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CHAPTER 8
Population Dynamics, Carrying Capacity, and
Conservation Biology
THINKING
Goals
See bulleted list of questions on p. 109 of text.
Objectives
1. Describe the three characteristics that describe a
biological community.
2. Distinguish among the following roles played by species
and give one example of each: native species, nonnative
species, indicator species, keystone species. Explain
why these labels are important.
3. Distinguish among the following species interactions and
give one example of each: interspecific competition,
predation, and symbiosis. Distinguish between
interference competition and exploitation competition.
Summarize the competitive exclusion principle. List two
strategies species use to reduce competition.
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4. List two strategies that predators use to capture their
prey. List at least five strategies that prey use to defend
themselves against predators.
5. Distinguish among three forms of symbiotic
relationships and give one example of each: parasitism,
mutualism, and commensalism.
6. Define succession. Distinguish between primary and
secondary succession. Describe how humans affect
communities.
7. Define birth rate, death rate, immigration, and
emigration. Write an equation to mathematically
describe the relationship between these rates and the rate
of population change.
8. Define limiting factor. Give an example of a resource
that would be limiting in an ecosystem.
9. Define exponential growth.
10. Compare a J-shaped growth curve with a S-shaped
growth curve and comment on the factors that produce
the sigmoid (S-shaped) curve.
11. Define carrying capacity and explain what determines
the carrying capacity of an ecosystem.
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12. Define r-selected species and K-selected species and
compare the two. Give an example for each type of
species reproductive pattern.
13. List the eight major ways that humans have altered
natural ecosystems and comment on the effects of these
alterations for the future of the planet.
Key Terms (Terms are listed in the same font style as they
appear in the text.)
physical appearance (p.
109)
species diversity (p. 109)
species richness (p. 109)
species evenness (p. 109)
niche structure (p109)
native species (p. 111)
nonnative species (p. 111)
invasive species (p. 111)
alien species (p. 111)
indicator species (p. 111)
keystone species (p. 111)
habitat loss and
fragmentation (p. 111)
pollination (p. 112)
top predator (p. 113)
interspecific competition
(p.114)
resource partitioning (p.
114)
predation (p. 114)
predator (p. 114)
prey (p. 114)
herbivores (p. 115)
carnivores (p. 115)
pursuit (p. 115)
ambush (p. 115)
camouflage (p. 115)
chemical warfare (p. 116)
warning coloration (p.
116)
mimicry (p. 116)
parasitism (p. 117)
parasite (p. 117)
host (p. 117)
mutualism (p. 117)
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gut inhabitant mutualism
(p. 117)
commensalism (p. 118)
epiphytes (p. 118)
ecological succession (p.
118)
primary succession (p.
118)
secondary succession (p.
118)
climax community (p. 119)
balance of nature (p. 119)
age structure (p. 120)
pre-reproductive age (p.
120)
reproductive stage (p. 120)
post-reproductive stage (p.
120)
biotic potential (p. 120)
intrinsic rate of increase
(p. 120)
environmental resistance
(p. 121)
carrying capacity (K) (p.
121)
exponential growth (p.
121)
logistic growth (p. 121)
overshoots (p. 121)
reproductive time lag (p.
122)
dieback (crash) (p. 122)
r-selected species (p. 122)
opportunists (p. 122)
competitor (p. 122)
K-selected species (p.
122)
precautionary principle
(p.126)
persistent organic
pollutants (POPs)
(p. 127)
Outline
6-1 Community structure and species diversity
A.
Ecologists use three characteristics to describe a
biological community:
1. Physical appearance- the relative sizes,
stratification, and distribution of its populations and
species
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a. Large terrestrial communities are patchy
b. Transition occurs around the edges, where two
community types interact.
c. Increased edge area may be harmful due to
habitat fragmentation; many species become
more vulnerable to predators, loss of colonization
ability.
2. Species diversity- is a combination of numbers of
different species (richness) and abundance of
individuals within each species (species evenness)
3. Niche structure- the number of ecological niches,
their resemblance or difference from each other, and
interaction of species with each other
a. High species richness often is correlated with low
species evenness (few numbers per species)
b. Two major factors affect diversity of species in a
community: latitude in terrestrial communities
and pollution in aquatic systems.
B. The number of species on an island is determined by
how fast new species arrive, old species become
extinct, the size of the island and distance from the
mainland.
1. The species equilibrium model, or the theory of
island biogeography was proposed in the 1960’s to
explain differences in species richness with island
size.
a. Balance between two factors is important in this
model: rate of immigration of species and rate of
extinction of existing species
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b. Two island features are important: size and
distance from mainland
2. This model has been expanded to include habitat
islands in protected areas such as national parks.
6-2 Types of species
Species in a community play many different roles in its
ecology.
A. Types of Species
1. Native species are those whose original home is in
this particular ecosystem.
2. Non-native species originally evolved in a different
ecosystem and migrated or were introduced to a
new ecosystem.
3. Indicator species alert us to harmful changes taking
place in biological communities.
a. Birds are excellent biological indicators because
they are everywhere and are quickly affected by
environmental change.
b. Some amphibians are, also, considered indicator
species.
1) Their disappearance may be a warning of
serious declining environmental quality.
2) Vulnerability in different parts of an amphibian
life cycle suggests degrees of environmental
decline/disruption.
a) They are born and live in water and prey to
water-born pollutants.
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b) As adults they live on land; they are
unprotected because their skins readily
absorb pollutants.
c) Frogs as a threatened species are harmed by
habitat loss, drought, pollution, parasitism,
disease, overhunting, ultraviolet radiation,
etc.
4. The importance of amphibian species’ becoming
extinct rests on three likelihoods.
a. This extinction suggestions that the health of the
environment is deteriorating.
b. Adult amphibians play important ecological roles
in the world community.
1) Extinction of one species leads to extinction of
other species.
2) Their biological niche controls insect
populations.
c. Amphibians represent the possibility of hundreds
of potential pharmaceutical products.
B. Keystone species help ecological communities run
smoothly; they determine the type and number of
community species.
1. Keystone species have a larger effect on both the
types and number of other species in a community.
2. Keystone species fill important ecological roles:
pollinating and regulating populations of other
species.
3. Loss of a keystone species has far-reaching
ramifications for other species in a community.
Example: alligators
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C. Foundation species shape communities by creating
and enhancing habitat that benefits other species.
1. Elephants, in breaking and uprooting trees, create
forest openings in African savanna grasslands and
woodlands.
2. Grazing species benefit from the new growth of
grasses and other forage plants.
3. The rate of nutrient cycling is increased.
6-3 Species Interactions.
Five basic species interactions are competition,
predation, parasitism, mutualism and commensalism.
A. Competition between species for food, sunlight, water,
soil, space, nest sites, etc. is interspecific competition.
1. With intense competition for limited resources, one
species must migrate; shift its feeding
habits/behavior or fact extinction.
2. As humans take more and more space, other species
are compromised.
B. In competitive situations, some species evolve
adaptations which reduce/avoid competition for
resources.
1. Over a long time, species evolve more specialized
traits that allow them to use shared resources at
different times or in different ways or in different
places; this is termed resource partitioning. An
example is that of insect-eating warblers in Maine
forests that eat insects in specific parts of a spruce
tree. Owls hunt at night; hawks hunt during the
day.
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2. Predator-prey relationships define one species (the
predator) feeding/preying on another. Individually
prey is harmed, but predation can help the
population by eliminating the sick, weak and old.
3. Predators have a variety of ways to capture prey.
Herbivores feed on immobile plant species;
carnivores use pursuit of prey or ambush to capture
prey. Some predators use camouflage and others
use chemical warfare (venom) to capture prey or
deter predators.
4. Prey species escape predators in a number of
different ways such as swift movement, protective
shells, camouflage or use of chemicals to repel or
poison.
C. Parasites live on or in another species. The host of
this arrangement is obviously harmed by it but the
parasite can contribute to biodiversity by controlling
the size of specific species populations.
D.
Mutually beneficial interactions also exist in
ecological environments.
1. Mutualism is a relationship which benefits both
species; these benefits can be in dispersing pollen
and seeds for reproduction, in receiving food or in
receiving protection.
2. Mutualism is not cooperation; each species exploits
the other.
a. Birds & African buffalo, elephants and
rhinoceroses
b. Clownfish and anemones
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c. Fungi and plant root associations called
mycorrhizae
E. Some species interaction helps one species but does
nothing for the other; this is commensalism.
Examples of this are the bromeliads and orchids
(epiphytes)
6-4 Ecological succession: Communities in Transition
A. With new environmental conditions, community
structures can change; one group of species is replaced
by another.
1. Ecological succession is the gradual change in
species composition of a given area.
2. Primary ecological succession is the gradual
establishment of biotic communities on lifeless
ground; in the soil there is no terrestrial community;
in an aquatic community, there is no bottom
sediment. This process generally takes a very long
time.
3. Secondary ecological succession defines a series of
communities with different species developing in
places with soil or bottom sediment. The soil or
sediment remains after the natural community of
organisms has been disturbed, removed or
destroyed.
B. The classic view of ecological succession is that it is
an orderly sequence, each stage leading to the next,
more stable stage until a climax community is
reached. Such a community would represent the
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balance of nature, one dominated by a few long-lived
plant species that is in balance with its environment.
C. Scientists can’t predict the course of a given
succession in a community toward a stable climax
community in balance with its environment.
6-5 Population dynamics and carrying capacity
A. Populations change in size, density, and age
distribution, most members of populations live
together in clumps or groups
B. Four variables influence/govern population size:
births, deaths, immigration, and emigration.
1. Increase in population occurs by birth and
immigration.
2. Decrease in population occurs by death and
emigration.
C. No population can grow indefinitely due to limited
resources such as light, water, and nutrients and
because of competitors or predators.
1. The biotic potential is the population’s capacity for
growth.
2. The intrinsic rate of increase (r) is the rate of
population growth with unlimited resources.
D. Environmental resistance consists of factors that limit
population growth.
1. Carrying capacity (K) is determined by biotic
potential and environmental resistance. This is the
number of a species’ individuals that can be
sustained indefinitely in a specific space.
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2. As a population reaches its carrying capacity, its
growth rate will decrease because resources become
more scarce.
E. A population can grow rapidly with ample resources
1. With few resource limitations, a population will
have exponential growth. This is a fixed rate of
growth that will take be a J-shaped growth curve as
the base size of population increases. This
represents its intrinsic rate of increase (r) or biotic
potential.
2. This exponential growth is converted to logistic
growth when the populations gets larger and face
environmental resistance. In logistic growth, the
growth rate levels off as population size reaches or
nears carrying capacity.
3. The sigmoid (s-shaped) population growth curve
shows that the population size is stable, at or near
its carrying capacity.
F. When population size exceeds its carrying capacity,
organisms die unless they move or switch to new
resources.
1. Exponential growth leads to logistic growth and
may lead to the population overshooting the
environment’s carrying capacity.
a. Overshooting an environment’s resources often is
a result of a reproductive time lag.
b. The reproductive time lag can produce a
dieback/crash of organisms unless the organisms
can find new resources or move to an area with
more resources.
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2. If the carrying capacity of an area is exceeded,
changes in the area itself can reduce future carrying
capacity. Reducing grass cover by over-grazing
allowed sagebrush to move in and reduce the
number of cattle that the land could support.
3. Technological, social, and cultural changes have
extended the earth’s carrying capacity for human
beings, for the time being.
G. Reproductive patterns can be classified into two
fundamental reproductive patterns, r-selected and Kselected species
1. r-selected species have a high rate of reproduction
with little parental care. They have many, usually
small offspring. Massive loss of offspring is
compensated for by the large numbers of offspring
each reproductive cycle.
2. r-selected species are opportunists; reproduce when
conditions are favorable or when disturbance opens
a niche for invasion. Most species of this type go
through irregular and unstable boom-and-bust
cycles in population size.
3 Competitor or K-selected species are at the other
extreme. They produce later in life, have a small
number of offspring, are born fairly large, are cared
for and protected by parents, mature more slowly.
4. K-selected species generally follow a logistic
growth curve. Many of the larger species with long
generation times/low reproductive rate are prone to
extinction.
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5. Availability of suitable habitat for individuals of a
population ultimately determines the population
size.
6-6 Human Impacts on Natural Systems: Learning from
Nature
A. Humans have altered nature in ways that threaten the
survival of many species including our own species.
1. Humans have directly affected changes on about
83% of earth’s land surface.
2. Humans have altered nature to meet needs and
wants in nine major ways.
a. Destruction, fragmentation, and degrading of
wildlife habitats have reduced biodiversity.
b. The simplification and homogenization of natural
ecosystems by clearing land and planting a single
species (monoculture) reduces numbers of
species and interactions. Opportunistic species
and pest organisms are costing time, energy and
money to control. Invasion of pathogenic
organisms is another threat.
c. Destruction of earth’s net primary productivity is
a third type of alteration.
d. Certain types of intervention have unintentionally
strengthened pest species and disease-causing
bacteria.
e. Some predator species have been deliberately
eliminated from ranching areas.
f. Alterations have occurred due to the introduction
of non-native or new species into an ecosystem.
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g. A number of renewable resources have been
over-harvested, such as overgrazing of
grasslands, over-hunting of wildlife, and
pumping out aquifers for fresh water faster than
they can recharge.
h. Some human activities also interfere with normal
chemical cycling and energy flows in
ecosystems.
i. Human-dominated ecosystems are increasingly
dependent on nonrenewable energy from fossil
fuels that produce pollution, add greenhouse
gases to the atmosphere.
3. Alteration of natural ecosystems needs to be slowed
down and we need to maintain a balance between
simplified, human-altered ecosystems and more
complex natural ecosystems.
B. By mimicking four major ways that nature has adapted
and sustained itself we can develop more sustainable
economies.
1. We are totally dependent on the sun and earth for
life. We are an expendable species.
2. Everything is interconnected and interdependent.
What connections are strongest, most important and
most vulnerable are those that we must discover.
3. Any intrusion into nature has unexpected and
unintended side effects. We must not deplete and
degrade earth’s natural capital.
C. The ‘precautionary principle’ suggests that we should
be careful or our environments and environmental
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resources; we have no sure knowledge of their interconnectivity nor of their long-term importance.
Summary
1. The number of species in a community is determined by
latitude in terrestrial systems, and depth and pollution in
aquatic systems.
2. According to their ecological roles within an ecosystem
species are classified as native, nonnative, indicator, and
keystone species.
3. Interspecific interactions include competition for
resources, predation, parasitism, mutualism, and
commensalism. Intraspecific interactions are represented
by competition for resources.
4. Communities and ecosystems change in response to
environmental change. The change of ecosystem species
composition is known as ecological succession.
5. High species diversity increases the stability of an
ecosystem since there are more ways to respond to
environmental stressors.
6. Reproductive patterns in species are influenced by
ample resources and limited resources. Once resources
become limited, the population must decrease over time.
The patterns among specific species vary but there are
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four general types of population fluctuations: stable,
irruptive, cyclic, and irregular.
7. Human activities and the use of technology now threaten
the survival of many species and may affect our own
quality of life. We have reduced biodiversity,
homogenized natural ecosystems, wasted/destroyed
earth’s net primary productivity, strengthened some pest
species and disease-causing bacteria, eliminate most
predators, introduced nonnative species into ecosystems,
overharvested renewable resources, interfered with
chemical cycling and energy flows in ecosystems, and
become dependent on nonrenewable resources.
8. We must maintain a balance between human-altered
ecosystems and natural ecosystems. We must
implement and live the spirit of these four guidelines:
…we are totally dependent on the sun and the earth.
…everything is interdependent.
…anything we do will have consequences.
…we must live off the biological income of the earth’s
natural capital, not squander it.
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