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45
Ecological Communities
Concept 45.1 Communities Contain Species That Colonize and
Persist
Community—a group of species that coexist
and interact with one another within a defined
area
Biologists may designate community boundaries
based on natural boundaries (e.g., the edge of
a pond) or arbitrarily.
They may restrict study to certain groups (e.g.,
the bird community).
Concept 45.1 Communities Contain Species That Colonize and
Persist
Communities are characterized by species
composition; that is, which species they
contain and the relative abundances of those
species.
A species can occur in a location only if it is able
to colonize and persist there.
A community contains those species that have
colonized minus those that have gone extinct
locally.
Concept 45.1 Communities Contain Species That Colonize and
Persist
Local extinctions can occur for many reasons:
• Species unable to tolerate local conditions
• A resource may be lacking
• Exclusion by competitors, predators, or
pathogens
• Population size too small; no reproduction
Concept 45.1 Communities Contain Species That Colonize and
Persist
In 1883 the volcano on Krakatau erupted, killing
everything on the island.
Scientists studied the return of living organisms.
Within 3 years, seeds of 24 plant species had
reached the island.
Later, as trees grew up, some pioneering plant
species that require high light levels
disappeared from the island’s now-shady
interior.
Species composition continues to change as
new species colonize and others go extinct.
Figure 45.1 Vegetation Recolonized Krakatau (Part 1)
Figure 45.1 Vegetation Recolonized Krakatau (Part 2)
Concept 45.2 Communities Change over Space and Time
Ecologists have documented recurring patterns
of species compositional change.
Species composition varies along environmental
gradients, after disturbances, and with
changing climate.
Concept 45.2 Communities Change over Space and Time
Species composition changes along
environmental gradients, such as elevation or
soil types.
A transect is a straight line used for ecological
surveys.
A transect along an environmental gradient will
show the species turnover through space.
Figure 45.2 Species Turnover along an Environmental Gradient
Concept 45.2 Communities Change over Space and Time
Many animal species are associated with
particular plant communities:
The plants they eat may be there; or because
plants modify physical conditions and
contribute to habitat structure.
Habitat structure determines the animal’s ability
to get food or avoid predators.
Figure 45.3 Many Animals Associate with Habitats of a Particular Structure (Part 1)
Figure 45.3 Many Animals Associate with Habitats of a Particular Structure (Part 2)
Concept 45.2 Communities Change over Space and Time
In any community there is ongoing colonization
and extinction, and thus a steady turnover in
species composition.
Species turnover can result from disturbance
events: volcanic eruptions, wildfires,
hurricanes, landslides, human activities.
Some or all the species are wiped out, and
environmental conditions are changed.
Concept 45.2 Communities Change over Space and Time
Species often replace one another in a
predictable sequence called succession.
Example: A patch of elephant dung is colonized
by a sequence of dung beetle species.
Figure 45.4 Dung Beetle Species Composition Changes over Time
Concept 45.2 Communities Change over Space and Time
Some species are better at colonizing than
others.
Early-arriving dung beetles tend to be strong
fliers with a good sense of smell, or
“hitchhikers” that ride on the dung-producers.
On Krakatau, the first plants were species that
have seeds that are easily dispersed by sea or
wind.
Concept 45.2 Communities Change over Space and Time
After a disturbance, environmental conditions
change with time.
Examples:
Dung starts out wet and dries over time.
As trees grow, the forest canopy closes and
light conditions change.
Concept 45.2 Communities Change over Space and Time
After a disturbance, succession often leads to a
community that resembles the original one.
Example: On Krakatau, tropical forests
eventually came back.
Concept 45.2 Communities Change over Space and Time
If the original community is not reestablished,
there is an ecological transition to a different
community.
Example: Conversion of grasslands to
shrublands in the U.S.–Mexico Borderlands
after intensive cattle grazing.
Concept 45.2 Communities Change over Space and Time
Climate change can also cause temporal
variation in communities.
As physical conditions change, the geographic
ranges of species necessarily change with
them.
One way to reconstruct such change is analysis
of fossilized plant remains in packrat middens.
Biologists can show how plant communities of
the Borderlands changed over the last 14,000
years as the climate became drier.
Figure 45.5 Species Composition Changes with Climate Change
Concept 45.3 Trophic Interactions Determine How
Energy and Materials Move through Communities
Each species in a community has a unique
niche.
This concept refers to the environmental
tolerances of a species, which define where it
can live.
Also refers to the ways a species obtains energy
and materials and to patterns of interaction with
other species in the community.
Concept 45.3 Trophic Interactions Determine How
Energy and Materials Move through Communities
Consumer–resource, or trophic interactions
cause energy and materials to flow through a
community.
Trophic levels—feeding positions
Primary producers, or autotrophs, convert
solar energy into a form that can be used by
the rest of the community.
Concept 45.3 Trophic Interactions Determine How
Energy and Materials Move through Communities
Heterotrophs get energy by breaking apart
organic compounds that were assembled by
other organisms.
Primary consumers (herbivores) eat primary
producers.
Secondary consumers (carnivores) eat
herbivores.
Tertiary consumers eat secondary
consumers.
Concept 45.3 Trophic Interactions Determine How
Energy and Materials Move through Communities
Omnivores feed from multiple trophic levels.
Decomposers, or detritivores, feed on waste
products or dead bodies of organisms.
Decomposers are responsible for recycling of
materials; they break down organic matter into
inorganic components that primary producers
can absorb.
Table 45.1 The Major Trophic Levels
Concept 45.3 Trophic Interactions Determine How
Energy and Materials Move through Communities
Trophic interactions are shown in diagrams
called food webs.
Arrows indicate the flow of energy and materials
—who eats whom.
Figure 45.6 A Food Web in the Yellowstone Grasslands
Concept 45.3 Trophic Interactions Determine How
Energy and Materials Move through Communities
Gross primary productivity (GPP)—total
amount of energy that primary producers
convert to chemical energy.
Net primary productivity (NPP)—energy
contained in tissues of primary producers and
is available for consumption.
Change in biomass of primary producers (dry
mass) per unit of time is an approximation for
NPP.
Concept 45.3 Trophic Interactions Determine How
Energy and Materials Move through Communities
Ecological efficiency is about 10%:
Only about 10% of the energy in biomass at one
trophic level is incorporated into the biomass of
the next trophic level.
This loss of available energy at successive levels
limits the number trophic levels in a community.
Concept 45.3 Trophic Interactions Determine How
Energy and Materials Move through Communities
Ecological efficiency is low because:
• Not all the biomass at one trophic level is
ingested by the next one.
• Some ingested matter is indigestible and is
excreted as waste.
• Organisms use much of the energy they
assimilate to fuel their own metabolism.
Figure 45.7 Energy Flow through Ecological Communities
Concept 45.3 Trophic Interactions Determine How
Energy and Materials Move through Communities
The per capita growth rate of a species is related
to the sum of positive and negative
contributions of species with which it interacts.
Succession can be driven by such interactions.
Examples: Late-colonizing dung beetles inhibit
early colonizers by competing for nutrients.
Late-arriving plant species shade out
pioneering species.
Concept 45.3 Trophic Interactions Determine How
Energy and Materials Move through Communities
Consumer–resource interactions can have ripple
effects across trophic levels, resulting in a
trophic cascade.
In Yellowstone National Park, wolves were
extirpated by hunting by 1926.
Elk were culled each year to prevent them from
exceeding carrying capacity, until 1968. Elk
population then rapidly increased.
The elk browsed aspen trees so heavily that no
young aspens could get a start.
Concept 45.3 Trophic Interactions Determine How
Energy and Materials Move through Communities
Elk also browsed streamside willows to the point
that beavers (who depend on willows for food)
were nearly exterminated.
Wolves were reintroduced in 1995 and preyed
primarily on elk.
Aspen and willows grew again, and the beaver
population increased.
Figure 45.8 Removing Wolves Initiated a Trophic Cascade (Part 1)
Figure 45.8 Removing Wolves Initiated a Trophic Cascade (Part 2)
Figure 45.8 Removing Wolves Initiated a Trophic Cascade (Part 3)
Concept 45.4 Species Diversity Affects Community Function
Species diversity has two components:
Species richness—the number of species in the
community.
Species evenness—the distribution of species’
abundances
Figure 45.9 Species Richness and Species Evenness Contribute to Diversity
Concept 45.4 Species Diversity Affects Community Function
Both aspects of diversity affect community
function.
A species’ influence in a community depends on
its interactions, and also its abundance.
Communities with a few very abundant species
are largely defined by them, rather than by the
many rare ones.
Concept 45.4 Species Diversity Affects Community Function
Communities can be thought of as systems with
inputs and outputs.
Important measures of community function are
the total flow of energy into the community
(GPP), and net energy available for
consumption by heterotrophs (NPP).
Concept 45.4 Species Diversity Affects Community Function
Community outputs vary with species diversity.
Within a community type, NPP is generally
greater and more stable as species diversity
increases.
A long-term study of prairie plant communities
found that above-ground biomass increased as
species diversity increased.
Figure 45.10 Species and Functional Group Diversity Affect Grassland Productivity (Part 1)
Concept 45.4 Species Diversity Affects Community Function
Possible reasons that species diversity affects
community function:
• Sampling: communities with more species are
more likely to have some with a strong
influence on community output.
• Niche complementarity: communities with more
species may be better able to use all available
resources.
Concept 45.4 Species Diversity Affects Community Function
In the prairie plant experiment, the most speciesrich plots also had the most plant functional
groups:
Plant groups differing in traits such as ability to
grow in warm versus cool seasons,
associations with N-fixing bacteria, allocation to
growth versus reproduction, etc.
Figure 45.10 Species and Functional Group Diversity Affect Grassland Productivity (Part 2)
Concept 45.5 Diversity Patterns Provide Clues to Determinants of
Diversity
Geographic patterns of species richness suggest
factors that affect diversity.
Early explorer–naturalists noticed that species
richness varies with latitude.
Greatest diversity of many plant and animal
groups occurs in the tropics.
Figure 45.11 Species Richness Increases toward the Equator
Concept 45.5 Diversity Patterns Provide Clues to Determinants of
Diversity
Why the tropics support more species is an
unresolved question.
Possibilities:
• Climatic conditions have been stable and not
affected by the glacial cycles that caused
massive shifts in geographic ranges in
temperate regions.
Concept 45.5 Diversity Patterns Provide Clues to Determinants of
Diversity
• Tropics have abundant solar energy and high
productivity. Greater energy flow through
communities could facilitate coexistence of
more species with narrow, specialized niches.
• Environmental heterogeneity: in general,
diversity is higher in more structurally complex
habitats.
Figure 45.12 Structurally Complex Habitats Support Greater Diversity
Concept 45.5 Diversity Patterns Provide Clues to Determinants of
Diversity
Oceanic islands have fewer species than areas
of comparable size on nearby mainlands.
Small islands contain fewer species than large
islands, and isolated islands contain fewer
species than comparable-size islands closer to
a mainland.
These patterns could not be explained by
productivity, habitat heterogeneity, or
disturbance rate.
Figure 45.13 Area and Isolation Influence Species Richness on Islands (Part 1)
Figure 45.13 Area and Isolation Influence Species Richness on Islands (Part 2)
Concept 45.5 Diversity Patterns Provide Clues to Determinants of
Diversity
In 1963, MacArthur and Wilson formulated the
theory of island biogeography:
Equilibrium species richness on islands depends
on relative rates of colonization and extinction.
Small islands—small populations sizes, greater
likelihood of extinctions
More isolated islands (farther from a colonizing
source)—less likely that colonizers will reach it
Figure 45.14 MacArthur and Wilson’s Theory of Island Biogeography
Concept 45.5 Diversity Patterns Provide Clues to Determinants of
Diversity
The theory of island biogeography has been
tested in many natural communities and has
been one of the most successful explanatory
theories in ecology.
Figure 45.15 The Theory of Island Biogeography Can Be Tested (Part 1)
Figure 45.15 The Theory of Island Biogeography Can Be Tested (Part 2)
Concept 45.6 Community Ecology Suggests Strategies
for Conserving Community Function
Ecological communities provide humans with
critical goods and services, which depend on
community diversity.
These ecosystem services include food, clean
water, clean air, fiber, building materials, fuel,
flood control, soil stabilization, pollination, and
climate regulation.
Table 45.2 Some Major Ecosystem Goods and Services
Concept 45.6 Community Ecology Suggests Strategies
for Conserving Community Function
The role of properly functioning communities in
providing ecosystem services is often taken for
granted.
Example: European settlers in Australia brought
cattle. Native dung beetles were adapted to
dry, fibrous dung of marsupials and ignored the
wet dung produced by cattle.
Cattle dung piled up, pastures lost productivity
(no recycling of nutrients) and populations of
flies that lay eggs in dung exploded.
Concept 45.6 Community Ecology Suggests Strategies
for Conserving Community Function
Ecosystem services have economic value.
Example: In the U.S. wild native pollinating
insects contribute $3 billion annually to crop
production.
Some services, such as greenhouse gas
regulation, are more difficult to place a value
on, but are very valuable.
Concept 45.6 Community Ecology Suggests Strategies
for Conserving Community Function
Faced with the need to improve drinking water
supplies, New York City considered a new
water treatment facility that would cost $6–$8
billion to build and $300 million annually to run.
Instead, they invested $1.5 billion in land
protection and better sewage treatment in the
Catskills where the water reservoirs are
located.
Concept 45.6 Community Ecology Suggests Strategies
for Conserving Community Function
Humans are rapidly converting natural
communities into less diverse, humanmanaged communities such as croplands,
pastures, and urban areas.
Large areas of habitat are being fragmented.
The fragments can be seen as habitat “islands”
surrounded by “seas” of human-modified
habitat.
Figure 45.16 Habitat Fragmentation in Tropical Forests
Concept 45.6 Community Ecology Suggests Strategies
for Conserving Community Function
Habitat fragmentation causes loss of species:
• Total amount of habitat decreases, average
patch size decreases, and patches become
more isolated from one another.
• Populations become smaller and more prone to
extinction.
• The human-modified habitat may be a barrier
to dispersal, reducing colonization.
Concept 45.6 Community Ecology Suggests Strategies
for Conserving Community Function
The theory of island biogeography suggests
ways to minimize effects of fragmentation.
Enhance colonization: cluster habitat fragments
together, connect fragments with dispersal
corridors
Reduce extinctions: retain large patches of
original habitat, maintain ability of the
fragments to support healthy populations.
Concept 45.6 Community Ecology Suggests Strategies
for Conserving Community Function
In a large-scale experiment in Brazil, land
owners agreed to preserve forest patches laid
out by biologists.
The patches were surveyed before and after
forest cutting.
Species began to disappear: monkeys that travel
over large areas of forest; army ants and the
birds that follow them.
Small, isolated patches lost species most rapidly.
Figure 45.17 A Large-Scale Study of Habitat Fragmentation
Concept 45.6 Community Ecology Suggests Strategies
for Conserving Community Function
Conservation efforts often target species that
have important roles in community structure
and function.
Example: the wolves in Yellowstone National
Park are critical in maintaining healthy aspen
forests and watersheds via trophic cascades.
The Yellowstone to Yukon Conservation
Initiative aims to maintain a continuous corridor
of wolf habitat between Yellowstone and similar
areas to the north.
Concept 45.6 Community Ecology Suggests Strategies
for Conserving Community Function
Restoration ecology focuses on restoring
function to degraded ecosystems.
One goal is to restore original species diversity,
drawing on our knowledge of the factors that
shape diversity.
Figure 45.18 Species Richness Can Enhance Wetland Restoration (Part 1)
Figure 45.18 Species Richness Can Enhance Wetland Restoration (Part 2)
Concept 45.6 Community Ecology Suggests Strategies
for Conserving Community Function
Disturbance sometimes results in an ecological
transition to a very different community.
It may be very difficult to reverse the transition
and restore the original community.
Answer to Opening Question
Coffee cultivation can be improved by using
principles of community ecology:
• Increase diversity by growing crops together in
functional groups
• Attract wild bee pollinators by planting coffee
close to intact forest patches and leave
flowering weeds in place
Figure 45.19 Traditional Coffee Cultivation and Community Diversity
Answer to Opening Question
Traditional low-intensity cultivation may yield
less per acre, but are profitable because they
avoid costs of chemicals and labor.
It also avoids pollution and helps maintain
natural communities and their species.