Chapter 33: Population
Growth and Regulation
33-1
Scope of Ecology
Ecology is the study of the interactions of
organisms with other organisms and with
the physical environment.
The study of ecological interactions can be
undertaken at many levels: the individual
organism, populations, communities,
ecosystems, and the biosphere.
A population is all the members of the same
species interacting with the environment
at a particular locale.
33-2
A community consists of all the various
populations in an area.
An ecosystem is the community plus its
nonliving habitat, including abiotic
(nonliving) and biotic (living)
components.
The biosphere is the portion of the entire
earth’s surface, including air, water, and
land, where living things exist.
Ecology describes the environment and
tests models.
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Ecological levels in a coral reef
33-4
Community Composition and
Diversity
The composition of a community is a
listing of populations present.
The diversity of a community adds in the
relative abundance of individuals.
Ecologists have ideas about why
populations assemble together in the
same place at the same time.
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The interactive model of community
structure views the community as a
stable assemblage that remains the
same over time.
The individualistic model views a
community as a collection of species
where each responds to its own
requirements and tolerance factors.
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Two terrestrial communities
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33-8
Community Stability and
Diversity
A change in community composition
over time is called ecological
succession.
Primary succession starts on areas
devoid of soil and secondary
succession starts with pioneer species
in areas where there is already soil
such as an abandoned field.
Succession also occurs in aquatic
communities.
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Secondary succession in a
forest
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Models of Succession
The climax-pattern model predicts that a
particular area will always lead to a
climax community characteristic for that
area (i.e., tropical rain forest at the
equator).
The facilitation model says that each
proceeding stage facilitates the
development of the next stage.
The inhibition model says that each
preceding stage tries to prevent the
arrival of the next stage.
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The tolerance model says that plants
from various stages try to colonize at
the same time and chance arrival of
seeds determines the outcome.
The length of time it takes trees to
develop gives the impression of a
series of plant communities from the
simple to the complex.
These models are not mutually exclusive,
and succession is probably a complex
process.
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Population Characteristics and
Growth
Population density is simply the number
of individuals per unit area or volume.
Distribution of these individuals can be
uniform, random, or clumped.
Most members of a population are
clumped, as are the members of a
human population.
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Patterns of distribution within a
population
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Abiotic factors such as water,
temperature, and availability of organic
nutrients often determine a
population’s distribution.
Biotic factors, such as the availability of
food, or presence of disease, affect the
distribution of populations.
Limiting factors are those factors that
determine whether an organisms lives
in an area.
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Patterns of Population Growth
Each population has a particular pattern
of growth.
The per capita rate of increase is
calculated by subtracting the number
of deaths from the number of births
and dividing by the number of
individuals in the population.
It is assumed that immigration and
emigration are equal.
33-16
Every population has a biotic potential,
the greatest possible per capita rate of
increase under ideal circumstances.
Two possible patterns of population
growth are considered.
Exponential growth results in a J-shaped
curve because as the population
increases in size so does the expected
increase in new members.
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Biotic potential
33-18
Exponential growth
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Environmental resistance occurs when
most environments restrict growth, and
exponential growth cannot continue
indefinitely.
Under these circumstances logistic
growth occurs and an S-shaped growth
curve results with four phases: lag,
exponential growth, deceleration, and
stable equilibrium.
When the population reaches carrying
capacity, the population stops growing
because environmental resistance
opposes biotic potential.
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Logistic growth
33-21
Survivorship
Populations are made up of individuals of
different ages.
Populations tend to have one of three
types of survivorship curves, depending
on whether most individuals live out the
normal life span (type I), die at a
constant rate regardless of age (type II),
or die early (type III).
Much can be learned about the life history
of a species through its survivorship
curve.
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Survivorship curves
33-23
Human Population Growth
The human population is expanding
exponentially.
The doubling time is the length of time it
takes for a population to double,
currently estimated at 53 years.
Only when birthrate equals death rate will
there be zero population growth.
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More-Developed Versus LessDeveloped Countries
Most of the expected increase in human
population will occur in certain lessdeveloped countries (LDCs) of Africa,
Asia, and Latin America.
Doubling time in more-developed
countries (MDCs) is about 100 years
because a decrease in death rate due to
medical advances was followed by a
decrease in birth rates.
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Standard of living
33-26
World population growth
33-27
The relationship between decreased
death rate followed by a slower birth
rate is called demographic transition.
Despite introduction of medical care,
LDCs still have twice the MDC growth
rate.
Support for family planning, social
progress, and delayed childbearing
could help prevent an expected
increase in population size.
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Age Distributions
Many MDCs have a stable age structure,
but most LDCs have a youthful profile—
a large proportion of the population is
younger than age of 15.
This means their populations will expand
greatly in the near future.
Zero population growth or replacement
reproduction does not occur when each
couple has only two children because
there is momentum from younger
women entering reproductive years.
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Age-structure diagram for MDCs
33-30
Age-structure diagram for LDCs
33-31
Regulation of Population Growth
Two life history patterns exist in
populations.
Opportunistic populations have a short
lifespan, small stature, and produce
many offspring to take advantage of
new resources.
Equilibrium species live longer, are larger,
and produce fewer young but have
greater parental care; they hold
population size near carrying capacity.
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Life history patterns
33-33
Population growth is limited by both
density-independent factors (e.g.,
weather) and density-dependent factors
(predation, competition, and resource
availability).
Density-independent factors operate
regardless of population density.
Density-dependent factors increase in
intensity as population size increases.
33-34
Competition
Competition occurs when two species try
to use a resource that is in limited
supply.
According to the competitive exclusion
principle, no two species can occupy
the same ecological niche at the same
time when resources are limiting.
Resource partitioning occurs when
resources are partitioned between two
or more species.
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Competition between two
populations of Paramecium
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Competition between two
species of barnacles
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Predation
Predation occurs when one living
organism, the predator, feeds on
another, the prey.
Predators include lions, whales that filter
feed, parasites that draw blood from
hosts, and herbivores that eat grass,
trees, and shrubs.
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Predator-Prey Population
Dynamics
Predator-prey interactions between two
species are influenced by environmental
factors.
Cycling of population densities may
occur, as in the case of the Canadian
lynx and hare; predators kill off prey and
then the predator population declines
when food is in short supply.
Predator-prey systems are not usually
simple two-species systems.
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Predator-prey interaction: lynx
and snowshoe hare
33-40
Prey Defenses
Coevolution occurs when two species
adapt to selective pressures of each
other.
Prey defenses against predation take
many forms: camouflage, use of fright,
and warning coloration are three prey
defense mechanisms.
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Antipredator defenses
33-42
Mimicry
Mimicry occurs when one species
resembles another that possesses an
antipredator defense.
Batesian mimicry occurs when one
species has the warning coloration but
lacks the antipredator defense of the
species it mimics.
Müllerian mimicry occurs when two
species with the same warning
coloration both have defenses.
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Mimicry
33-44
Symbiosis
Symbiosis refers to close interactions
between members of two populations.
Three types of symbiosis occur:
parasitism, commensalism, and
mutualism.
Symbiotic associations do not
necessarily fall neatly into these three
categoties.
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Parasitism
In the symbiotic relationship called
parasitism, the parasite benefits and
the host is harmed.
Parasites derive nourishment from their
host and the effect can be mild or fatal
to the host.
Many parasites use a secondary host to
disperse or complete their stages of
development, as is the case in the life
cycle of a deer tick.
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Commensalism
In commensalism, one species benefits
and the other is neither benefited nor
harmed.
Often a host provides a home or
transportation for another species.
For example, barnacles attach to backs
of whales, remoras attach to sharks,
clown fishes live within the tentacles of
sea anemones, and cattle egrets eat
insects off large grazing mammals.
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Egret symbiosis
33-48
Mutualism
In mutualism, both members benefit.
Lichens have traditionally been regarded
as mutualistic but experiments suggest
that the fungus may be parasitic on the
algae.
The bullhorn acacia tree provides a home
for the ant Pseudomyrmex ferruginea,
in swollen acacia thorns.
Ants feed from nectaries at base of
leaves and also eat Beltian bodies at
leaf tips.
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In return ants protect this tree from
herbivores.
Cleaning symbiosis involves crustaceans,
fish, and birds that act as cleaners of a
variety of vertebrate clients.
In some cases, the cleaners may exploit
the situation and feed on host tissues,
but cleaning appears to improve the
fitness of the client.
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Cleaning symbiosis
33-51
Chapter Summary
Ecology is the study of the interactions
of organisms with each other and the
physical environment.
Ecology includes the organism, the
population, the community, the
ecosystem, and the biosphere.
Communities are assemblages of
interacting populations that differ in
composition and diversity.
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Environmental and biotic factors
influence community composition and
diversity.
Ecological succession is a change in
species composition and community
structure over time.
Population size depends upon births,
deaths, immigration, and emigration.
Exponential and logistic patterns of
population growth have been
developed.
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Mortality rates within a population can be
illustrated by a survivorship curve.
Life history patterns range from one in
which many young receive little care to
one in which few young receive much
care.
The human population is still growing
exponentially, and how long this can
continue is not known.
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Factors that affect population size are
classified as density-independent and
density-dependent.
Competition often leads to resource
partitioning, which reduces competition
between species.
Predation often reduces prey population
density, which in turn can lead to a
reduction in predator population density.
Symbiotic relationships include
parasitism, commensalism, and
mutualism.
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