability in elk movements results from variability in wolf

detection by elk. Elk are more likely to occupy sites, such as grassy areas, where they can forage

with confidence that wolves are absent (that is, they haven’t detected them). Conversely, elk are

more likely to seek protective cover in coniferous forests when they detect wolves nearby.

Altering habitat preference based on the presence of wolves is likely to have several effects,

including a decrease in the energetic resources obtained by elk (not eating grass all the time), an

increase in energetic costs by elk (more vigilance when eating grass), and a possible release of

herbivore pressure on tree seedlings (fewer elk feeding in the grass where new seedlings could




sprout). Recall that the grassy areas are often at lower elevations, in flood plains or near rivers,

and this is exactly where the groves of cottonwood, aspen, and willows are often located.

Now that you have made a connection between trees, elk and wolves, it would be wise to

determine if any other factors influence where the elk eat and indirectly what they eat. To

determine whether the changes in elk behavior that occur in the presence of wolves have an

indirect effect on tree growth, you would need to know whether more trees like cottonwood,

aspen and willow are growing in areas where elk are spending less time. In another study,

Beschta and a colleague, William Ripple, measured stands of aspen trees in elevated and

floodplain sites. Like the cottonwoods, there are gaps in the growth of aspen forests that

correspond to the time when wolves were absent from the park (see Figures 20.3 and 20.5). The

scientists determined the recent history of browsing and measured the height of the five tallest

young aspen in each of several stands. They estimated the annual heights of these trees for the

nine previous years based on the

pattern of browsing damage of the

highest branches (Figure 20.8).

Figure 20.8 Comparison of terrain on

tree growth. a. Percentage of leading

shoots of aspen eaten. b. Average

aspen height. From Ripple and

Beschta 2007 Figures 1c and d.

At each tree they also measured the

number of downed large logs that

were within 3 meters of the tree

because logs might impede elk

escaping through the forest. The

scientists hypothesized that aspen

trees would be taller nearer streams

than in upland areas if wolves are

frequenting upland areas. Aspens also were predicted to be taller at sites with downed logs than

sites with fewer or no downed logs due to behavior modification of the elk caused by the

reintroduction of wolves. Of four habitat types studied, they predicted that stream-side sites with

logs would have the highest predation risk, whereas upland sites without logs would have the

lowest predation risk. Elk might be able to assess the risk of predation in different habitats, and

predation risk on elk should then correlate with aspen growth (Figure 20.8).


20. Restate the predictions of Ripple and Beschta regarding aspen tree height and elk foraging, as

they relate to predation risk on elk. If the foraging pattern of elk relates to their perceived

predation risk, such that they forage less in areas where they perceive more risk, then




according to Figure 20.8, which habitat has the highest predation risk? Does your conclusion

match the predictions of the scientists?

21. What is the effect of downed logs on predation risk? Compare the general effect of logs

within a habitat from Figure 20.8: upland with logs vs. upland without logs, and stream-side

with logs vs. stream-side without logs. Then compare the two uplands curves with and

without logs vs. the two stream-side curves with and without logs. Is the effect of logs

within a site greater than or less than the effect of habitat type?

It is uncommon for a policy change to be measured so carefully to determine whether the

policy was a success or not. Many scientists studying Yellowstone National Park ecological

system concluded that the regrowth of the vegetation was likely due to a combination of altered

elk behavior and reduced elk numbers in response to the reintroduction of wolves. Much of the

regrowth occurred near rivers, which wolves frequently use, and would thus present a high

predation risk to the elk. Aspen stands with downed logs would present a further risk of

predation as areas with lots of downed logs would be difficult to run through while escaping

wolves.

The change in elk behavior leads to indirect responses throughout the system, due to

emergent properties. As aspen, willow and cottonwood regenerate and begin reproducing again,

other aspects of ecological systems are predicted to change. Beschta and Ripple analyzed the

communities of berry-producing shrubs that often grow under aspen trees (Figure 20.9). Among

other things, they examined the relationship between shrub height and aspen height and the

number of different species of shrubs under aspen stands that were classified into different height

classes – some stands contained many small aspens, others had aspens that were taller than what

elk could usually reach and browse upon, as discussed for Figure 20.8.

Figure 20.9 Effects of

aspen regrowth on shrubs.

a. Statistically significant

regressions of berry-

producing shrub height vs.

understory aspen height. b.

Average number of berry-

producing shrub species in

aspen stands (±95%

confidence intervals) in

three height categories.

From Beschta and

Ripple 2012 Figures 3

and 4.




Figure 20.10 Direct and indirect interactions without wolves in the northern ecosystems of Yellowstone

National Park (a) and with wolves present (b). Solid arrows indicate documented responses; dashed

arrows indicate predicted responses. From Ripple and Beschta 2004 Figure 5.

Other emergent properties appeared after wolves were reintroduced to Yellowstone National

Park (Figure 20.10). For instance, coyotes had enjoyed being the top predator in Yellowstone

National Park during the wolves’ long absence. Wolves interact with coyotes, and they may

compete for resources, because they are both predators. Coyotes will scavenge for food, feeding

on carcasses left behind by other predators. Wolves are larger and hunt in packs, whereas

coyotes are often solitary, with a smaller body size. When wolves came back, coyote numbers

plummeted and the average size of surviving coyotes also increased. In addition, beavers have

been sighted in Yellowstone National Park where they have not been seen in a long time. This

recolonization of beavers is a consequence of regrowth of a prime beaver food source, willow

trees, along rivers (Figure 20.10).


22. You might have predicted correctly that coyote population size would decline with the

reintroduction of the wolves. However, you might not have expected an increase in

surviving coyote body size. How do you explain the drop in coyote numbers that goes along

with an increase in average body size?

23. While emergent properties are not always predictable, make some predictions about the

effects of beavers on the Yellowstone National Park ecosystem? Figure 20.10 makes the

connection from wolf to beaver. Beavers dam up stream and feed on buds, leaves, and bark




of trees, so you might be able to predict further changes to this ecological system. For

instance, what would be an emergent property associated with increased beaver feeding and

habitat alteration due to new beaver dams?

You know that biological systems, from wolves, to elk, to beaver, require resources. In order

to obtain their resources, wolves hunt cooperatively in packs, although different packs must

compete for territory. Elk have to maintain vigilance in order to avoid predation while foraging

for the food they need; as they alter their behavior to reduce the risk of predation, they may

exhibit a trade-off. Here, elk exchange abundant food consumption for increased survival.

{Definition: A trade-off is a compromise that occurs as organisms exchange one behavior or

resource for another.}

The entire ecological system responded to the presence of wolves in ways that no one

predicted when wolves were reintroduced to the park. Beaver were not present in the park for

many years in part because their preferred trees (young willow trees) were not present in high

abundance; the number of beaver colonies went from one to 12 between 1999 and 2010. The

ecological system contained within Yellowstone National Park exceeds the sum of the individual

species living in the park.

The randomness of this large ecological system provides flexibility of response. For

instance, the locations of downed logs, the meandering of a river, or the movements of a herd of

elk and a pack of wolves, may all play a role in whether a cottonwood or aspen can grow to

maturity. Depending on the abiotic conditions at a particular location such as an upland area or a

floodplain, trees may or may not grow regardless of the elk. The carcasses left behind by wolves

provide a bounty to other scavengers, including coyotes, which may explain the larger sizes of

individual surviving coyotes that were not killed due to resource competition with wolves. There

may be more variability of response among scavengers depending on abundance and location of

the resource, possibly leading to more competitive interactions as scavengers attempt to gain

these resources. You have learned how the interactions between predators and their prey can lead

to emergent properties. In the next section, you will examine how emergent properties arise

from competitive interactions.

Ethical, Legal, and Social Implications Box 20.1

What are the arguments for and against species re-introductions?

Over the last several decades, many species have disappeared throughout all or part of their

original ranges. The most common reasons for loss of species are habitat loss, pollution,

competition with invasive species and global climate change, all primarily caused by human

overpopulation and overconsumption of resources. Many people argue that to restore ecological

systems humans must reintroduce species to their natural ranges, assuming they still exist

somewhere. Perhaps the most successful case of a species that was restored to its habitat is the

California condor, with the largest wing span of any bird (9 feet). All 22 of the remaining




condors were captured in 1987 and placed into a captive breeding program with the intent to re-

release them into the wild. Since 1991, over 150 young adults have been released and 150 are

still in the breeding program. Other species, such as gray wolves, may require some assistance

from humans to expand back into their original ranges.

Reintroducing the gray wolf to Yellowstone National Park was the result of a long debate

over the value of reintroducing a predator. Wolves have been reintroduced in other places, and

similar debates have occurred in these places, too, including Scandinavia and the Adirondack

Park in New York State. In addition to the biological data required to make a successful

reintroduction, social scientists conducted many surveys of public attitudes and perceptions

towards wolf reintroductions. While 60% of the general population supports reintroduction, only

35% of ranchers do. As you might imagine, ranchers were nervous and opposed to the idea,

fearing for their livestock. While a majority of the general population has a positive attitude

towards wolves, more of those people live in urban areas and have little or no experience with

wolves. People who have a negative attitude towards wolves live near the wolves. Others argue

against reintroduction for other reasons, saying that money spent on captive breeding and

reintroduction would be better spent solving societal problems, like homelessness and poverty.

As you consider these arguments, you can appreciate the complexities of conservation.

Because humans eliminated top predators, do we now have an ethical obligation to restore those

species to their former ranges? When predators were eliminated, not much was known of the

ecological roles of those species. The direct and indirect roles of wolves have been demonstrated

in their return to Yellowstone National Park. Gray wolves were the only native vertebrate

species that was not still present in Yellowstone prior to its reintroduction. However, local

residents are concerned about loss of life or economic damage to crops or livestock that may

result from wolf reintroduction. Humans eradicated gray wolves from the park and the

surrounding area because they perceived the wolves as a threat to livestock and family, and that

perception persists. Should federal tax dollars be used to conduct additional research to

determine whether these perceived threats were justified or not?

Ethicists have argued that if we honor our ethical duty to a past ecosystem by reintroducing a

missing species, then we create a new conflict with our ethical duties to present and future

ecosystems, in that a missing species may cause harm to other species. “Do our duties to existing

ecosystems outweigh our duty to past ecosystems?” If we reintroduce a species to an area where

it is no longer present, species that currently thrive there may be harmed. In the case of wolves,

prey populations were harmed, but some species increased in abundance after reintroduction. As

you know, indirect effects and emergent properties are difficult to predict. How can you weigh

these possible outcomes when you can’t even predict them?

Finally, there is a concern among ecologists that a plan by humans to “restore” an ecosystem

leads the public to believe that ecosystems are naturally static. Ecological systems are naturally

dynamic, and constantly undergo changes. Extinction of a species can be caused by non-human

processes, and if humans are perceived as a natural part of an ecosystem, then humans are just

another natural cause of extinction, as natural as an ice age or a volcano. Thus, both




reintroductions and eliminations may be viewed as equally harmful or beneficial to ecological

systems. Where do you fall on this ethical issue?

These arguments and the emotions they generate have led to changes in laws and policy. The

concern of the late 1960’s and early 1970’s was caused, in part by the publication of Silent

Spring, by Rachel Carson, several environmental disasters, and the first Earth Day in 1970.

Important environmental laws were enacted during those times, including the Endangered

Species Act (ESA) in 1973. The ESA is a very powerful law aimed at preserving endangered

species. The federal government defines an endangered species as one at risk of extinction

through all or a significant portion of its natural habitat. A threatened species is at less risk, but

is likely to become endangered in the foreseeable future. The ESA requires the U.S. government

to have a recovery plan for any species listed as endangered or threatened. Biologists from the

Fish and Wildlife Service within the U.S. Department of Interior are charged with developing

recovery plans to protect species and increase their populations.

Gray wolves were listed as endangered in the lower 48 states but not in Alaska or Canada.

The reintroduction of wolves into Yellowstone National Park was part of the federal recovery

plan. Part of the reintroduction debate centered on the question of whether humans should

restore a species that is abundant in Alaska to an area where they have become locally extinct.

In addition to questions based on emotions and value judgments, the recovery plan presented

many legal questions, too.

The cost of the reintroduction program to Yellowstone National Park was a few hundred

thousand dollars with no clear economic benefit at the time. Now that the recovery is in full

swing, the benefits are estimated to be millions of dollars due to increased tourism to

Yellowstone National Park. The financial benefit takes into consideration lost income from

reduced hunting and compensation for ranchers who lost livestock. Interestingly, tourism is also

an emergent property that has an economic impact that had not been predicted when recovery

plan was implemented.

In early 2008, the gray wolf was removed from the ESA’s threatened list for the lower 48

states even though wolves are only in a few states. Gray wolves were removed because about

1,500 wolves are thriving in the greater Yellowstone ecosystem and several other areas in the

Northern Rockies. Ironically, the success of the wolves has produced unexpected controversy

because their removal from the endangered species list meant that wolves can be shot and killed

once they step out of Yellowstone National Park. An emergent property argument could be

made that the only way to protect the wolves is to keep their population low. Even while

acknowledging the success of wolf conservation, environmental groups feared that losing their

protected status could be a step backward, especially if hunters and ranchers begin killing wolves

with impunity. Even while ranchers complain that wolves eat their livestock and that they

should be allowed to kill wolves to defend their stock, environmental groups have sued the

federal government over the decision to remove protection of the gray wolf. The judge reversed

the Bush Administration’s decision to remove protection for the wolves. Rather than appeal the

decision, the Bush Administration removed its support for the de-listing, effectively placing gray




wolves back on the endangered species list. During the time that protection was removed, more

than 100 wolves were killed in Idaho, Wyoming, Montana and parts of Oregon. This series of

events is an ongoing drama and debate about the ethical, legal and social implications of humans

trying to be good stewards of an endangered species and the entire ecosystem.

ELSI Integrating Questions

1. Given what you now know about the Yellowstone National Park ecosystem, was

reintroducing the wolves a beneficial move? Can you place a dollar figure on species

recovery in a national park? Can you predict the benefit to unforeseen emergent properties

such as tourism and beaver dams?

2. When a species is removed from the endangered species list, they are no longer afforded

protection under the ESA. Will a species’ success lead to population declines, wiping away

the past successes of the ESA and the earlier success of a once-endangered species? Is the

possibility of extinction an emergent property of a successful recovery plan?

20.2 What determines the outcome of competition for a resource?

Context: Competitive interactions affect populations and can be affected by ecological

conditions.




Bottom Line: The outcome of a competitive interaction is often determined by other factors

in the habitat.

Within ecological systems, different species compete for resources that are in short supply.

Over evolutionary history, competition for resources has led to evolution of specific adaptations

for efficient gathering of those resources. {Connections: The Big Idea of Evolution is examined

in Chapters 6-10.} For instance, in your examination of the effects of wolves on Yellowstone

National Park, you learned that they evolved social behavior which led to cooperation within a

pack. When social behavior of wolves first evolved, the wolf ancestors that exhibited

cooperative behavior were more successful than those that hunted alone. Cooperative behavior,

just like any other trait, has a genetic component to it. Wolves that hunted cooperatively

produced more offspring, who in turn inherited the alleles associated with cooperative hunting.

Cooperation spread throughout the wolf population due to increased resource acquisition, which

increased the efficiency of food gathering for all subsequent wolves. From this example, you

might predict that the organism most efficient in gathering a specific resource will outcompete

other organisms, and drive less efficient populations toward extinction. If wolves are better

hunters than coyotes, why hasn’t the wolf driven the coyote to extinction? The answer was

explored in IQ #22 and will be explored further in BME 20.2. {Definition: Cooperative

behavior is behavior that involves several individuals and is mutually beneficial.}




Two species such as wolves and coyotes that consume similar resources have high resource

use overlap. Coexistence is possible, but how do they strike a balance? According to the

competitive exclusion principle, one competitor will exclude the other from a commonly used

resource when resource use overlap is high. {Definitions: Resource use overlap is a measure of

the resources shared between any pair of species. The competitive exclusion principle states

that no two species that consume the exact same set of resources can coexist.} In other words,

the two species need to evolve a mechanism to eat slightly different foods (or otherwise divide

up the resources) or else one of them will become extinct. Resource overlap will lead to either

extinction of the losing competitor species or an evolutionary adaptation to use a different

resource, thus reducing overlap. The competitive exclusion principle assumes that other

ecological factors such as resource abundance and abiotic conditions are constant. Competitive

exclusion has many exceptions which are almost always based on the fact that ecological factors

frequently change over time because biological systems are dynamic.


24. If wolves are more efficient hunters than coyotes, how is it possible that wolves and coyotes

can coexist? Before proceeding to BME 20.2, speculate on the diets of these two predators

and how much overlap there is now that they coexist in the same habitat.

25. What amount of overlap in resource use is required for two species to exist in the same

habitat? How might the abundance of unshared resources affect a competitive interaction?

---------------------------------------------------------------------------------------------------------------------

Bio-Math Exploration 20.2 How can you measure resource use overlap?

Concepts: Pianka measure of resource use overlap.

Objective: Predict competition and behaviors of predators based on extent of resource use

overlap.

Required Skills: Graphing points, summation notation, basic trigonometry

The competitive exclusion principle tells us what happens when multiple species compete for

a single resource, but the situation is more complex when they compete simultaneously for

multiple resources. To describe the extent of competition between two species, scientists use

various measures of resource use overlap. One of these measures, called the Pianka measure

(after a scientist of the same name), can be illustrated by a simple graph when there are only two

resources. The following questions guide you through the discovery of this concept by

considering the following two hypothetical resource use mixes:

(1) The typical wolf diet is 60% elk and 40% deer, while the typical coyote diet is 45% elk and

55% deer.

(2) The typical wolf diet is 75% elk and 25% deer, and the typical coyote diet is 30% elk and

70% deer.




Bio-Math Integrating Questions

20.2a: Explain why the resource use overlap between wolf and coyote is very high in the first

hypothetical resource use mix.

20.2b: Graph the wolf and coyote dietary percentages in the first hypothetical resource use mix

as (x,y) points, where the x-axis represents the elk percentage and the y-axis represents the

deer percentage. The scale on each axis should be from 0 to 100. You should have one point

for the wolf and one point for the coyote. Now draw two lines, one line to each of these

points starting from the origin (0, 0).

20.2c: Explain why the resource use overlap between wolf and coyote is low in the second

hypothetical resource use mix.

20.2d: Repeat the graphing process for the second hypothetical resource use mix, and compare

the graphs for the two situations. Describe how the angle between the two lines for a given

species is different between the high and low resource overlap conditions.

The Pianka measure of resource use overlap formula quantifies the observation you made in

the BioMath integrating questions, that the angle between the two lines in the graph is large

when the resource use overlap is low, and small when the resource use overlap is high.

Specifically, the Pianka measure is the cosine of the angle between the two lines formed in the

above graphs. Because the cosine of an angle is small if the angle is large, a small Pianka

measure means low overlap. Similarly, the cosine is large if the angle is small, so a large Pianka

measure means high overlap. In general, if elk is Wx percent and deer is Wy percent of the wolf’s

diet, and elk is Cx percent and deer is Cy percent of the coyote’s diet, then the formula for the

Pianka measure of resource use overlap between wolf and coyote is

WxCx WyCy

Wx

2Wy

2 Cx2Cy

2 (20.1)

For example, in the first hypothetical resource use mix (i.e., high overlap situation) the Pianka

measure of resource use overlap is

0.6 0.450.4 0.55

0.62 0.42 0.452 0.552 0.995


20.2e: Use Equation 20.1 to determine the Pianka measure of resource use overlap in the second

hypothetical resource use overlap (i.e., low overlap situation).

20.2f: By experimenting with Equation 20.1, or recalling the values of the cosine function when

the angle is between 0 and 90 degrees (see the cosine tutorial on Wiley Plus), what is the

maximum possible value of the Pianka resource use overlap measure? What is the minimum

possible value? By experimenting with the dietary percentages, determine values of Wx, Wy,




Cx and Cy that produce both the maximum and minimum possible values for the Pianka

measure of resource use overlap.

You may be wondering how Equation 20.1 could possibly be the cosine of the angle between

the lines in these graphs, because you learned that the cosine of an angle is “adjacent over

hypotenuse.” However, the adjacent over hypotenuse calculation only works in a right triangle,

and the triangles you are working with do not necessarily have right angles, so a different

method is needed. If you are interested in the details, you can go to

http://www.mathwarehouse.com/trigonometry/law-of-cosines-formula-examples.php to see how

the Law of Cosines and the formula for the distance between two points produces the formula in

Equation 20.1.

The Pianka measure of resource use overlap can be extended to any number of food

resources. It is harder to visualize the amount of overlap when there are more than two resources,

and impossible when there are more than three resources because you cannot graph points in

more than three dimensions. The formula for the Pianka measure when two species compete for

10 resources is like Equation (1), but with 10 percentages plugged in for the two competing

species. For example, if Wi represents the percentage of the wolf’s diet allocated to the ith

resource, and Ci represents the percentage of the coyote’s diet allocated to the ith

resource, then

the Pianka measure of competition between the wolf and coyote over these 10 resources is:

Wi Cii1

10

Wi

2

i1

10

Ci2

i1

10


20.2g: The Excel spreadsheet Pianka.xls lists ten food resources for wolves and coyotes in an

area similar to Yellowstone National Park and the approximate percentages of each resource

in each predator’s diet. What is the Pianka measure of resource use overlap for wolves and

coyotes in this region?

20.2h: Using Pianka.xls, answer the following questions: Which two of these ten food resources

would result in minimal resource use overlap between wolves and coyotes, assuming all

other food resources became locally extinct? Which two food resources would result in

maximum resource use overlap in the absence of all other resources?

You found that resource use overlap for wolf and coyote with ten prey types was 0.66. The

two food resources that would result in the lowest overlap of 0.40 you found to be squirrels and

bison. The two that led to the highest overlap were deer and coyotes. Neither of these scenarios

is likely to lead to long term sustainability of the predator populations. Measuring resource use

overlap helps scientists assess the stability of an ecosystem like Yellowstone National Park,

predict future resource usage, and plan conservation efforts. For example, using the data

http://www.mathwarehouse.com/trigonometry/law-of-cosines-formula-examples.php




provided in Pianka.xls, ecologists could predict ways in which wolf and coyote behaviors might

evolve to reduce the degree of resource use overlap in Yellowstone National Park. Similarly,

conservation biologists could assess the resources and predict the possibility of extinction of

wolf competitors or the success of wolves when wolves were reintroduced to the Park.

---------------------------------------------------------------------------------------------------------------------

Species that have high resource use overlap coexist by altering their resource use when the

competing species is present. Coyotes may include a higher proportion of elk in their diet when

wolves are absent, but then switch to consuming more squirrels, rabbits and mice when wolves

are present. Resource use overlap can be relatively high in two coexisting species, but cannot be

100%. As overlap increases, species involved in competition may switch to unused, less

desirable resources, especially if they are abundant in the environment. This switching behavior

is a mechanism for species coexistence.

The outcome of competition depends on the species involved, but may also depend on

variation in abiotic factors in the environment. Ecological systems change across space, where

conditions related to an abiotic factor change as location changes. For example, average

temperatures become cooler as one moves from the equator toward the poles, or the moisture in

soil may decrease as you move upslope from a body of water towards higher ground. Another

type of change is across time, where the condition changes as time passes. A common example

is the variation in the amount of rain as seasons change from dry to rainy season. Some changes

take place over time and space. Consider an estuary, the region where rivers meet oceans. The

salt content of estuaries may change not only across space, as you move from a river delta

toward the ocean, but also across time, tides push in salty water or yield to fresh water in the

river. Changes in environmental properties across time or space are called environmental

gradients, and biologists often study how organisms respond to these gradients. {Definition: A

gradient is a change in an abiotic variable across time or space.}

Some conditions are harsher than others – for most plants, drier soils with fewer nutrients are

more stressful than wet, fertile soils. Disturbances such as fires or strong storms may also affect

the ability of an organism to survive. {Connections: Evolutionary adaptations to disturbance

are explored in Section 10.3.} Some scientists have hypothesized that organisms adapted to

living in harsh conditions, either because of high levels of disturbance or low levels of essential

nutrients, are poor competitors in less harsh conditions. If this hypothesis is correct, species

adapted to conditions of high nutrients or high resources would be better competitors when living

in conditions with high nutrients or resources.

You will examine one study that investigates this hypothesis about adaptation to harsh

conditions, but before you do, you should review the three main ideas about emergent properties

and apply them to this hypothesis. One main idea about emergent properties is that biological

systems require resources, which results in competition or cooperation for resources. Another

main idea about emergent properties is that randomness and variation within an ecological

system allow coexistence of species. Different species that consume the same resource (e.g.,




plants competing for sunlight) may be adapted to live in different environmental conditions. For

example, plants compete for access to sunlight, but some plants are adapted to direct sunlight

while others prefer indirect light, which minimizes resource use overlap. Tall oak and pine trees

are adapted to more direct light conditions while shade-loving mosses and ferns specialize in low

light photosynthesis and yet all four plants consume the same resource. {Connections:

Adaptations to varying light conditions is explored in Section 9.4.} These plants don’t coexist at

the exact same location, but they can live near each other within in a wider ecological system,

such as a forest. The third main idea about emergent properties is that biological systems exceed

the sum of their parts. These ecological systems do so by increasing biodiversity in the forest,

causing indirect interactions, and changing the abiotic conditions of the forest.

Ecologists Scott Wilson and Paul Keddy tested the hypothesis that there is a trade-off

between competitive ability and adaptations to harsh conditions. They studied the distributions of

seven plant species along the shores of Axe Lake in Ontario, Canada where there exists an

environmental gradient. This environmental gradient included both changes in disturbance, in

the force of waves hitting the shore, and the amount of organic matter and nutrients in the soil.

The scientists quantified the organic matter content in many plots as a measure of the gradient

because they had found in previous research that organic matter negatively correlated with wave

action intensity – shores exposed to high wave action have lower levels of nutrients. Their study

sites ranged from exposed beaches with high wave action and low nutrient concentrations to

sheltered shores in small inlets with nutrient-

rich soils.

Wilson and Keddy divided the plots into

seven organic matter categories, represented on

a logarithmic scale in Figure 20.11. Within

each category, the researchers determined the

percentage of plots of that contained each

species, and represented those values as a

frequency distribution in Figure 20.11. Because

plots often contained more than one species, the

percentages in any one category do not add up

to 100%. Regardless, they were able to

compare frequency distributions of the different

species against the organic matter content.

Figure 20.11 The distributions of seven plant

species along the organic matter gradient of a

lakeshore. The average sediment organic content

associated with each species is indicated by an

arrow. Species' common names are given. From

Wilson and Keddy 1986 Figure 1.





26. Consider the following factors and situations and describe how the factor changes across

space: water depth from the shore of a lake to its center, temperature from a grassy area to

the edge of a forest to the forest interior, salt concentration from the mouth of a river to an

estuary to the open ocean, moisture content of soil from the edge of a river to the top of a hill.

27. Compare and contrast the frequency distributions of the seven plants. A frequency

distribution in this case would be the percentage of plots in which each of the seven plants

was found at each concentration of organic matter.

28. Are there any plants that appear similar in resource and habitat use, and if so, which ones?

29. Organic matter content is an indication of harshness and disturbance. According to the trade-

off hypothesis stated previously, which species do you predict will be poor competitors?

Superior competitors?

Once Wilson and Keddy plotted their data, they observed clear differences in the

distributions of the plants. Some plants had higher frequencies of occurrence at lower organic

matter levels while others had higher frequencies at the high end of the organic matter scale.

Wilson and Keddy designed an experiment to determine the competitive abilities of each plant

species compared to the other six species. The scientists placed two plants, one from each of two

different species or two from the same species, in a small bucket filled with sand and organic

shoreline sediment, simulating a sheltered shore with high organic matter. They had ten buckets

of each pair-wise combination of species for a total of 490 buckets.

The scientists measured relative growth, growth in the presence of another species compared

to growth in the presence of another member of the same species, as an indication of competitive

ability. They quantified their comparative growth using a value they called relative increase per

plant (RIP). Growth was determined by measuring biomass at the time of planting and again at

the time of harvest about three months later. Since the biomass measurement kills the plants, the

initial measurements were made on a random sample of plants of the same size as those planted,

sacrificed for that purpose. To calculate RIP at the end of the experiment, they determined the

average final biomass of plants of species i grown in the presence of plants of species j minus the

average initial biomass of plants of species i. They then divided that quantity by the average

final biomass of a plant of species i grown in the presence of a plant of species i minus initial

biomass of species i.

(final biomass of i with j) – (initial biomass of i)

RIPij = --------------------------------------------------------------

(final biomass of i with i) – (initial biomass of i)

Table 20.1 lists their results. {Definitions: Biomass is the total mass of a living organism after

removal of water, often expressed or referred to as dry mass.}




Table 20.1. RIPij values for each pairwise combination of seven plant species grown in experimental

buckets. The species listed in the rows are i, the species being compared to others; those in columns are j,

the species to which species i is being compared. Target score is the mean RIP of a species in comparison

to all six other species. Neighbor score is the mean RIP of a species when it is the neighbor of each of the

six other species. From Wilson and Keddy 1986 Table 1.

Three-

way sedge

Brownfruit

rush

Loose-

strife

St. John’s

wort

Breaksedge Sundew Pipewort Target

Three-way sedge 1 1.33 1.18 1.17 1.25 1.18 1.34 1.21

Brownfruit rush 0.63 1 1.34 1.46 1.46 1.52 1.49 1.27

Loose-strife 0.88 0.87 1 1.63 1.63 1.78 1.57 1.34

St. John’s wort 1.09 0.99 0.91 1 1.22 1.29 1.23 1.10

Breaksedge 1.05 0.73 0.93 0.91 1 1.21 1.36 1.03

Sundew 0.98 0.91 0.93 1.02 1.02 1 1.11 1.00

Pipewort 0.65 0.71 0.88 0.89 0.87 1.48 1 0.93

Neighbor 0.90 0.93 1.02 1.15 1.21 1.35 1.30

RIPii, which assigns a value for plants grown in the presence of another plant of the same

species, will always equal 1, as seen in the diagonal of Table 20.1. If RIPij < 1, the plant of

species i accumulated less biomass when grown with a plant of species j than when grown with a

plant of its own species. For instance, brownfruit rush accumulated less biomass in the presence

of three-way sedge than when grown with another brownfruit rush. Conversely if RIPij > 1,

species i accumulated more biomass when grown with a plant of species j than when grown with

another plant of species i. Three-way sedge accumulated more biomass when grown with

brownfruit rush than it did when grown with another three-way sedge.

In order to determine the performance of each species compared to all others, Wilson and

Keddy calculated the target score for each species. They did this by averaging the RIPij values

for all other species j in comparison to the target species, that is, the values across the rows in

Table 20.1. A target value above 1 indicates a species is a good competitor and outcompetes all

or most of the other 6. Three-way sedge did well in the presence of other species because the

sedge accumulated more biomass on average in the presence of other species than it did in the

presence of other sedge (average target score = 1.21).

Similarly, Wilson and Keddy calculated a neighbor score. To find this value they averaged

the RIPij values of all six other species i grown in the presence of a particular species j (i.e.,

down the columns in Table 20.1). A neighbor score above 1 indicates that other species i fared

well when paired with species j. If a neighbor is a good competitor on the other hand, other

species will do poorly and the mean neighbor score will be less than one. Sedge is not a good

neighbor, as the average neighbor did worse in its presence than it did in the presence of a plant

of its own species (average neighbor score = 0.90).

Finally, as shown in Figure 20.12, the scientists compared each target score and each

neighbor score to their average position on the sediment organic matter scale (from Figure

20.11), and calculated the correlation coefficient for each set of scores vs. the percent sediment




organic matter content. Recall from BME x.x that correlation coefficients range from -1 to +1,

and measure the strength and direction of a linear relationship between two variables.

a value of -1 indicates a strong negative linear relationship; one variable increases as the

other decreases;

a value of 0 indicates no relationship;

a value of +1 indicates a strong positive linear relationship; one variable increases as the

other increases.

Figure 20.12 a. The relationship

between target scores and percent

sediment organic matter content for the

seven species at Axe Lake, Ontario. b.

The relationship between neighbor

scores and percent sediment organic

matter content for the seven species.

Correlation coefficients are given for

each panel. PW = pipewort, BS =

breaksedge, BFR = brownfruit rush,

LS = loosestrife, TWS = three-way

sedge, SJW = St John’s wort, and SD

= sundew. From Wilson and Keddy

1986 Figure 2.


30. Which species is a better

competitor, brownfruit rush or

three-way sedge (see Table 20.1

and the description in the text)?

31. The sundew was excluded from

the correlation analysis in Figure

20.12. Examination of both graphs

may provide a clue as to why the

analysis omitted sundew. In

addition, sundew is insectivorous.

What does insectivorous mean, and what are the resource implications for competition

between sundew and the other plants in this ecological system? Search the textbook website

or the Encyclopedia of Earth site (http://www.eoearth.org) for information on insectivorous

plants. Specifically, look for information on sundews.

32. How do target and neighbor scores in the plant community change as percent sediment

organic matter increases? On each graph in Figure 20.12, indicate the dependent and the

independent variable. Which dependent variable of competitive advantage has a negative





relationship with the independent variable, and which one has a positive relationship? Do the

relationships fit the pattern predicted by the scientists?

You began this case study by asking whether there were emergent properties associated with

competition for limited resources. You have discovered that plants with high competitive ability

tend to occupy sheltered, nutrient-rich shores, while species with low competitive ability occupy

disturbed shorelines with low nutrient content soils. This make sense since those who compete

best get the best growing conditions, while those that compete poorly must adapt to worse soil

conditions or face extinction.

From the analysis of Wilson and Keddy, you saw that the sundew is not a very good

competitor in high organic matter soils, having the second lowest target score, and only does

better in head-to-head competition with pipewort. And sundew is also a very good neighbor,

with the other six species outcompeting sundew in the experiment. Because of sundew’s ability

to acquire extra nutrients from insects, the researchers did not include this species in the

correlation analysis. Under the conditions of the experiment, sundew did not perform well, but

along the shores of the lake, sundew actually does compete well, and tends to be found in more

favorable conditions. The nutrients from insects give it a competitive edge.

Adaptations to particular environmental conditions such as disturbance and soil nutrient

concentrations determine where species can live, which directly or indirectly determine the

outcome of competition. The property that emerges from all this is a diverse community of plant

species that coexist at a location (such as the shore of a lake) and yet use a variety of

microhabitats. {Connections: Microhabitats are explored in Section 8.3.} Plants provide

resources to many microbial and animal species that benefit from these different plants. The

beneficiaries of the plants have, in turn, different competitive abilities and adaptations that they

need to feed on particular plants. All these different interactions lead to increased complexity and

the potential for indirect interactions in ecological systems – a good example of an emergent

property at the ecological system scale.

Although the seven plants exhibit a significant amount of overlap in distributions at Axe

Lake, Ontario, the sundew has adapted very well to living in conditions in which the other

species cannot exist. You would not expect plants to evolve to be less competitive and adapt to

low-nutrient growth conditions. But growth in nutrient poor conditions is better than going

extinct after competing and losing out to a more competitive species. You have uncovered an

evolutionary mechanism involving competing species: a species may adapt to exploit a resource

more efficiently, and become a superior competitor (target score > 1). Alternatively, a species

with low target scores may adapt to live in an environment from which superior competitors are

excluded, or it may adapt to exploit a different resource altogether. Adapting to an alternative

means to acquire a limited resource is an explanation for sundew, the carnivorous plant. In the

next study you will examine competition for a different type of resource, but one that also

involves the introduction of new species to an area and habitat alterations by humans.




To examine the impacts of an introduced species and habitat alteration on the outcome of

competition, researchers at The Australian National University studied birds that set up nests in

tree cavities. These cavities are often in short supply, as many other animals, such as mammals

and bees also use these cavities as habitat, so competition may be strong. If forests are cleared

by humans that may well reduce the number of available cavities. If a tree-cavity nesting bird

species is introduced to an area where competition is already fierce, the outcome of competition

may be altered. Researchers studied the invasive common myna bird (Acridotheres tristis) and

two native parrot species, the crimson rosella (Platycerus elegans) and the eastern rosella

(Platycerus eximius).

The goal of the research was to determine the impact of common myna use of nest boxes,

which are artificial cavities set up by the researchers, on the abundance of the two parrots in

forests with different tree densities, from highly modified open grassy woodlands with low tree

densities to native forests with high tree densities. The researchers studied the abundance,

percent occupancy of nest boxes and egg success (% of laid eggs that hatched) of each species

across the gradient of tree density (Figure 20.13). They determined that abundances of common

myna and crimson rosella were correlated with the percentages of nest boxes occupied across all

forest types (compare Figures 20.12a and b). This is important to know to determine how

competition for nest boxes affects abundance of species. If a superior competitor or poor habitat

is shown to reduce nest box occupancy, then that reduction leads to a decrease in the population.

They then compared abundance of the two parrot species to nest box occupancy of myna

birds across forests of different densities (Figure 20.14). Overall, the researchers hypothesized

that tree density would influence the three species

differently and that increased occupancy of nest

boxes by common myna would reduce the

abundance of the parrots, especially at low tree

densities.

Figure 20.13 Responses of three cavity-nesting bird

species to forests of different density. a. Mean bird

densities. Changes in abundance across forest density

types was statistically significant for all three species. b.

Mean percent occupancy of nest boxes. Changes in

occupancy across forest density types was statistically

significant for all three species. c. Mean percent egg

success. Changes in egg success across forest density

types was statistically significant for common myna and

crimson rosella. From Grarock et al. 2013 Tables 2, 3

and 4, original art by CJP.




Figure 20.14 Relationships between the percentage

of nest boxes occupied by myna birds at a site and

abundance of rosellas. a. Crimson rosella. b.

Eastern rosella. The best fit regressions were

statistically significant for both rosella species.

From Grarock et al. 2013 Figures 6 and 7,

original art by CJP.


33. Describe the relationship between tree

density and bird species responses.

34. If scientists observe variation in the ways

two species respond to different

environmental conditions, such as tree

density, they can begin to describe the

habitat requirements of those species and

predict success of introduced species. From

Figure 20.13, describe the habitat

requirements of the common myna and predict its success.

35. Is competition between these bird species evident? What evidence supports your conclusion?

Similar species often overlap in their geographic ranges and their diet. This leads to potential

competition between the two species. As you saw with the wolf and coyote, different

combinations of prey items in the diet lead to more or less overlap in diet, and thus more or less

competition. When wolves were reintroduced to Yellowstone National Park, there was suddenly

competition in the park between wolves and coyotes. This led to a change in the coyote

population that was present in the park. Competition can lead to adaptations to environmental

conditions: a species may adapt to some environmental condition, such as high temperature or

high tree density, in order to avoid competition with another species.

With the myna and two parrots, the overlap in nest cavity use was 100%, or very close to it.

All three species utilized the nest boxes, but they may be more selective in natural cavities,

reducing competition. The introduction of a new species, the common myna, which overlapped

in nesting requirements so much with native species could, and did, led to competition.

However, the strength of competition varied with tree density, and the variability in tree

density at different sites allowed all three species to coexist, despite the dramatic disturbances

associated with introduction of a strong competitor and forest clearing by humans. These two

disturbances appear to work together, in that clearing of forests leads to habitats with lower

densities of trees and thus fewer nesting cavities. The common myna preferred low density

forests and had densities about ten times higher in those forests. High densities of common




myna led to high percent occupancy of nest boxes and greater success of eggs. Something in the

low density forests allowed myna birds to be more successful.

Low tree density forests might naturally have low densities of rosellas. However, just as you

concluded from your examination of the data, the researchers were able to show that in those

habitats high tree densities of myna birds led to high occupancy of nest boxes and that reduced

the abundance of crimson and eastern rosellas. When mynas were less abundant, as in the

medium and high tree density forests, abundance, nest box occupancy and egg success all went

up, to different degrees, in the two rosella species.

You have learned that changes in an environmental condition, in this case forest tree density,

strongly influenced the outcome of competition for a limiting resource, in this case, nest boxes.

The outcome of competition was determined partly by the differential use of habitat by the birds

determined by their different genetically encoded responses to variable forest types. {Definition:

a limiting resource is a resource, such as food, light, nutrients, or space, which is in short supply

and restricts the growth of an organism or population.}

You began this section by considering competition for resources and the variability in

environmental conditions. The results of the studies demonstrated all three of the main ideas of

emergent properties. Adaptations to different habitats led to the evolution of diverse species that

utilize similar resources within an ecosystem, in such a way that they can coexist in a broad

geographical region. Species diversity is an emergent property of ecological systems – the

ecosystem, with its individual species all living in the same region, is more than the sum of its

parts. Competition for resources led species to evolve adaptations to exist in different

microhabitats – the three bird species have adapted to slightly different conditions, which often

helps avoid direct competition and possible extinction. The three bird species were able to

coexist together, even though densities of rosellas were reduced in the presence of high myna

density, possibly because the birds competed less for other resources, such as food.

20.3 How efficient are ecological systems at transferring energy from the sun and carbon

dioxide from the air to predators?

Context: Energy from the sun and carbon dioxide from the atmosphere enter and move

through ecological systems.




Bottom Line: Energy and nutrients are transferred between trophic levels via feeding

relationships.

In the first two sections of this chapter, you focused on emergent properties that arise in

ecological systems as a result of predator-prey or competitive interactions. Each of these types

of interactions affects the flow of energy and nutrients within an ecological system. Energy

flows are emergent properties and you will examine them in more detail here. Ecological




systems are characterized by interactions and interdependent relationships. As you have

discovered, emergent properties at the ecological systems level arise when species live together

in the same habitat, obtaining their resources in diverse ways.

Energy, in the form of sunlight, carbohydrates or fats, as well as nutrients, such as carbon

dioxide, nitrates and phosphates, are the resources that all organisms must obtain to survive and

maintain stable or constant internal conditions (homeostasis). Ultimately, energy from the sun

powers the growth of animals at the top of most food webs (see Figure 20.1 or the food web you

drew in IQ #3). All of the interactions you have discussed in this chapter revolve around

obtaining resources, from wolves eating elk or plants competing for soil nutrients. {Connection:

Homeostasis is the Big Idea in Chapters 21-25.}

Each individual plant, bacteria, fungus, and animal goes about its business, obtaining its own

resources, confronting its own predators, prey, competitors, parasites, etc. When an elk chews

on a cottonwood sapling, there is a transfer of energy and material from the plant to the

herbivore. Plants convert light energy from the sun and carbon dioxide from the atmosphere into

the chemical energy of carbohydrates and this conversion of energy is called primary

production. When you add up all the light energy in primary production, you get the amount of

energy available in the first trophic level. In Chapter 22, you will investigate the molecular and

cellular aspects of this conversion, but here you will consider the properties that emerge in

ecological systems when you consider all plants collectively. {Connections: we discuss

photosynthesis in Chapter 22.} Light energy and carbon dioxide flow from the abiotic part of the

environment to the living primary

producers (Figure 20.15).

{Definitions: Primary production

is the production of organic

compounds from carbon dioxide,

principally through the process of

photosynthesis. A trophic level is a

feeding position in a food web,

which describes what an organism

eats and what eats it.}

Figure 20.15 The carbon cycle,

showing how light energy and carbon

dioxide (CO2) are taken up by plants as

the first step of producing chemical

energy that can later flow into higher

trophic levels (the consumers). From

Thoughts on Global Warming http://thoughtson globalwarming.

blogspot.com /2007/02/ factsheet.html.




Summing up all individuals and species, resources of energy and matter move from the

bottom of trophic pyramids to the top. When considering matter, scientists tend to measure the

amount of biomass, or the amount of living matter mass minus the mass of water. For energy,

scientists will measure the amount in different food sources, often in units of calories. When all

the energy is added up, there is a consistent relationship observed across different trophic levels

(Figure 20.16). {Definition: A trophic pyramid is a representation of all the energy or biomass

at each trophic level.}

Figure 20.16 Distribution of biomass or energy at different trophic levels in an ecological system.

Primary consumers are those animals that eat plants, also known as herbivores. A secondary consumer

eats primary consumers, and a tertiary consumer eats secondary consumers. Secondary consumers and

above are also known as carnivores or predators. Original art by CJP.


36. Based on your personal observations of plants, does sunlight or carbon dioxide most

frequently limit growth? Is there geographic variation in the conditions that favor plant

growth?

37. Investigate the concept of “trophic level” on Encyclopedia of Earth (http://www.eoearth.org)

or ScienceDaily (http://www.sciencedaily.com).

38. Describe the pattern in the distribution of biomass or energy in the trophic levels of a typical

ecological system (as shown in Figure 20.16).

39. Investigate the different trophic levels – provide examples of organisms at different levels,

using Yellowstone National Park or the food web you put together in IQ #3 as a case study.

For your trophic pyramid, consider where you should put decomposers in your scheme. This

latter question is challenging – how could you fit those organisms into the scheme presented

in Figure 20.16?

Spiders, dragonflies, and raccoons are carnivores and they could be secondary, tertiary, or

quaternary consumers. Primary consumers eat plants or other primary producers, secondary

consumers eat primary consumers, and tertiary consumers eat secondary consumers. Quaternary


http://www.sciencedaily.com/




consumers are usually the top predators that consume tertiary (and often secondary and primary)

consumers. Going back to our original question, how does light energy from the sun get

incorporated into a dragonfly’s body? Let’s consider a classic study in ecology by John Teal

who studied salt marshes of Georgia along the southeastern coast of the United States. A salt

marsh is part of an estuary, the waters between oceans and rivers. You will consider some

research on these ecological systems performed by Teal and his colleagues. {Definitions:

Primary consumers are organisms that get their energy from primary producers (plants), also

known as herbivores. Secondary, tertiary, and quaternary consumers are organisms that get

their energy from other consumers. These are often called carnivores. Secondary consumers eat

primary consumers, tertiary consumers eat secondary consumers, etc.}

Salt marshes are bounded on one side by sea islands and on the other, by the mainland. The

salt concentrations of the marsh fluctuate with the amount of rainfall and the height of the tides.

You know from examination of killifish that some species have adapted to these harsh and

dynamic conditions. Salt-tolerant smooth cordgrass makes up most of grass in the salt marshes.

However, there is a gradient of habitat types (Figure 20.17).

Figure 20.17 Range of habitats typically observed in a salt marsh and surrounding terrestrial zone.

Notice how the vegetation type changes as distance from the tidal creek increases. The tidal creek rises

and falls with the tides, and can reach as far as the high marsh, but only rarely, leaving salty water behind

when it recedes. From Sherpa Guides, The Natural History of Georgia's Barrier Islands

http://www.sherpaguides.com/georgia/barrier_islands/natural_history/index.html.

Cordgrass dominates in tidal creek, levee, and low marsh zones and thus is the dominant primary

producer. Algae species floating in the water are also important primary producers, whereas

other plants dominate the drier zones.

Teal listed the obvious species occurring in each zone, although he focused his research on

the low saltmarsh. From knowledge of these animals, plants, and microbes, he constructed the

food web shown in Figure 20.18.

http://www.sherpaguides.com/georgia/barrier_islands/natural_history/index.html




Figure 20.18 Recreation of Teal’s saltmarsh foodweb. The lower group of herbivores, as classified by

Teal, are feeding on both living algae and dead cordgrass that sinks to the bottom of the marsh. Bacteria

are feeding on decomposing organic matter – the cordgrass and animals that die are decomposed by

bacteria. From Teal 1962 Figure 2, original art by CJP.

Teal and his colleagues estimated 34,580 kilocalories / m2 / year of primary production in the salt

marsh, which is how much energy was converted from carbon dioxide and sunlight into

carbohydrates. The actual amount of plant biomass growth, however, was estimated to be only

6,580 kilocalories / m2 / year. Dividing the biomass energy (6,580) by total chemical energy

(34,580) tells us that only 19% of the total chemical energy is retained in the plant biomass. Teal

measured algal production and biomass, too. The efficiency of conversion of primary production

to biomass was higher for algae, although the total amount of production was lower (1,620

(biomass growth) ÷ 1,800 (total primary production) kilocalories / m2 / year = 90%). The total

biomass growth for all primary producers (6,580 + 1,620 = 8,200 kilocalories / m2 / year) is what

is available to primary consumers (herbivores).

Let’s focus only on the upper pathway in Figure 20.18, especially the small jumping insects

called planthoppers and katydids, a type of grasshopper. Teal and his colleagues estimated the

annual amount of energy in the insects’ consumption, production, respiration, assimilation, and

waste (Figure 20.19).




Figure 20.19 Energy relationships in saltmarsh planthoppers and katydids. Estimates are from Teal

(1962). He estimated 85% assimilation efficiency for planthoppers because they feed on liquid sugar

solutions from plants, which are easy to digest, not cellulose, which is a plant structural chemical that is

not easy to digest. The assimilation efficiency for grasshoppers, which consume mostly cellulose, was

estimated to be 30%. From Teal 1962, data in text, original art by CJP.

After eating, the food is either assimilated into the body via the digestive system, or eliminated

as feces. Of the assimilated food, some goes to growth (production) and some goes to respiration

(ATP production and CO2 waste). Respiration is sort of the opposite of photosynthesis – carbon

dioxide is a waste product of cellular respiration, which puts carbon dioxide back into the

environment for plants to absorb again during photosynthesis. {Connections: Cellular

respiration is discussed in Chapter 21.} Thus, carbon cycles in the environment (i.e., CO2 →

biomass → CO2). Nutrient cycling is an emergent property of ecological systems and occurs as

organisms grow, consume, and respire (Figure 20.20).

323.5

275.0

70.0

205.0

48.5

99.4

29.410.8

18.6

70.0

0.0

50.0

100.0

150.0

200.0

250.0

300.0

350.0

Consumption Assimilation Production Respiration Feces

Ene

rgy (

kca

l /

m2

/ y

ear

Planthoppers

Katydids




Figure 20.20 One pathway of energy and carbon in the Georgia saltmarsh. Carbon cycles (atmosphere

plants herbivores carnivores atmosphere), and each component sends carbon dioxide back to the

atmosphere. Decomposers also respire or provide biomass to predators. The pathway of carbon is

combined with energy flow (sunlight plants herbivores carnivores, with respiration energy used

for metabolism and lost as heat). The size of the boxes for cordgrass, insects herbivores, and predators is

proportional to the energy content in each trophic level. From Teal 1962 Figure 4 and data in text,

original art by CJP.

The amount of production of the two herbivorous insect species is available for consumption

by spiders, wrens, and dragonflies (Figure 20.18). Spiders are the most abundant predator of

these two insects, but Teal lumped all predators together for his analysis. Teal estimated that

together predators consumed 28 out of 80.8 kilocalories / m2 / year available to them in the form

of planthopper and katydid biomass. The rest of the insect herbivore biomass (52.8 kilocalories /

m2 / year) ended up as food for bacteria or was washed out of the saltmarsh when the insects

died. The predators converted those 28 kilocalories into only 5 kilocalories / m2 / year of

biomass production.


40. Examine Figure 20.17 and develop hypotheses to explain what factors determine the type of

habitat that appears along this gradient. Consider what you know about saltmarshes, or

conduct an internet search if you need more information




(http://www.sherpaguides.com/georgia/barrier_islands/natural_history/index.html has a good

description of the saltmarsh ecosystem). Can you design an experiment to test your

hypotheses?

41. Teal estimated that only 6.1% of the energy in sunlight was absorbed by the primary

producers in the Georgia saltmarsh. Why are plants only able to absorb this small fraction of

the sunlight? For this question, you may have to consult the WileyPlus tutorial on

photosynthesis and consider the properties of light and the wavelengths that plants are able to

use to convert carbon dioxide to carbohydrates.

42. What happens to the 81% of chemical energy produced by cordgrass that is not turned into

biomass? Consider other ways that plants use energy.

43. Which species has a higher assimilation efficiency (assimilation energy ÷ energy in

consumption), planthoppers or katydids? Which species has a higher production efficiency

(production energy ÷ assimilation energy), planthoppers or katydids? What do you conclude

about these efficiencies? Where does the energy go that is not assimilated? Where does the

energy go that is not used for production?

44. Calculate the percentage of energy in the cordgrass that is incorporated into predator

biomass. What value did you obtain? Why is this number so low?

As you have no doubt discovered, ecological systems are not very efficient. Most of the

energy captured by primary producers is lost through respiration. No organisms can use all of

the energy they consume to grow bigger – each must use energy to maintain basic metabolic

functions, to move or to reproduce. Much of the chemical energy used for metabolism or

reproduction is lost to as heat, which is waste energy. Because of this waste, the amount of

energy or biomass in predator trophic levels will always be smaller than the amount of energy or

biomass of the producer trophic level. After using the energy in carbohydrates, the carbon is

released as carbon dioxide into the atmosphere, where it is available for plants to consume again

in a continuous cycle. The heat energy is no longer available for organisms to use as a source of

energy for creating biomass and so this energy is biologically out of circulation. You have seen

how energy flows and nutrients cycle through ecological systems and their movement are

emergent properties that arise from the species that interact within an ecological system.

Conclusions

Emergent properties at any level of biological hierarchy cannot be studied in isolation. You

have learned that the other big ideas of biology, such as evolution, cells as functional units,

information, and homeostasis all contribute to emergent properties. For instance, any interaction

between two species may result in selective pressures and possible extinction, which is an

emergent property. Information is used by individuals to assess the environment while foraging

or maintaining homeostasis. What have you learned about emergent properties in ecological

systems? First, indirect effects arise from biotic interactions – the wolf-elk predator-prey

interaction led to effects on trees and several other animal species in Yellowstone National Park.

http://www.sherpaguides.com/georgia/barrier_islands/natural_history/index.html




Next, you learned that cooperation is an emergent property exhibited by packs of wolves as they

compete in an ecological system. You have also learned that species adapt to local environmental

conditions, which allows species to coexist even when they are competing for similar resources.

This leads to increased species diversity within an ecological system. Finally, you have learned

that nutrients cycle and energy flows in ecological systems, and those movements of matter and

energy arise from interactions between species, as herbivores eat plants and carnivores eat

herbivores.

End of Chapter Review Materials

Review Questions

1. What is a resource, and what are the consequences of a limiting resource?

2. Give an example of an indirect effect.

3. How does variation in ecological systems lead to “flexible responses”?

4. How do prey (individuals or populations) change as a result of exposure to a predator?

5. Explain how management of large mammalian herbivores by humans is unlike regulation of

those populations by their natural prey.

6. What is the competitive exclusion principle?

7. What hypothesis did scientists develop regarding competitive ability and adaptations to

abiotic conditions?

8. How do environmental conditions affect the outcome of competitive interactions?

9. What is a negative correlation between two variables?

10. What is primary production?

11. Why are transfers of energy between trophic levels less than 100% efficient?

12. What are trophic levels and trophic pyramids?

Apply What You Know

1. Predict the indirect effects of loss of another top predator from a marine ecosystem. What do

you think will happen to kelp forests in the ocean when sea otters are reduced in number,

considering that sea otters eat sea urchins, which eat kelp?

2. We learned that competitive interactions may be altered by environmental conditions. We

also learned that wolves and coyotes compete in Yellowstone National Park, and that wolves

appear to be outcompeting coyotes. Design an experiment to test the hypothesis that coyotes

are the superior competitors in some habitats.

3. Construct a trophic pyramid, like the one depicted in the Georgia saltmarsh, for the

Yellowstone ecosystem.

4. We learned how energy and carbon moved in tandem within ecological systems, from

primary producers to top predators. Predict how energy is also linked to other essential

biological elements, such as nitrogen and phosphorous.




5. Speculate about an emergent property that arises when a squirrel collects and buries acorns

from oak trees.

6. How do the decisions made by individual predators or prey affect an entire population of the

same species?

7. Consider the evolution of cooperative behavior. If individual birds, for instance, attempt to

maximize the number of offspring they produce, relative to other individuals of the same

species, why would individuals choose to cooperate? There are only so many resources to go

around, and if one bird helps another, doesn’t it lose out on potential resource gains?

References

Wolves and Yellowstone National Park

Arjo WM, Pletscher DH, and Ream RR (2002) Dietary Overlap between Wolves and Coyotes in

Northwestern Montana. Journal of Mammalogy 83(3):754-766.

Beschta RL (2003) Cottonwoods, Elk, and Wolves in the Lamar Valley of Yellowstone National

Park. Ecological Applications 13(5):1295-1309.

Beschta RL (2005) Reduced Cottonwood Recruitment Following Extirpation of Wolves in

Yellowstone's Northern Range. Ecology 86(2):391-403.

Beschta RL and Ripple WJ (2012) Berry-producing shrub characteristics following wolf

reintroduction in Yellowstone National Park. Forest Ecology and Management 276:132–

138.

Creel S, Winnie J. Jr., Maxwell B, Hamlin K, and Creel M (2005) Elk Alter Habitat Selection as

an Antipredator Response to Wolves. Ecology 86(12):3387-3397.

Pianka ER (1973) The structure of lizard communities. Annual Review of Ecology and

Systematics 4:53-74.

Ripple WJ and Beschta RL (2004) Wolves and the Ecology of Fear: Can Predation Risk

Structure Ecosystems? BioScience 54(8):755-766.

Ripple WJ and Beschta RL (2007) Restoring Yellowstone’s aspen with wolves. Biological

Conservation 138:514 –519.

Robbins J (2004) Lessons from the wolf. Scientific American June 2004:76-81.

Smith DW, Stahler DR, and Guernsey DS (2006) Yellowstone Wolf Project: Annual Report,

2005. National Park Service, Yellowstone Center for Resources, Yellowstone National

Park, Wyoming, YCR-2006-04.

White PJ, and Garrott RA (2005) Northern Yellowstone Elk after Wolf Restoration. Wildlife

Society Bulletin 33(3):942-955.

Ethical, Legal, Social Implications

Bangs EE and Fritts SH (1996) Reintroducing the Gray Wolf to Central Idaho and Yellowstone

National Park. Wildlife Society Bulletin 24:402-413.

Carroll N, Dunning, JB Jr., Freeman A, Hallberg CA, and Morgan J (1997) Reintroduction of the

Gray Wolf to Yellowstone National Park: A Case Study. Presented at the May 1997




Purdue Bioethics Workshop. Accessed on March 30th

, 2008 at

http://www.bioethics.iastate.edu/classroom/graywolf.html

Pearson B. The Recall of the Wild. TPM (The Philospher’s Magazine) Online. Accessed on

March 30th

, 2008 at http://www.philosophersnet.com/magazine/article.php?id=992

Williams CK, Ericsson G, and Heberlein TA (2002) A Quantitative Summary of Attitudes

toward Wolves and Their Reintroduction (1972-2000). Wildlife Society Bulletin

30(2):575-584.

Outcome of Competition

Grarock K, Lindenmayer DB, Wood JT, Tidemann CR (2013) Does human-induced habitat

modification influence the impact of introduced species? A case study on cavity-nesting

by the introduced common myna (Acridotheres tristis) and two Australian native parrots.

Environmental Management 52:958–970. DOI 10.1007/s00267-013-0088-7

Wilson SD, and Keddy PA (1986) Species competitive ability and position along a natural stress

disturbance gradient. Ecology 67:1236-1242.

Energy and Nutrient Flow

Teal JM (1962) Energy Flow in the Salt Marsh Ecosystem of Georgia. Ecology 43(4): 614-624.

http://www.bioethics.iastate.edu/classroom/graywolf.html

http://www.philosophersnet.com/magazine/article.php?id=992

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