chapter 14: population ecology planet at capacity: patterns of population growth lectures by mark...
TRANSCRIPT
Chapter 14: Population Ecology
Planet at capacity: patterns of population growthLectures by Mark Manteuffel, St. Louis Community College
14.1–14.6 Populationecology is the study of how populationsinteract with theirenvironments.
14.1 What is ecology?
Take-home message 14.1
Population ecology is the study of the interaction between populations of organisms and their environment, particularly their patterns of growth and how they are influenced by other species and by environmental factors.
14.2 A population perspective is necessary in ecology.
Take-home message 14.2
Most ecological processes cannot be observed or studied within an individual.
Rather, when studying them it is necessary to consider the entire group of individuals that regularly exchange genes in a particular locale.
14.3 Populations can grow quickly for a while, but not forever.
There is no exception to the rule that every organic being naturally increases at so high a
rate that, if not destroyed, the Earth would soon be covered by the progeny of a single pair.
—Charles Darwin, The Origin of Species
In stable populations,
How many of the five million eggs that a female cod might lay over the course of her life will, on average, survive and grow to adulthood?
Who leaves more surviving offspring, a pair of elephants or a pair of rabbits?
Figuring Out How a Population Grows (or Shrinks)
Two pieces of information are needed:
• Growth rate, abbreviated as “r”
• Number of individuals in the population (N)
• r N
Population Growth Rate Calculation
500 individuals in a population.
Over the course of the year 125 offspring are born.• Birth rate is 125/500 or .25 births per
person.
If 25 out of the 500 individuals die during the course of the same year, • the death rate is 25/500 or .05 deaths
per person.
The growth rate is .25 .05 or .20 individuals per person.
Take-home message 14.3
Populations tend to grow exponentially, but this growth is eventually limited.
14.4 A population’s growth is limited by its environment.
Density-dependent Factors
The limitations on a population’s growth that are a consequence of population density
This ceiling on growth is the carrying capacity, K, of the environment.
How the Carrying Capacity of an Environment Influences a
Population’s Growth r * N
Multiply by [(K – N)/K]• varies between 0 and 1
If the new term, [(K – N)/K], is close to 1, population growth is essentially unchanged.
How the Carrying Capacity of an Environment Influences a
Population’s Growth r * N
Multiply by [(K – N)/K]varies between 0 and 1
If the new term, [(K – N)/K], is close to 0, the environment is nearly full to capacity, and population growth reduces to almost zero.
Density-independent Forces
Factors that strike populations without regard for the size of the population
Mostly weather-based
How many people can earth support?
Why does the answer keep increasing?
Take-home message 14.4
A population’s growth can be reduced both by density-dependent factors related to crowding and density-independent factors such as natural or human-caused environmental calamities.
14.5 Some populations cycle between large and small.
Take-home message 14.5
Although the logistic growth pattern describes the general growth pattern of populations better than any other model, some populations cycle between periods of rapid growth and rapid shrinkage.
14.6 “Maximum sustainable yield” is a useful but impossible-to-implement concept.
Almost all natural resource managers working for the U.S. government fail to do their job exactly as mandated.
Why?
What We Often Do Not Know…
Population carrying capacity
Number of individuals alive
Stability of carrying capacity from year to year
Which individuals to harvest
Take-home message 14.6
Based on models of population growth, it seems easy to efficiently and sustainably utilize natural resources.
In practice, however, difficulties such as estimating population size and carrying capacity complicate the implementation of such strategies.
14.7–14.9 A life history is like a species summary.
14.7 Life histories are shaped by natural selection.
Do any animals mate themselves to death?
Why?
Why all the variation? Is one strategy better than others,
evolutionarily?
There are many possible responses to the challenge of:• when to reproduce • how often to reproduce• how much to reproduce
Life History
The vital statistics of the species
Includes: age at first reproduction, probabilities of survival and reproduction at each age, litter size and frequency, and longevity
Reproductive Investment
The material and energetic contribution that an individual will make to its offspring
Single episode of reproduction
Repeated episodes of reproduction
Which life history strategy is best?
1. What is the cost of reproductive investment during any reproductive episode?
2. What is an individual’s likelihood of surviving to have future reproductive episodes?
Natural selection favors lifetime reproductive success.
Why do humans put off mating so much longer than cats or mice?
Take-home message 14.7
An organism’s investment pattern in growth, reproduction, and survival is its life history.
14.8 Populations can be described quantitatively in life tables and survivorship curves.
Life Tables and Survivorship Curves
Life table• Allow biologists to predict an individual’s
likelihood of either dying within a particular age interval or surviving the interval.
Life Tables and Survivorship Curves
Survivorship curves• graphs of the proportion of individuals of
a particular age that are alive in a population
Take-home message 14.8
Life tables and survivorship curves summarize the survival and reproduction patterns of the individuals of a population.
Take-home message 14.8
Species vary greatly in these patterns: the highest risk of mortality may occur among the oldest individuals or among juveniles or mortality may strike evenly at all ages.
14.9 There are tradeoffs between reproduction and longevity.
Designing an Organism
To structure its life history for maximum fitness, create one that could: produce many offspring, beginning just after birth, continuing every year, while growing tremendously large, to reduce the predation risk and living forever.
Evolutionary Constraints
These traits are not all possible because selection that changes one feature tends to adversely affect others.
Evolutionary tradeoffs
Three areas to which an organism can allocate its resources:
Growth
Reproduction
Survival
Take-home message 14.9
Because constraints limit evolution, life histories are characterized by tradeoffs between investment in growth, reproduction, and survival.
14.10–14.12 Ecology influences the evolution of aging in a population.
14.10 Things fall apart: What is aging and why does it occur?
Physiological Deterioration over Time
Aging: an increased risk of dying with increasing age.
Why do organisms age?
The force of natural selection lessens with advancing age.
Many genetic diseases kill old people, but almost none kill children.
Why not?
Mutations That Arise and Cause Their Carrier to Be More Likely to Die Later in Life
Such mutations include those that increase the risk from cancers or heart disease or other types of ailments.
Do not affect reproductive output.
Consequently, these mutants are never cleaned out of a population.
A cure for cancer may be discovered but not a cure for aging.
Why the difference?
Take-home message 14.10
Natural selection cannot weed out bad alleles that do not diminish an individual’s relative reproductive success.
Consequently, they accumulate in the genomes of nearly all species.
This leads to multiple physiological breakdowns that we see as aging.
14.11 What determines the longevity of different species?
Hazard Factors
High-risk worlds• Death from external sources• Reproduce early
Low-risk worlds• Death from external sources low
Age at Time of Reproduction
A key factor determining longevity.
Early reproduction will also favor early aging.
Later reproduction will also favor later aging.
Take-home message 14.11
The rate of aging and pattern of mortality in a species is determined by the hazard factor of that organism's environment.
In environments characterized by low mortality risk, populations of slowly aging individuals with long life spans evolve.
In environments characterized by high mortality risk, populations of early-aging, short-lived individuals evolve.
14.12 Can we slow down the process of aging?
Life extension is possible.
Take-home message 14.12
By increasing the strength of natural selection later in life, it is possible to increase the mean and maximum longevity of the individuals within a population.
This occurs in nature and has also been done under controlled laboratory conditions.
14.13–14.15 The humanpopulation is growingrapidly.
What is the baby boom?
Why is it bad news for young people today?
14.13 Age pyramids reveal much about a population.
Describing Populations
In terms of the proportion of individuals from each age group
The population age distribution
Age groupings called cohorts
Take-home message 14.13
Age pyramids show the number of individuals in a population within any age group.
They allow us to estimate birth and death rates over multi-year periods.
14.14 As less-developed countries become more developed, a demographic transition often occurs.
Population growth is alarmingly slow in Sweden and alarmingly fast in Mexico.
Why is there a difference?
Take-home message 14.14
The demographic transition tends to occur with the industrialization of countries.
It is characterized by an initial reduction in the death rate, later followed by a reduction in the birth rate.
14.15 Human population growth: How high can it go?
How high can it go?!
Very difficult to assess just how many resources each person needs.
Ecological footprints• Evaluating how much land, how much
food and water, and how much fuel, among other things, are necessary.
Take-home message 14.15
The world population is currently growing at a very high rate, but limited resources will eventually limit it, most likely at a population size between 7 and 11 billion.