Lecture #K4 – Population Ecology – Dr. Kopeny 4/19/02 – NOT on Test #4 – revised 4/22, as delivered

Ecology is the study of interactions among organisms (biotic factors) and their physical environment (abiotic factors).

Jaguar in Brazilian rain forest

Solomon et. al. 1999

Population Ecology

Humans on a crowded urban street

Freeman 2002

Organism

Community

Ecosystem

Population

Raven & Johnson 1999

Population refers to individuals within a species living in the same place at the same time

Populations as Units of Structure and Function

•The population has unique, discernable properties and is an important unit of biological investigation.

•Interests of population ecologistsinclude

•to understand how and why population sizes change.

•to understand populations as functional subunits of communities.

•The population is an important level of organization for the study of evolution

•a population has common gene pool

•evolutionary change happens to populations

Population ecology has important applications:

•Conservation biology - predicting extinction risk, managing populations

•Understanding, predicting human population growth

Population of Mexican poppies (Eschscholzia mexiana) blooming in the desert after the winter rains

Solomon et al 2002)

Density and Dispersion are important attributes of

populations

Species Range Geographic extent of the occurrence of individuals in a species

Population Individuals within a species living in the same placeat the same time;

Species consist of populations of varying size and number, with varying degrees of proximity and connectedness (think gene flow)

Population Density Number of conspecific individuals per unit area or volume at a given time

Dispersion Distribution, or spacing of individuals within a population, relative to each other

Aerial census for African Buffalo (Syncerus caffer) in the Serengeti of East Africa. (Campbell 2002)

Clumped

Uniform dispersion Not common in general, but is not uncommon among birds nesting on small islands. This is a breeding colony of King Penguins on South Georgia Island in the South Atlantic Ocean.

Clumped dispersion Many fishes, including these butterfly fish, often clump in schools. Adaptive function may relate to hydrodynamic efficiency of swimming, decreased predation risk, increased feeding efficiency. Within a school, individuals are fairly evenly spaced

Random dispersion Populations of trees in tropical rain forests are often randomly dispersed, but in general, this dispersion pattern is rather rare in nature

Patterns of dispersion of individuals in a population

Uniform dispersion: Individuals are dispersed more evenly than expected from random occurrence in a habitat. Explanations include:

•uniform territory sizes in relatively homogenous environments (e.g. penguins)

•allelopathy -- production of toxins that inhibit growth of nearby plants (e.g.desert creosote bush and saltbush)

Campbell 1993

(Solomon et. al. 1999)

Campbell 1993

Clumped dispersion (aggregation) by far most common in nature.

•Environmental conditions seldom uniform throughout even relatively small area. Clumping of individuals often a response to clumping of resources

Clumped distribution of “Microhabitats” often explains clumping; local soil moisture, rotting log, limestone outrcrop…

•Reproductive behavior patterns including sexual attraction, often favor clumping

•Nonreproductive behavior patterns often lead to active congregation in loose groups or in more organized colonies, schools, flocks, or herds

Population demographics affect increase (growth) or decrease (decline) in population size

Demography; study of changes in size and structure of populations

Important demographic variables include:

•Sex Ratio

•Age structure (cohort=individuals of same age, age class)

•Age specific fecundity (birth) rate and mortality (death) rate

Ecologists assemble such demographic data in life tables, then use the life tables as the basis for further analysis of population dynamics

Population Dynamics; Changes in population size and demographics over time

Populations size changes in response to births and deaths, and to the movement of individuals into (immigration) our out of (emigration) the population

N1=N0 + B - D + I - E

•Mortality high during first year of life; mortality then dropped for several years, followed by a general increase

•Some of the fluctuation between years was probably related to annual rainfall; survival is related to seed production, which is closely correlated with rainfall

Life Table of the 1978 Cohort of Ground Finches on Isla Daphne

In drought years on the archipelago, seed production is low, nesting is low (most adults don’t breed), and adult survival is low

In years of heavy rainfall, seed production is high, most birds breed several times, and adult survival is high

Survivorship curves are graphical representations of age-specific survivorship

Survivorship = 1-mortality, = percent of cohort surviving to a given age

Top left figure shows generalized, hypothetical survivorship curvesacross a range of possibilities, and that are recurring in nature

Curve I characterizes humans, other large mammals, that have high parental care but produce relatively few young

Curve III characterizes some species with low parental care, large number of young (eg many fishes, marine invertebrates

Curve II Individuals roughly equally likely to die at any age. Characterizes some annual plants, some invertebrates (eg Hydra), some lizards, rodents, birds…

•Deer live up to 16, and females can breed at 4

•Type I survival curve indicates a relatively consistent increase in the risk of mortality with age

•The growth rate for most populations is strongly dependent on age structure

Life table for Red Deer on island of Rhum, Scotland.

http://cervid.forsci.ualberta.ca/library/taxonomy/cervus_elaphus.htm

Raven and Johnson 1999

Raven and Johnson 1999

Mathematical modeling is an important and widely-used approach to studying population dynamics

• Using equations to simulate population dynamics over time

•Illuminate complex processes and guide further research

•Vary in predictive ability; rarely are perfect approximations

Biotic Potential and the Exponential Growth Model

•All species have potential for explosive, exponential growth; absent resource limitations, growth would be exponential

•Biotic potential (rmax) is the maximum rate at which a population could increase under ideal conditions

•Exponential growth has been demonstrated experimentallyin bacterial and protist cultures and in some insects.

•At some point, environmental resisitance will curtail exponential growth; environmental limitations cause decreases in birth rates and increases in death rates Time

Popu

latio

n Si

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Example of rapidly increasing population. European loosestrife is now naturalized over thousands of square miles ofNorth American wetlands. Introduced in 1860, it has had a negative effect on may native plant and animal species

Exponential growth in bacteria. Bacteria, dividing every 20 minutes, experience exponential population growth

Raven & Johnson 1999

Solomon et al 1999

Biotic potential and the Exponential Growth Model

Time

Popu

latio

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•Rate of increase (slope of the curve) steadily accelerates, population increases exponentially.

b=per capita birth rated=per capita death rate

Think about (bN-dN) ...

•difference between births and deaths in absolute numbers

•determines if population will grow, be stable, or decline

Keeton & Gould 1993

dN = (bN-dN)dt

dN/dt = rate of change in size of population

dt

dN

Time

Popu

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(Keeton & Gould 1993)

(b-d) is the per capita growth rate. It is the net rate of population change per individual

factor N out on right side of equation

dN = (b-d)Ndt

divide both sides by N to “see” the per capita growth rate (b-d)

dNdt = (b-d)

-----N

back to...

dN = (b-d)Ndt

Time

Popu

latio

n Si

zedN = (b-d)Ndt

substitute r for b-d

dN = (r)Ndt

r = b-d = net rate of population change per individual at a given moment

Under these conditions of maximum birth rate and minimum death rate r is designated rmax

dN = rmaxNdt

• rmax represents the intrinsic rate of increase, or biotic potential of the population

• rmax varies widely among species

(Keeton & Gould 1993)

Time

Popu

latio

n Si

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dN = rmaxNdt

Rate of population growth is a function of r and N; N is related to the number of breeders

N increases with each generation; therefore so does the rate of increase -- dN/ d t

Its due to this accelerating rate of increase that the slope of the curve becomes steeper and steeper

(Keeton & Gould 1993)

Population growth predicted by the exponential model. The exponential growth model predicts unlimited populations increase under conditions of unlimited resources. This graph compares growth in populations with two different values of rmax:1.0 and 0.5

•All other things being equal, a population with a higher intrinsic rate of increase will grow faster than one with a lower rate of increase

•The value of rmax for a population is influenced by life history features, such as

•age at onset of reproductive capability

•number of young produced

Human population growth. During the last 1000 years, the human population (globally) has been growing nearly exponentially

(Solomon et al 1999)