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Community and Population Ecology
Key Terms (Terms are listed in the same font style as they appear in the text.)
physical appearance (p. 144)
species diversity (p. 144)
species richness (p. 144)
species evenness (p. 144)
niche structure (p. 144)
species equilibrium model (p. 145)
theory of island
biogeography (p. 145)
habitat islands (p. 146)
native species (p. 146)
nonnative species (p. 146)
invasive species (p. 146)
alien species (p. 146)
indicator species (p. 147)
keystone species (p. 148)
pollination (p. 148)
top predator (p. 148)
dung beetle (p. 148)
Piaster orchaceus (p. 148)
foundation species (p. 149)
interspecific competition (p. 150)
resource partitioning (p. 150)
predation (p. 151)
predator (p. 151)
prey (p. 151)
predator-prey relationship (p. 151)
chemical warfare (p. 153)
bioprospectors (p. 154)
warning coloration (p. 154)
mimicry (p. 154)
behavioral strategies (p. 154)
parasitism (p. 154)
parasite (p. 154)
host (p. 154)
mutualism (p. 154)
pollination mutualism (p. 154)
nutritional mutualism (p. 154)
lichens (p. 154)
clownfish species (p. 154)
gut inhabitant mutualism (p. 155)
commensalism (p. 156)
redwood sorrel (p. 156)
epiphytes (p. 156)
ecological succession (p. 156)
primary succession (p. 156)
secondary succession (p. 156)
pioneer species (p. 156)
early successional plant
species (p. 156)
midsuccessional plant species (p. 156)
late successional plant
species (p. 156)
facilitation (p. 159)
inhibition (p. 159)
tolerance (p. 159)
disturbance (p. 159)
intermediate disturbance
hypothesis (p. 159)
climax community (p. 159)
balance of nature (p. 159)
biotic change (p. 160)
mature community (p. 160)
vegetation patches (p. 160)
inertia (p. 160)
persistence (p. 160)
constancy (p. 160)
resilience (p. 160)
complexity (p. 160)
stability (p. 160)
precautionary principle (p. 161)
coevolution (p. 161)
Outline
8-1 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
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 and loss of colonization ability.
2. Species diversity: 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, how fast old
species become extinct, the size of the island, and the distance from the mainland.
1. The species equilibrium model, or the theory of island biogeography, was proposed in the 1960s
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.
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.
8-2 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. Nonnative 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 are prey to water-born pollutants.
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.
C.
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 (for
example, alligators).
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.
8-3 Species Interactions: Competition and Predation
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 face extinction.
2. As humans take more and more space, other species are compromised.
B. In competitive situations, some species evolve adaptations that 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.
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.
8-4 Species Interactions: Parasitism, Mutualism, and Commensalism
A. 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.
B. Mutually beneficial interactions also exist in ecological environments.
1. Mutualism is a relationship that 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
c. Fungi and plant root associations called mycorrhizae
C. Some species interaction helps one species but does nothing for the other; this is commensalism.
Examples of this are the bromeliads and orchids (epiphytes).
8-5 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; and in an aquatic community, there is no
bottom sediment. This process generally takes a very long time.
a. Pioneer species attach themselves to patches of bare rock to begin the process. Lichens and
moss begin the building of soil particles.
b. Early successional plants include tiny annuals that reseed, which are then followed by small
perennial grasses, herbs, and ferns that grow close to the ground.
c. Mid-successional plants include low shrubs and trees that require more soil and lots of
sunlight. This process takes hundreds of years.
d. Late successional plant species are generally tree species that create shade and can tolerate
shade to become a complex forest community.
e. Primary succession can also take place in newly created small ponds that, over a long period
of time, will be transformed to a marsh and finally to dry land.
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.
a. Forest fires or deforestation, for example, can convert a particular stage of succession to an
earlier stage.
b. Changes in vegetation during secondary succession also change the numbers and types of
animals and decomposers.
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
balance of nature, one dominated by a few long-lived plant species that are in balance with their
environment. Three factors have been identified that affect how and at what rate succession occurs.
1. Facilitation: an area is made suitable for a second species by the actions of the first
2. Inhibition: early species delay establishment of later species
3. Tolerance: later species are unaffected by plants at earlier stages of succession
C. Changes in environmental conditions that disrupt a community can set back succession.
1. Disturbances such as fire, drought, mining, plowing, climate change, etc. can set back succession
to an earlier stage.
2. Large catastrophic disturbances can devastate a community, but in the long run, such
disturbances create unfilled niches and may encourage greater biodiversity.
3. The intermediate disturbance hypothesis contends that infrequent, moderate disturbances are
large enough to create openings for colonizing species, but mild enough for the survival of some
mature species in undisturbed areas. There is some evidence to support this hypothesis, but not
enough to state that this applies to all types of communities.
D. Scientists can’t predict the course of a given succession in a community toward a stable climax
community in balance with its environment.
1. The equilibrium model of succession was thought of as the balance of nature.
2. Ecologists now view a community as a continuous change, with instability rather than
equilibrium.
3. Succession reflects a struggle for each species to obtain food, light, nutrients, and space to gain
an advantage by occupying as much of its fundamental niche as possible.
4. The term biotic change better describes the changes occurring than does succession, and mature
community fits better than climax community when describing a relatively stable community
with a mosaic of vegetation patches.
8-6 Ecological Stability, Complexity, and Sustainability
Living systems maintain some degree of stability or sustainability through constant change in response to
changing environmental conditions.
A. All systems, from the smallest to the largest, are constantly changing in response to changing
environmental conditions. There are a series of positive and negative loops that interact to provide
stability or sustainability over the life span of a system. There are three aspects of stability or
sustainability in living systems.
1. A system that resists being disturbed or altered is said to have persistence or inertia.
2. A population that can keep its numbers within limits imposed by the resources shows constancy.
3. A system that is able to repair damage after a moderate external disturbance shows resilience.
B. Some communities obtain ecological stability or sustainability by having many different species
present.
1. One measure of a community’s biodiversity is to consider the complexity, or number of species
present, at each trophic level of the community. Exceptions to this view of stability have been
found.
2. There is a minimum threshold of species diversity, since no community can survive without some
producers and decomposers.
3. Recent research indicates that communities with more species tend to have a higher net primary
productivity
4. How much biodiversity is needed to maintain stability is still being debated. Part of the problem
lies in the disagreement on how to define stability.
5. Populations, communities, and ecosystems are rarely in a state of equilibrium.
C. We should take precautionary measures to prevent serious harm even if some of the cause-andeffect relationships have not been established.
1. Some developers feel there is no good reason to maintain biological diversity if it does not
necessarily lead to ecological stability.
2. Ecologists point out that environmental degradation is and has been occurring due to human
disturbances. This has disrupted some of the environmental services that support and sustain all
life and all economies.
3. The wise course is to use caution regarding the making of potentially harmful changes to
communities and ecosystems.
4. The precautionary principle can be taken too far. Some risk is necessary in order to discover what
works and what doesn’t. We must consider possible short- and long-term expected and
unintended effects.
5. Interactions of organisms in a community determine their abundance and distributions and serve
as natural selection on one another through coevolution.
Key Terms (Terms are listed in the same font as they appear in the text.)
emigration (p. 164)
population dynamics (p. 164)
age structure (p. 164)
size (p. 164)
prereproductive stage(p. 164)
density (p. 164)
reproductive stage (p. 164)
age distribution (p. 164)
postreproductive stage (p. 164)
population distribution (p. 164)
biotic potential (p. 165)
dispersion (p. 164)
intrinsic rate of increase (r) (p. 165)
clumping (p. 164)
environmental resistance (p. 165)
uniform dispersion (p. 164)
carrying capacity (K) (p. 165)
exponential growth (p. 166)
random dispersion (p. 164)
logistic growth (p. 166)
births (p. 164)
overshoots (p. 166)
deaths (p. 164)
reproductive time lag (p. 166)
immigration (p. 164)
dieback (crash) (p. 166)
population density (p. 167)
density-independent population controls (p. 167)
density-dependent population controls (p. 167)
bubonic plaque (p. 167)
stable population fluctuations (p. 167)
irruptive population fluctuations (p. 167)
cyclic fluctuations (p. 167)
irregular fluctuations (p. 167)
top-down control (p. 168)
bottom-up control (p. 168)
asexual reproduction (p. 169)
sexual reproduction (p. 169)
r-selected species (p. 169)
opportunists (p. 169)
K-selected species (p. 169)
life expectancies (p. 170)
survivorship curve (p. 170)
late loss curves (p. 170)
early loss curves (p. 170)
constant loss curves (p. 170)
life table (p. 170)
founder effect (p. 171)
demographic bottleneck (p. 171)
genetic drift (p. 171)
inbreeding (p. 171)
metapopulations (p. 171)
monoculture (p. 172)
opportunist species (p. 172)
pests (p. 172)
pathogens (p. 172)
Outline
9-1 Population Dynamics and Carrying Capacity
A. Populations change in size, density, and age distribution; most members of populations live together in
clumps or groups.
1. Three general patterns of population distribution occur in a habitat: clumping, uniform distribution,
and random dispersion. Most species live in clumps or groups.
a. Availability of resources varies from place to place.
b. Living in groups offers better protection from predators.
c. Some predator species live in packs to better have a chance to get a meal.
d. Temporary groups may form for mating and caring for young.
2. Uniform pattern distribution may occur where a resource, such as water, is scarce.
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.
3. Age structure of a population is usually described as the pre-reproductive stage, the reproductive
stage and the post-reproductive stage. A population with a large reproductive stage is likely to
increase, while a population with a large post-reproductive stage is likely to decrease.
C. No population can grow indefinitely due to limited resources such as light, water, and nutrients and also
due to competitors and/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.
3. Rapidly growing populations have four characteristics.
a. Individuals in the population reproduce early in life.
b. Individuals have short periods between generations.
c. Individuals have long reproductive lives.
d. Individuals produce multiple offspring each time they reproduce.
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.
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 get 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.
2. If the carrying capacity of an area is exceeded, changes in the area itself can reduce future carrying
capacity. Reducing grass cover by overgrazing allows sagebrush to move in and reduces the number
of cattle that the land can support.
3. Technological, social, and cultural changes have extended the earth’s carrying capacity for human
beings, for the time being.
G. The density of a population may or may not affect how rapidly it can grow.
1. Density-independent population controls affect a population’s size regardless of its density. These are
abiotic factors in the community.
2. Density-dependent factors or population controls have a greater affect on the population as its
density increases. Infectious disease is an example of density-dependent population control.
H. Population sizes may stay about the same, suddenly increase and then decrease, vary in regular cycles, or
change erratically.
1. Four general types of population fluctuations in nature are: stable, irruptive, cyclic, and irregular.
2. A stable population fluctuates slightly above and below carrying capacity and is characteristic of many
species living under fairly constant environmental conditions.
3. Some species have a fairly stable population size that may occasionally irrupt to a high peak and then
crash to below carrying capacity. This is characteristic of short-lived, rapidly reproducing species.
4. Cyclic fluctuations occur over a regular time period, generally a multiple year cycle.
5. Irregular behavior is poorly understood. Some scientists attribute irregular behavior to chaos in the
system; others disagree.
I. Interactions between predators and their prey change in cycles and appear to be caused by species
interactions, but other factors may be involved.
1. The hypothesis of top-down control of prey by predators may not be the only explanation for the
boom-and-bust cycles seen in these populations. This may also be related to the food supply of prey.
2. The bottom-up control hypothesis states that plants are consumed too rapidly by prey for
replacement to keep up. This may lead to a crash of plant predators, and that may lead to a crash of
higher predators of the herbivores.
3. These are not mutually exclusive hypotheses; more probably have interaction between predation and
food supplies.
9-2 Reproductive Patterns and Survival
A. Some species reproduce without having sex, and others reproduce by having sex.
1. Asexual reproduction does not utilize sex; each cell can divide and produce two identical cells that are
replicas of the original cell.
2. Sexual reproduction occurs when gametes from each parent combine to produce offspring with a
combination of genetic traits from each parent.
3. There are three disadvantages to sexual reproduction.
a. Males do not give birth; females have to produce twice the offspring to maintain the same
number of young as an asexual organism.
b. Chance of genetic errors/defects increase during splitting/recombination.
c. Courtship/mating consume energy and time, transmit disease, and inflict injury on males in some
cases.
4. Two important advantages are genetic diversity for survival of species in the face of changes in
environment and males may help with food gathering and/or rearing of young.
B. 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. The massive loss of offspring is compensated for by the large numbers of offspring in
each reproductive cycle.
2. r-Selected species are opportunists and 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, and mature more
slowly.
4.
K-selected species generally follow a logistic growth curve. Many of the larger species with long
generation times and a low reproductive rate are prone to extinction.
5. Availability of a suitable habitat for individuals of a population ultimately determines the population
size.
C. Populations of different species vary in how long individual members typically live.
1. A survivorship curve is one way to represent age structure of a population.
2. Three generalized types of survivorship curves are: late loss, early loss, and constant loss.
3. A life table shows the numbers of individuals at each age on a survivorship curve. Insurance
companies use life tables to determine the cost of insurance policies.
9-3 Effects of Genetic Variations on Population Size
A. Variations in genetic diversity can affect the survival of small, isolated populations.
1. Several factors can play a role in loss of genetic diversity and survival of a small population.
2. The founder effect is when a few individuals move to a new location that is isolated from the original
population. There is limited genetic diversity in such a population.
3. A demographic bottleneck occurs when only a few individuals survive a catastrophe.
4. Genetic drift is a third factor and involves random changes in gene frequencies in a population. This
may help or hurt the survival of the population.
5. Inbreeding occurs when members of a small population mate one another; this may increase the
numbers of defective genes in a population.
B. Metapopulations occur where some mobile populations occasionally exchange genes when some
members get together. Conservation biologists use this information to establish migration routes that will
enhance population size, genetic diversity, and survival of related local populations.
9-4 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 the 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 the 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 nonnative or new species into an
ecosystem.
g. A number of renewable resources have been over-harvested, such as overgrazing of grasslands,
over-hunting of wildlife, and pumping out aquifers for freshwater 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 and 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 the earth’s natural capital.