Chapter 52 Population Ecology
Overview: Earth’s Fluctuating Populations
• To understand human population growth, we must consider the
general principles of population ecology.
• Population ecology is the study of populations in relation to the
environment, including environmental influences on population
density and distribution, age structure, and population size.
Concept 52.1 Dynamic biological processes influence population
density, dispersion, and demography
• A population is a group of individuals of a single species that live in
the same general area.
• Members of a population rely on the same resources, are influenced
by similar environmental factors, and have a high likelihood of
interacting with and breeding with one another.
• Populations can evolve through natural selection acting on heritable
variations among individuals and changing the frequencies of various
traits over time.
Two important characteristics of any population are density and the
spacing of individuals.
• Every population has a specific size and specific geographical
• The density of a population is measured as the number of
individuals per unit area or volume.
• The dispersion of a population is the pattern of spacing among
individuals within the geographic boundaries.
• Measuring density of populations is a difficult task.
• We can count individuals, but we usually estimate population
numbers. • It is almost always impractical to count all individuals in a
• Instead, ecologists use a variety of sampling techniques to
estimate densities and total population sizes.
• For example, they might count the number of individuals
in a series of randomly located plots, calculate the
average density in the samples, and extrapolate to
estimate the population size in the entire area.
• Such estimates are accurate when there are many sample plots
and a homogeneous habitat.
• A sampling technique that researchers commonly use to
estimate wildlife populations is the mark-recapture method.
• Individuals are trapped and captured, marked with a tag,
recorded, and then released.
• After a period of time has elapsed, traps are set again,
and individuals are captured and identified.
• The second capture yields both marked and unmarked
• From these data, researchers estimate the total number
of individuals in the population.
• The mark-recapture method assumes that each marked
individual has the same probability of being trapped as
each unmarked individual.
• This may not be a safe assumption, as trapped
individuals may be more or less likely to be trapped a
• Density results from dynamic interplay between processes that add
individuals to a population and those that remove individuals from it.
• Additions to a population occur through birth (including all forms
of reproduction) and immigration (the influx of new individuals
from other areas). • The factors that remove individuals from a population are death
(mortality) and emigration (the movement of individuals out of a
• Immigration and emigration may represent biologically
significant exchanges between populations.
• Within a population’s geographic range, local densities may vary
• Variations in local density are important population
characteristics, providing insight into the environmental and
social interactions of individuals within a population.
• Some habitat patches are more suitable that others.
• Social interactions between members of a population may
maintain patterns of spacing.
• Dispersion is clumped when individuals aggregate in patches.
• Plants and fungi are often clumped where soil conditions favor
germination and growth.
• Animals may clump in favorable microenvironments (such as
isopods under a fallen log) or to facilitate mating interactions.
• Group living may increase the effectiveness of certain
predators, such as a wolf pack.
• Dispersion is uniform when individuals are evenly spaced.
• For example, some plants secrete chemicals that inhibit the
germination and growth of nearby competitors.
• Animals often exhibit uniform dispersion as a result of
territoriality, the defense of a bounded space against
encroachment by others.
• In random dispersion, the position of each individual is independent
of the others, and spacing is unpredictable.
• Random dispersion occurs in the absence of strong attraction
or repulsion among individuals in a population, or when key physical or chemical factors are relatively homogeneously
• For example, plants may grow where windblown seeds land.
• Random patterns are not common in nature.
Demography is the study of factors that affect population density and
• Demography is the study of the vital statistics of populations and how
they change over time.
• Of particular interest are birth rates and how they vary among
individuals (specifically females), and death rates.
• A life table is an age-specific summary of the survival pattern of a
• The best way to construct a life table is to follow the fate of a cohort,
a group of individuals of the same age, from birth throughout their
lifetimes until all are dead.
• To build a life table, we need to determine the number of individuals
that die in each age group and calculate the proportion of the cohort
surviving from one age to the next.
• A graphic way of representing the data in a life table is a survivorship
• This is a plot of the numbers or proportion of individuals in a
cohort of 1,000 individuals still alive at each age.
• There are several patterns of survivorship exhibited by natural
• A Type I curve is relatively flat at the start, reflecting a low
death rate in early and middle life, and drops steeply as death
rates increase among older age groups.
• Humans and many other large mammals exhibit Type I
survivorship curves. • The Type II curve is intermediate, with constant mortality over
an organism’s life span.
• Many species of rodent, various invertebrates, and some
annual plants show Type II survivorship curves.
• A Type III curve drops slowly at the start, reflecting very high
death rates early in life, then flattens out as death rates decline
for the few individuals that survive to a critical age.
• Type III survivorship curves are associated with
organisms that produce large numbers of offspring but
provide little or no parental care.
• Examples are many fishes, long-lived plants, and marine
• Many species fall somewhere between these basic types of
survivorship curves or show more complex curves.
• Some invertebrates, such as crabs, show a “stair-stepped”
curve, with increased mortality during molts.
• Reproductive rates are key to population size in populations without
immigration or emigration.
• Demographers who study sexually reproducing populations
usually ignore males and focus on females because only
females give birth to offspring.
• A reproductive table is an age-specific summary of the
reproductive rates in a population.
• The best way to construct a reproductive table is to
measure the reproductive output of a cohort from birth
• For sexual species, the table tallies the number of female
offspring produced by each age group.
• Reproductive output for sexual species is the product of
the proportion of females of a given age that are breeding and the number of female offspring of those breeding
• Reproductive tables vary greatly from species to species.
• Squirrels have a litter of two to six young once a year for
less than a decade, while mussels may release hundreds
of thousands of eggs in a spawning cycle.
Concept 52.2 Life history traits are products of natural selection
• Natural selection favors traits that improve an organism’s chances of
survival and reproductive success.
• In every species, there are trade-offs between survival and traits such
as frequency of reproduction, number of offspring produced, and
investment in parental care.
• The traits that affect an organism’s schedule of reproduction and
survival make up its life history.
Life histories are highly diverse, but they exhibit patterns in their
• Life histories entail three basic variables: when reproduction begins,
how often the organism reproduces, and how many offspring are
produced during each reproductive episode.
• Life history traits are evolutionary outcomes reflected in the
development, physiology, and behavior of an organism.
• Some organisms, such as the agave plant, exhibit what is known as
big-bang reproduction, in which an individual produces a large
number of offspring and then dies.
• This is known as semelparity.
• By contrast, some organisms produce only a few offspring during
repeated reproductive episodes.
• This is known as iteroparity.
• What factors contribute to the evolution of semelparity versus
iteroparity? • In other words, how much does an individual gain in reproductive
success through one pattern versus the other?
• The critical factor is survival rate of the offspring.
• When the survival of offspring is low, as in highly variable or
unpredictable environments, big-bang reproduction
(semelparity) is favored.
• Repeated reproduction (iteroparity) is favored in dependable
environments where competition for resources is intense.
• In such environments, a few, well-provisioned offspring
have a better chance of surviving to reproductive age.
Limited resources mandate trade-offs between investment in
reproduction and survival.
• Organisms have finite resources, and limited resources mean trade-
• Life histories represent an evolutionary resolution of several
• Sometimes we see trade-offs between survival and
reproduction when resources are limited.
• For example, red deer females have a higher mortality rate in
winters following summers in which they reproduce.
• Selective pressures also influence the trade-off between number and
size of offspring.
• Plants and animals whose young are subject to high mortality
rates often produce large numbers of relatively small offspring.
• Plants that colonize disturbed environments usually
produce many small seeds, only a few of which reach
• Smaller seed size may increase the chance of seedling
establishment by enabling seeds to be carried longer
distances to a broader range of habitats. • In other organisms, extra investment on the part of the parent
greatly increases the offspring’s chances of survival.
• Oak, walnut, and coconut trees all have large seeds with
a large store of energy and nutrients to help the seedlings
• In animals, parental care does not always end after incubation
• Primates provide an extended period of parental care.
Concept 52.3 The exponential model describes population growth in
an idealized, unlimited environment
• All populations have a tremendous capacity for growth.
• However, unlimited population increase does not occur indefinitely for
any species, either in the laboratory or in nature.
• The study of population growth in an idealized, unlimited environment
reveals the capacity of species for increase and the conditions in
which that capacity may be expressed.
• Imagine a hypothetical population living in an ideal, unlimited
• For simplicity’s sake, we will ignore immigration and emigration and
define a change in population size during a fixed time interval based
on the following verbal equation.
Change in population size = Births during - Deaths during
during time interval time interval time interval
• Using mathematical notation, we can express this relationship more
• If N represents population size, and t represents time, then δN
is the change is population size and ?t is the time interval.
• We can rewrite the verbal equation as:
δN/δt = B - D where B is the number of births and D is the number of
deaths. • We can convert this simple model into one in which births and deaths
are expressed as the average number of births and deaths per
individual during the specified time period.
• The per capita birth rate is the number of offspring produced per unit
time by an average member of the population.
• If there are 34 births per year in a population of 1,000
individuals, the annual per capita birth rate is 34/1000, or 0.034.
• If we know the annual per capita birth rate (expressed as b), we can
use the formula B = bN to calculate the expected number of births per
year in a population of any size.
• Similarly, the per capita death rate (symbolized by m for mortality)
allows us to calculate the expected number of deaths per unit time for
a population of any size.
• Now we will revise the population growth equation, using per capita
birth and death rates:
δN/δt = bN - mN
• Population ecologists are most interested in the differences between
the per capita birth rate and the per capita death rate.
• This difference is the per capita rate of increase or r, which
equals b ? m.
• The value of r indicates whether a population is growing (r > 0) or
declining (r < 0).
• If r = 0, then there is zero population growth (ZPG).
• Births and deaths still occur, but they balance exactly.
• Using the per capita rate of increase, we rewrite the equation for
change in population size as:
δN/δt = rN
• Ecologist use differential calculus to express population growth as
growth rate at a particular instant in time:
dN/dt = rN • Population growth under ideal conditions is called exponential
• Under these conditions, we may assume the maximum growth
rate for the population (rmax), called the intrinsic rate of
• The equation for exponential population growth is:
dN/dt = rmaxN
• The size of a population that is growing exponentially increases at a
constant rate, resulting in a J-shaped growth curve when the
population size is plotted over time.
• Although the intrinsic rate of increase is constant, the
population accumulates more new individuals per unit of time
when it is large.
• As a result, the curve gets steeper over time.
• A population with a high intrinsic rate of increase grows faster than
one with a lower rate of increase.
• J-shaped curves are characteristic of populations that are introduced
into a new or unfilled environment or whose numbers have been
drastically reduced by a catastrophic event and are rebounding.
Concept 52.4 The logistic growth model includes the concept of
• Typically, resources are limited.
• As population density increases, each individual has access to an
increasingly smaller share of available resources.
• Ultimately, there is a limit to the number of individuals that can
occupy a habitat.
• Ecologists define carrying capacity (K) as the maximum stable
population size that a particular environment can support.
• Carrying capacity is not fixed but varies over space and time
with the abundance of limiting resources. • Energy limitation often determines carrying capacity, although other
factors, such as shelters, refuges from predators, soil nutrients,
water, and suitable nesting sites can be limiting.
• If individuals cannot obtain sufficient resources to reproduce, the per
capita birth rate b will decline.
• If they cannot find and consume enough energy to maintain
themselves, the per capita death rate m may increase.
• A decrease in b or an increase in m results in a lower per capita
rate of increase r.
• We can modify our mathematical model to incorporate changes in
growth rate as the population size nears the carrying capacity.
• In the logistic population growth model, the per capita rate of increase
declines as carrying capacity is reached.
• Mathematically, we start with the equation for exponential growth,
adding an expression that reduces the rate of increase as N
• If the maximum sustainable population size (carrying capacity) is K,
then K ? N is the number of additional individuals the environment
can accommodate and (K ? N)/K is the fraction of K that is still
available for population growth.
• By multiplying the intrinsic rate of increase rmax by (K ? N)/K, we
modify the growth rate of the population as N increases.
• dN/dt = rmaxN((K ? N)/K)
• When N is small compared to K, the term (K ? N)/K is large and
the per capita rate of increase is close to the intrinsic rate of