Chapter 36

Lecture Outline
The Spread of Shakespeare’s Starlings
A. Originally native to Europe and Asia, starlings are now abundant and destructive pests across
North America.
Preview: The impact of introduced species is discussed in Module 38.3.
B. Starlings were brought to New York City by a private group of citizens for arbitrary reasons at a
time when importing foreign species was not regulated. Without native predators, parasites, or
competitors, the starling population rapidly spread across the United States.
C. Such uncontrolled population growth has features in common with that of the human population
on a global scale.
NOTE: In general, this picture is also one seen commonly wherever nonnative species are
introduced. Other examples include the introduction of rabbits into Australia and the kudzu vine
to the southern United States.
D. Population ecology is concerned with the changes in population size and structure over time
and the factors that regulate these changes.
Module 36.1 Population ecology studies how and why populations change.
Review: Populations and their place in the hierarchical organization of life are first discussed in
Module 1.1.
A. Ecologists define a population as an interacting group of individuals of a single species that use
common resources, most likely will interbreed, and are regulated by the same natural
B. Ecologists might restrict their definition to just the individuals in a very small but contained
area, such as all the sea anemones of one species in one tidepool. Or they may expand their
view to include all the individuals over the face of Earth—for instance, the human population as
it is exposed to the disease-causing HIV.
I. Population Structure and Dynamics
Module 36.2 Density and dispersion patterns are important population variables.
A. Population density is the number of individuals of a species per unit area or volume.
B. Density and dispersion patterns can be compared between populations occupying different
areas to contrast growth, stability, or other parameters.
C. Population density is usually measured by counting the number of individuals in a subsample
and estimating the number (relative to a unit of volume or area) in the whole population.
D. Dispersion pattern refers to how the individuals in a population are spaced within their areas.
E. A clumped pattern shows local aggregations in patches, usually resulting from an unequal
distribution of resources for plants and animals or from associations with mating and social
behavior for animals (Figure 36.2A).
NOTE: An example of a clumped distribution is the higher concentration of humans found in
and near cities.
F. A uniform pattern shows an even distribution over an area and usually results from interactions
among individuals, such as competition for resources by plants, secretion of chemicals by plants
that inhibit growth, or territorial behavior of animals (Figure 36.2B).
Preview: Territoriality, which can be responsible for a uniform population distribution, is
discussed in Module 35.16.
G. A random type of dispersion shows a patternless, unpredictable distribution. Such patterns are
rare but might be seen in, for example, clams distributed across a sandy ocean bottom, where
the factors affecting them are numerous and complexly interrelated.
H. Dispersion estimates are important tools used to analyze populations. They are used to monitor changes in populations
and to compare growth and stability of one area to another.
Module 36.3 Life tables track mortality and survivorship in populations.
A. Life tables relate these rates to size classes, usually grouped by decades for human life tables
(Table 36.3).
B. Data from life tables can be graphed to illustrate three basic survivorship curves (Figure 36.3).
C. Type I is characteristic of species (whales, elephants, and humans) that have low birth rates,
low infant mortality, and good care for the young.
D. Type II is characteristic of intermediate species (squirrels, lizards, and hydras).
E. Type III is characteristic of species (oysters and sea lettuce) that have high birth rates, high
infant mortality, and little or no care for the young.
F. Mathematical models are used by population ecologists to describe and predict population
Module 36.4 Idealized models help us understand population growth.
A. Mathematical equations of population growth provide useful starting points for studying
populations and have stimulated many experiments and controversies. For the following two
models, these abbreviations are used:
1. G 5 growth rate.
2. N 5 number of individuals in the population at the time the growth rate is studied.
3. r 5 intrinsic rate of increase (estimated by subtracting the death rate from the birth rate).
4. K 5 carrying capacity (the maximum number of individuals of a particular species that can
be supported by a particular environment).
B. Exponential Growth Model. This models the growth of a population under ideal conditions with
unlimited resources. The rate of growth is exponential and depends on the number of
individuals in the population:
G 5 rN
The graph shows a J-shaped curve, representing population size increasing without limit
(exponential growth model). As N increases, so does G. This type of growth, if exhibited by a
bacterium growing in an unlimited environment, would result in an inconceivably large number
of bacteria in less than two days (Figure 36.4A).
C. In nature, population growth is rarely, if ever, best modeled by an exponential growth equation.
There are always factors in the environment that limit population growth (population-limiting
D. Limiting Factors and Logistic Growth Model. As with the exponential growth model, this is an
idealized model of population growth. However, the logistic model takes limits to population
growth into consideration. The rate of growth is exponential in the beginning, but limited by
how close the population size (N) is to a critical size, the carrying capacity (K):
G 5 rN[(K 2 N)yK]
E. More accurately defined; carrying capacity is the largest population that a defined area can
sustain during a given period without harming the environment. The graph shows a lazy-Sshaped (logistic growth model) curve (Figure 36.4C). At first population size increases slowly;
N is very small relative to K, and (K 2 N)yK is near 1; then the population size increases
rapidly. As the population size approaches K, the growth rate slows down; as N approaches K,
the value of (K 2 N)yK approaches zero and the value of G also approaches zero. This type of
growth is typical of all organisms growing in limited environments. For example, actual data
from a population of fur seals follow this model (Figure 36.4B).
F. The growth rate of a population results from the combination of birth rate and death rate at any
one time. Growth rate rises if birth rate rises or death rate falls. Growth rate falls if birth rate
falls or death rate rises.
NOTE: The growth rate discussed above is assuming there is no emigration or immigration.
Module 36.5 Multiple factors may limit population growth.
A. Density-dependent factors affect a greater percentage of individuals as the density of
individuals (number per unit area) increases. Food supplies that become limited and the buildup
of toxic wastes often depress growth rates by increasing the death rate, decreasing the birth rate,
or both. The rate of the population growth is said to be density-dependent.
B. An example of the action of density-dependent factors is the decrease in clutch size as the
number of female song sparrows increased (Figure 36.5A).
C. Clear-cut cases of density-dependent factors operating in nature are sometimes hard to
determine because of the many conflicting factors. The white-footed mouse is a good example
of how the density of a population can limit the population growth.
D. Density-independent factors limit population size, no matter what the size, and are often abiotic
factors such as fires, floods, storms, seasonal changes in temperature or moisture, or disruption
by human activity.
E. An example of the action of density-independent factors is the exponential growth then sudden
decline in mid-summer of populations of leaf-sucking aphids due to drying conditions (Figure
F. Over the long term, most populations are regulated by a mixture of density-dependent and
density-independent factors, and the distinction between these two types may not be clear
(Figure 36.5C).
Module 36.6 Some populations have “boom-and-bust” cycles.
A. Potential density-dependent factors suggested for population cycles (in addition to food supply
and predation) include stress from crowding (e.g., lemmings).
B. Populations of predators and prey often show periodic cycles, such as the 10-year cycle for the
lynx and the snowshoe hare in the taiga of North America. Note that the cycles of both animals
have about a 10-year period (Figure 36.6).
NOTE: The numbers of prey are much larger than those for predators (note the two different Y
axes), relative differences that hold true for these two different trophic levels across community
types (see Module 37.13). Also notice that the hare population generally peaks before the lynx,
and once the hare population falls, the lynx population follows. This leads into the following
C. For the lynx and many predators, the availability of prey often determines population changes.
D. For the hare, however, the force of predation may not be the determining factor. Hare
populations cycle whether or not lynxes are present. There is evidence that the 10-year cycle of
the snowshoe hare population is due to both the effects of predation and fluctuations in the
hare’s food supply.
E. Predator population cycles always lag behind prey population cycles. Predators reproduce more
slowly than prey and take longer to increase the total population. As the predator population
increases, more pressure is placed on the prey population, thus decreasing their numbers. This
causes stress on the predator population, which then starts to decline.
II. Life Histories and Their Evolution
Module 36.7 Evolution shapes life histories.
A. Life history is the series of events of an organism’s existence—birth, reproduction, and death.
Aspects of life history that influence growth rate include age at first reproduction, number of
offspring, and amount of parental care. Each of these traits is shaped by natural selection.
B. Life histories that are characterized by reproducing early in life, yielding many offspring (bigbang reproduction), generally exhibiting high mortality rates (a Type III survivorship curve;
Figure 36.3), and having populations showing exponential growth during favorable times are
indicative of r-selection, the intrinsic rate of population increase. Such populations live in
unpredictable environments and are controlled by density-independent factors. Examples
include dandelions, molds, and the agave (Figure 36.7A).
Preview: r-selection-type species are often first to recolonize in secondary succession (Module
C. Life histories that are characterized by reproducing later in life, yielding fewer offspring,
generally experiencing low mortality rates (a Type I survivorship curve; Figure 36.6), and
having populations showing stability over time are of the K-selection type, close to carrying
capacity. Such populations are held near carrying capacity by density-dependent factors.
Examples include many larger vertebrates such as polar bears and whales.
D. An example is given of two different guppy populations evolving under two different regimes
of predation. Killifish and cichlids both prey on guppies. Killifish eat mainly small, immature
guppies. Cichlids eat mainly large, mostly mature, guppies. When preyed upon by cichlids,
guppies tend to be smaller, mature earlier, and produce more offspring than they do in areas
without cichlids. The heritability of these life history traits is demonstrated by the retention of
these characteristics over the course of several generations in predator-free environments.
Another experiment involved taking guppies from populations preyed upon by cichlids,
removing them from that environment, and subjecting them to a pattern of predation in which
small guppies were preyed upon. The results were that within two years the female guppies
matured later and produced fewer, larger offspring (Figure 36.7B).
Review: These experiments are good examples of the scientific approach to problem solving
(Modules 1.7 and 1.8) and the effects of directional selection (Module 13.16).
Module 36.8 Connection: Principles of population ecology have practical applications.
A. The practice of harvesting crops without damaging the resources is called sustainable resource
management and is practiced and promoted by wildlife managers, fishery biologists, and
B. Maximum sustained yield promotes the concept of harvesting at a level that is constant without
forcing the population into decline. This idea is illustrated in Module 36.4. Populations grow
best when resources are plentiful and the number of individuals in the population is at an
intermediate level.
C. Ecological principles are unfortunately not used when decisions are being made to overharvest
a crop. A prime example is the overharvesting of northern cod. As a result of poor planning and
a total disregard of ecological principles, the cod fisheries of Newfoundland have collapsed
(Figure 36.8A).
D. Integrated pest management (IPM) programs use several approaches in an attempt to control the
population of a pest. A combination of chemical, biological, and cultural methods is employed
with the objectives of minimizing environmental damage and reducing health risks while
controlling the pest population.
III. The Human Population
Module 36.9 Connection: Human population growth has started to slow after centuries of exponential
A. The worldwide human population grew relatively slowly (with a 25% setback during the
bubonic plague) until it was about 500,000,000 in 1650. Between 1650 and 1850 (200 years),
the population doubled to 1 billion. Between 1850 and 1930 (80 years), the population doubled
to 2 billion. Between 1930 and 1975 (45 years), the population doubled to 4 billion. Presently
there are approximately 6.4 billion people on Earth (Figure 36.9A). The world population is
projected to reach 9.3 billion by 2050.
B. Most of human history saw replacement reproduction. But with the advent of better nutrition,
medicine, and sanitation facilities (particularly clean drinking water), more people are reaching
maturity. The world growth rate has been exponential for more than 360 years.
C. Human population cannot continue to grow forever in a limited environment; the human
population will stop growing. The question is, at what carrying capacity or as a result of what
unforeseen biotic or abiotic catastrophe will the population stop growing? Fortunately, or we
can hope (depending on one’s view of human nature and ingenuity), humans can deliberately
choose at what level to control the human population.
NOTE: Regarding carrying capacity, debate must also concern the quality of life of humans and
of the remaining species on Earth (Chapter 38).
D. A new method of estimating the potential for human population growth is called an ecological
footprint. This is a multiparameter method of estimating the amount of land needed to support
one human in a particular lifestyle (Figure 36.9B). In 1997, Americans required 5 times the
ecological footprint available per person on earth.
E. Several questions can be raised and discussed by the class.
1. What will the maximum carrying capacity be for the Earth, or have we reached it already?
2. Are Americans and people of other developed nations, such as those in Europe, willing to
change their lifestyle to fit into the available ecological footprint?
3. How close to each other are humans willing to live (Figure 36.8C)?
Module 36.10 Birth and death rates and age structure affect population growth.
A. A country (e.g., Sweden) obtains zero population growth (ZPG) when the rate of birth equals
the death rate. Most developed countries are approaching or have obtained such growth
patterns, but others, such as Mexico, are still in the process of demographic transition (Figure
36.10A). Even in a country such as China (which has very strict population laws), with a birth
rate of 1.7 children per female, the population will continue to increase due to a lag in the rate
of decline.
B. A typical population has three main age groups: prereproductive (under 15 years old),
reproductive (15–45 years old), and postreproductive (45 years and older). In each diagram, all
the bars add up to 100, the vertical “steps” represent age groups by 5-year intervals, and the left
and right halves divide the population by gender (Figure 36.10B).
C. Age structure is the portion of individuals of a given age within a population. Human
population age structures can also show differences in social conditions, as in the comparison of
age structures of the populations of Italy, the United States, and Afghanistan. Italy has a stable
structure and relatively low birth rate, with older people continuing to represent a larger portion
of the entire population than even the United States. Afghanistan population structure shows the
effects of a predominantly young population and explosive population growth (Figure 36.10B).
D. Other factors that impact age structure and the comparison among countries are infant mortality
and life expectancy at birth. Afghanistan has an infant mortality rate of 143 and a life
expectancy at birth of 47 years. Japan has an infant mortality rate of 3 and life expectancy at
birth of 81 years.
E. Human populations are unique in being able to voluntarily regulate reproduction. Reduced
family size and, especially, delayed reproduction decreases population growth rates. A key
component to reducing a developing county’s population is the education of its women. As
women become better educated, they choose to have fewer children and start at a later age. This
is politically and socially a hot button.
F. The impact of the global human population growth on Earth is discussed in Chapter 38.