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Population Ecology
52
BIOLOGICAL SCIENCE
FOURTH EDITION
SCOTT FREEMAN
Lectures by Stephanie Scher Pandolfi
© 2011 Pearson Education, Inc.
Key Concepts
Life tables summarize how likely it is that individuals of each age
class in a population will survive and reproduce.
The growth rate of a population can be calculated from life-table
data or from the direct observation of changes in population size
over time.
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Key Concepts
Researchers observe a wide variety of patterns when they track
changes in population size over time, ranging from growth rates
that slow when populations are at high density, to regular cycles,
to continued growth independent of population size.
Data from population ecology studies help biologists evaluate
prospects for endangered species and design effective
management strategies, as well as to predict changes in human
populations.
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Introduction
• A population is a group of individuals from the same species that
live in the same area at the same time.
• Population ecology is the study of how and why the number of
individuals in a population changes over time.
• The mathematical and analytical tools used in population ecology
help biologists predict changes in population size and design
management strategies to save threatened species.
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Demography
• The number of individuals present in a population depends on four
processes: birth, death, immigration, and emigration.
• Populations grow due to birth and immigration, which occurs
when individuals enter a population by moving from another
population.
• Populations decline due to deaths and emigration, which occurs
when individuals leave a population to join another population.
• Demography is the study of factors such as these that determine
the size and structure of populations through time.
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Demography
• To make predictions about the future of a population, biologists
need to know how many individuals of each age are alive, how
likely individuals of different ages are likely to survive to the
following year, and how many offspring are produced by females
of different ages.
• They also need to know how many individuals of different ages
immigrate and emigrate each generation—the average time
between a mother’s first offspring and her daughter’s first
offspring.
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Demography
• If a population consists primarily of young individuals with a high
survival rate and reproductive rate, the population size should
increase over time.
• On the other hand, if a population comprises chiefly old individuals
with low reproductive rates and low survival rates, then it is almost
certain to decline over time.
• To understand a population’s dynamics, biologists turn to the data
contained in a life table.
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Life Tables
• A life table summarizes the probability that an individual will
survive and reproduce in any given time interval over the course of
its lifetime.
• The lizard Lacerta vivipara is a common resident of open, grassy
habitats in western Europe. Most populations give birth to live
young.
• Biologists were able to calculate the number of individuals that
survived each year in each particular age group as well as how
many offspring each female produced by monitoring a population
daily for seven years.
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© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Survivorship
• Survivorship is a key component of a life table and is defined as
the proportion of offspring produced that survive, on average, to a
particular age.
• To recognize general patterns in survivorship and make
comparisons among populations or species, biologists create a
graph called a survivorship curve.
• The survivorship curve is a plot of the logarithm of the number of
survivors versus age.
• There are three general types of survivorship curves.
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Survivorship
• In a type I curve, survivorship throughout life is high, and most
individuals approach the maximum life span of the species; humans
show this type of survivorship curve.
• In a type II curve, most individuals experience relatively constant
survivorship over their lifetimes; songbirds have this curve.
• Type III curves result from high death rates early in life, with high
survivorship after maturity; many plants have type III curves.
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© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Fecundity
• Fecundity is also a key part of a life table; it is defined as the
number of female offspring produced by each female in the
population.
• Age-specific fecundity is the average number of female offspring
produced by a female in a given age class—a group of individuals
of a specific age.
• Data on survivorship and fecundity allow researchers to calculate
the growth rate of a population.
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The Role of Life History
• In many species, key aspects of the life table vary dramatically
among populations.
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What Are Fitness Trade-Offs?
• Fitness trade-offs occur because every individual has a restricted
amount of time and energy at its disposal―its resources are limited.
• For example, if a female devotes a great deal of energy to
producing a large number of offspring, it is not possible for her to
devote that same energy to her immune system, growth, nutrient
stores, or other traits that increase survival.
• A female can maximize fecundity, maximize survival, or strike a
balance between the two.
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Life History Is Based on Resource Allocation
• An organism’s life history describes how an organism allocates its
resources to growth, reproduction, and activities or structures
related to survival.
• Traits such as survivorship, age-specific fecundity, age at first
reproduction, and growth rate are all aspects of an organism’s life
history.
• Understanding variation in life history is all about understanding
fitness trade-offs.
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Patterns across Species
• Life-history traits form a continuum.
• In general, organisms with high fecundity tend to grow quickly,
reach sexual maturity at a young age, and produce many small eggs
or seeds.
• In contrast, organisms with high survivorship tend to grow slowly,
and invest their energy and time in traits that reduce damage from
enemies and increase their own ability to compete for resources.
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© 2011 Pearson Education, Inc.
Population Growth
• The most fundamental questions that biologists ask about
populations involve growth or decline in numbers of individuals.
• For conservationists, analyzing and predicting changes in
population size is fundamental to managing threatened species.
• A population’s overall growth rate is a function of birthrates, death
rates, immigration rates, and emigration rates.
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Quantifying the Growth Rate
A population’s growth rate is the change in the number of
individuals in the population (N) per unit time (t).
• If no immigration or emigration is occurring:
growth rate = N  r
• The per-capita rate of increase (r) is the difference between the
birthrate and death rate per individual.
r=b−d
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Quantifying the Growth Rate
• If the per-capita birthrate is greater than the per-capita death rate,
then r is positive and the population is growing. If the per-capita
death rate begins to exceed the per-capita birthrate, then r is
negative and the population declines.
• Within populations, r varies through time. Its value can be positive,
negative, or 0.
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Quantifying the Growth Rate
• When birthrates per individual are as high as possible and death
rates per individual are as low as possible, r reaches a maximum
value called the intrinsic rate of increase, rmax.
• When this occurs, the population's growth rate is expressed as:
N/t = rmaxN
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Quantifying the Growth Rate
• Each species has a characteristic rmax that does not change. But at
any specific time, the per-capita rate of increase of each population
of that species is likely to be much lower than rmax.
• A population’s r is also likely to be different from r values of other
populations of the same species and to change over time.
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Exponential Growth
• Exponential population growth occurs when r does not change
over time. It does not depend on the number of individuals in the
population—it is density independent.
• In nature, exponential growth is observed in two circumstances:
1. A few individuals found a new population in a new habitat.
2. A population has been devastated by a storm or some other
type of catastrophe and then begins to recover, starting with a
few surviving individuals.
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Exponential Growth
• Exponential growth cannot continue indefinitely.
• When population density—the number of individuals per unit
area—gets very high, the population’s per-capita birthrate should
decrease and the per-capita death rate increase, causing r to decline.
– This type of growth is density dependent.
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Logistic Growth
• Carrying capacity, K, is the maximum number of individuals in a
population that can be supported in a particular habitat over a
sustained period of time. K can change depending on conditions.
• The carrying capacity of a habitat depends on a large number of
factors: food, space, water, soil quality, and resting or nesting sites.
Carrying capacity can change from year to year, depending on
conditions.
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A Logistic Growth Equation
• If a population of size N is below the carrying capacity K, the
population should continue to grow. Specifically, a population’s
growth rate should be proportional to (K – N)/K:
N/t = rmaxN((K – N)/K)
• This expression is called the logistic growth equation.
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A Logistic Growth Equation
• The logistic growth equation describes logistic population
growth—a change in growth rate that occurs as a function of
population size.
• Logistic growth is density dependent. As a population approaches a
habitat’s carrying capacity, its growth rate should slow.
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Graphing Logistic Growth
• In a hypothetical population, density-dependent growth has three
distinct stages:
1. Initially, growth is exponential (r is constant).
2. Growth rate begins to decline (N increases) when competition
for density-dependent factors begins to occur.
3. Growth rate reaches 0 at the carrying capacity (N vs. t is flat).
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What Limits Growth Rates and Population Sizes?
• Population sizes change as a result of density-independent and
density-dependent factors.
• Density-independent factors, such as variation in weather patterns,
are usually abiotic; they change birthrates and death rates
irrespective of population size.
• Density-dependent factors, such as increased predation when a deer
population increases, are usually biotic; they change in intensity as
a function of population size.
• Density-dependent changes in survivorship and fecundity cause
logistic population growth.
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A Closer Look at Density Dependence
• An experimental study of a coral-reef fish called the bridled goby
showed a strong density-dependent relationship in survivorship.
• Likewise, a long-term study of song sparrows on Mandarte Island,
British Columbia, showed a strong density-dependent relationship
in fecundity.
• Density-dependent changes in survivorship and fecundity cause
logistic population growth.
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Carrying Capacity Is Not Fixed
• K varies among species and populations.
• K varies because for any particular species, some habitats are better
than other habitats due to differences in food availability, space,
and other density-dependent factors. Stated another way, K varies
in space.
• It also varies with time, as conditions in some years are better than
in others.
• In addition, the same habitat may have a very different carrying
capacity for different species.
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Modeling Population Growth
Web Activity: Modeling Population Growth
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Population Dynamics
• Population dynamics are the changes in populations through time.
Research on population dynamics has uncovered a wide array of
patterns in natural populations, in addition to exponential and
logistic growth.
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How Do Metapopulations Change through Time?
• If individuals from a species occupy many small patches of habitat
so that they form many independent populations, they represent
metapopulations―a population of populations.
• Because humans are reducing large, contiguous areas of forest and
grasslands to isolated patches or reserves, more and more species
are being forced into a metapopulation structure.
• Glanville fritillaries—an endangered species of butterfly native to
the Åland islands off the coast of Finland—exist naturally as
metapopulations.
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Metapopulations Should Be Dynamic
• Ilkka Hanski and colleagues determined the number of Glanville
fritillary breeding pairs in each patch within a metapopulation.
• Over time, each population within the larger metapopulation is
expected to go extinct due to any number of potential causes.
– However, migration can reestablish populations in empty
patches.
• There is thus a balance between extinction and recolonization
within a metapopulation. Subpopulations may blink on and off over
time, but the overall population is maintained at a stable number of
individuals.
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Metapopulations Should Be Dynamic
• Experimental studies supported the dynamic nature of
metapopulations in Glanville fritillaries; the overall population size
was relatively stable even though constituent populations came and
went.
• To summarize, the history and future of a metapopulation is driven
by the birth and death of populations, just as the dynamics of a
single population are driven by the birth and death of individuals.
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Why Do Some Populations Cycle?
• Some populations exhibit population cycles—regular fluctuations
in size.
• Most hypotheses put forward to explain population cycles depend
on some density-dependent factor. Predation, disease, or food
shortages intensify dramatically at high population density and
cause population numbers to crash.
• A long-term study of the hare and lynx illustrates population cycles.
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Is It Food or Predation?
• To explain the hare-lynx cycle, biologists proposed two hypotheses
based on density dependent factors:
1. Hares use up all their food when their populations reach high
density and starve; in response, lynx also starve.
2. Lynx populations reach high density in response to increases
in hare density. At high density, lynx eat so many hares that
the prey population crashes.
• So the question was, do hares control lynx population size, or do
lynx control hare population size?
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A Field Experiment
• To test the hypotheses that predation, food availability, or a
combination of those two factors controls the hare-lynx cycle,
researchers set up a series of study plots in similar boreal forests.
• Three plots were used as controls, one plot excluded lynx but not
hares, two plots provided additional food for hares, and one plot
both excluded predators and provided extra food for hares.
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A Field Experiment
• Plots that excluded predators showed higher hare populations than
did control plots during the “decline” phase of the cycle, supporting
the hypothesis that predation by lynx reduces hare populations.
• Plots with supplemental food also had much higher populations
than controls during the peak and decline phases. But the plot with
supplemental food and predators excluded had a hare population as
much as 35 times more dense than the population in the control
plots.
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A Field Experiment
• These data support the hypothesis that hare populations are limited
by availability of food as well as by predation, and that food
availability and predation intensity interact—meaning the combined
effect of food and predation is much larger than their impact in
isolation.
• The leading hypothesis to explain this combined effect is that when
hares are at high density, individuals are weakened by nutritional
stress and are more susceptible to predation.
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Population Dynamics
BLAST Animation: Population Dynamics
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How Does Age Structure Affect Population Growth?
• A population’s age structure—the proportion of individuals that are
at each possible age—has a dramatic influence on the population’s
growth over time.
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Age Structure in a Woodland Herb
• The common primrose lives in sunny gaps in shady woodlands.
• Its age structure varies over time.
– Rapidly growing juveniles dominate populations in sunny new
gaps.
– As trees grow to shade these gaps, the growth of the population
declines, leaving mostly adults in the population.
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Age Structure in a Woodland Herb
• The dynamics of primrose populations are expected to follow three
trends:
1. Populations that are dominated by juveniles should
experience rapid growth followed by a decline due to
shading by trees.
2. The long-term trajectory of the overall primrose population
in an area may depend primarily on the frequency and
severity of windstorms that knock down trees and create
sunlit gaps.
3. In a large tract of forest, the primrose population will have
metapopulation structure.
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Age Structure in Human Populations
• The age structure of human populations in different countries,
which varies dramatically, can be represented by age pyramids—
graphs with horizontal bars representing the numbers of males and
females of each age group.
• The age structure of a population tends to be uniform in developed
countries and bottom-heavy in developing countries.
• Analyzing an age pyramid can give biologists important
information about a population’s history, and also help them predict
a population’s future.
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Population “Momentum”
• Because of recent and rapid population growth in developing
countries, overall population size will increase dramatically in these
nations over the course of your lifetime.
• Part of this increase will be due to increased survivorship. In
addition, these populations now have so many young women that
the overall number of births will stay high, even though average
fecundity is much less than it was a generation ago.
• Biologists say that these populations have momentum or inertia.
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Analyzing Change in Human Population Growth Rate
• The rate of human population growth has increased over the past
250 years, leading to a very steeply rising curve over the past few
decades.
• It is almost impossible to overemphasize just how dramatically the
human population has grown recently.
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How Large Is the Current Human Population?
• In 2009, the world population is estimated at over 6.8 billion, and
about 77 million additional people are being added each year.
• It is not possible to overstate the consequences to us and to the
planet of these recent and current increases in human population.
• In addition to being the primary cause of habitat loss and species
extinction, overpopulation is linked to declines in living standards,
political instability, and acute shortages of basic resources.
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How Large Is the Current Human Population?
• The one encouraging trend in the data is that the growth rate of
the human population has already peaked and begun to decline.
• Since 1970, the growth rate of human populations has been
dropping. Between 1990 and 1995, the worldwide growth rate
averaged 1.46 percent per year; currently the annual rate is 1.2
percent.
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Will Human Population Size Peak in Your Lifetime?
• The UN has projected human population growth to the year 2050
based on current fertility rates, and 2.5 (high), 2.1 (medium), or 1.7
(low) children per woman.
• When fertility at the replacement rate is sustained for a
generation—each woman producing exactly enough offspring to
replace herself and her offspring’s father—zero population
growth (ZPG) results.
• The future of the human population hinges on fertility rates—on
how many children each woman living today decides to have.
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Human Population Growth and Regulation
Web Activity: Human Population Growth and Regulation
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Can Population Ecology Help Endangered Species?
When designing programs to save species threatened with
extinction, conservationists draw heavily on concepts and
techniques from population ecology.
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Using Life-Table Data
• Collecting and analyzing demographic data such as age-specific
survivorship and fecundity are important for saving endangered
species and for other applied problems.
• Life-table data can be used to project the future of a population.
• Projecting a population based on life-table data allows biologists to
alter values for survivorship and fecundity at particular ages and
assess the consequences.
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Altering Values for Survivorship and Fecundity
• Analyses like these allow biologists to determine which aspects of
survivorship and fecundity are especially sensitive for particular
species, and plan accordingly.
• For example, many endangered species have high juvenile
mortality, low adult mortality, and low fecundity. In these species,
the fate of a population is sensitive to increases in adult mortality.
• Based on this, conservationists have begun an intensive campaign
to reduce the loss of adult female sea turtles in fishing nets.
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Altering Values for Survivorship and Fecundity
• In humans and other species with high survivorship, rates of
population growth are sensitive to changes in age-specific
fecundity.
– Programs to control human population growth focus on
lowering fertility rates and delaying the age of first
reproduction.
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Preserving Metapopulations
• Because humans are reducing large, contiguous areas of forest and
grasslands to isolated patches or reserves, more and more species
are being forced into a metapopulation structure.
• The subpopulations within a larger metapopulation that are most
likely to survive are those that are larger, occupy larger
geographical areas, are closer to neighboring populations, and have
more genetic diversity.
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Population Viability Analysis
• Although traditional population growth models such as the
expressions for exponential and logistic growth are simple and
elegant, the factors they ignore—immigration and emigration—are
crucial to understanding the dynamics of most populations.
• Because metapopulation structure is common, biologists must use
more sophisticated models to predict the fates of populations.
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