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Ecological Communities
Chapter 44 Ecological Communities
Key Concepts
44.1 Communities Contain Species That
Colonize and Persist
44.2 Communities Change over Space and Time
44.3 Community Structure Affects Community
Function
44.4 Diversity Patterns Provide Clues to What
Determines Diversity
44.5 Community Ecology Suggests Strategies
for Conserving Community Function
Chapter 44 Opening Question
Can we use principles of community
ecology to improve methods of coffee
cultivation?
Concept 44.1 Communities Contain Species That Colonize and
Persist
Community: group of species that occur
together in a geographic area
We depend on communities for natural
resources and services.
A community’s species and their interactions
determine how well it functions.
Understanding how communities are put
together and how they work is essential to
conserving them.
Concept 44.1 Communities Contain Species That Colonize and
Persist
Ecologists must make practical decisions on the
boundaries of the community under study.
Boundaries may be based on natural
boundaries (e.g., the edge of a pond).
They may restrict study to certain groups (e.g.,
the bird community) or study a representative
portion of a habitat.
Concept 44.1 Communities Contain Species That Colonize and
Persist
Communities are characterized by species
composition—which species they contain,
the number of species, and the abundance of
each species.
These attributes are components of the
community structure.
Concept 44.1 Communities Contain Species That Colonize and
Persist
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.
Species may fail to colonize a community, or be
lost from it, for many reasons.
Concept 44.1 Communities Contain Species That Colonize and
Persist
Local extinctions can occur for many reasons:
• Inability of species to tolerate local
conditions
• A resource may be lacking
• Exclusion by competitors, predators, or
pathogens
• Population size too small; no reproduction
Concept 44.1 Communities Contain Species That Colonize and
Persist
In 1883 the volcano on Krakatau in Indonesia
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 early pioneer
plant species that require high light levels
disappeared from the island’s now-shady
interior.
Concept 44.1 Communities Contain Species That Colonize and
Persist
Once forests developed, fruit-eating birds and
bats were attracted to the island, bringing new
animal-dispersed seeds with them.
Even today, species composition continues to
change as new species colonize and others
go extinct.
Figure 44.1 Vegetation Recolonized Krakatau
Concept 44.2 Communities Change over Space and Time
Ecologists have noted repeated patterns of
spatial and temporal change, or turnover, in
species composition of communities.
Species composition varies along
environmental gradients, after disturbances,
and with changing climate.
Concept 44.2 Communities Change over Space and Time
Species composition changes along
environmental gradients, such as elevation or
soil types.
• Example: As you go up a tropical mountain,
there are gradients in temperature and
moisture; because different species have
different tolerance limits, there are
constantly changing plant communities.
Concept 44.2 Communities Change over Space and Time
Change in plant species composition was
measured along a transect (a straight line
used for ecological surveys) running from nonserpentine to serpentine soils.
The turnover of plant species along the transect
reflects their tolerance or intolerance to the
heavy metals that characterize serpentine
soils.
Figure 44.2 Change in Species Composition along an Environmental Gradient
Concept 44.2 Communities Change over Space and Time
Many animal species are associated with
particular plant communities:
• Plants they eat may be there
• Plants modify physical conditions,
contributing to habitat structure
Morphological, physiological, and behavioral
traits of animals adapt them for the structure
of the habitats with which they are associated.
Figure 44.3 Many Animals Are Associated with Habitats of a Particular Structure
Concept 44.2 Communities Change over Space and Time
Species composition also changes over time.
All communities are dynamic.
There is ongoing colonization and local
extinction and thus a steady turnover in
species composition.
Dispersal delivers a constant influx of new
individuals to all but the most isolated
locations.
Concept 44.2 Communities Change over Space and Time
Disturbance—events that cause sudden
environmental change can change species
composition
Disturbances include volcanic eruptions,
wildfires, hurricanes, landslides, human
activities.
Some or all species are wiped out, and
environmental conditions are changed.
Concept 44.2 Communities Change over Space and Time
New environments can also appear without
disturbance; for instance, when a glacier melts
away, a depression fills with rainwater, or a
mammal deposits dung.
Species often replace one another in a
predictable sequence called succession.
• Example: A patch of elephant dung is
colonized by a series of dung beetle
species.
Figure 44.4 Dung Beetle Species Composition Changes over Time
Figure 44.4 Dung Beetle Species Composition Changes over Time (Part 1)
Figure 44.4 Dung Beetle Species Composition Changes over Time (Part 2)
Concept 44.2 Communities Change over Space and Time
Factors that result in successional sequences:
• 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 dungproducers.

On Krakatau, the first plants were
species that have seeds that are easily
dispersed by sea or wind.
Concept 44.2 Communities Change over Space and Time
• After a disturbance, environmental
conditions change with time.

Dung starts out wet and dries over time.

As trees grow, the forest canopy closes
and light conditions change.
Concept 44.2 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.
But return of the original community is not
guaranteed.
Concept 44.2 Communities Change over Space and Time
Some disturbances may push the system past a
threshold, or tipping point, causing an
ecological transition to a distinctly different
community.
• Example: conversion of grasslands to
shrublands in the U.S.–Mexico Borderlands
after intensive cattle grazing
Concept 44.2 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.
Concept 44.2 Communities Change over Space and Time
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 44.5 Species Composition Changes as the Climate Changes
Concept 44.3 Community Structure Affects Community Function
An ecological community can be thought of as a
system with inputs, “internal workings,” and
outputs.
Inputs include energy and materials from the
abiotic environment.
Internal workings include the metabolism of its
individuals, dynamics of its populations, and
interactions among species.
Outputs are transformed energy and materials.
Concept 44.3 Community Structure Affects Community Function
Community function is measured by the
amount of energy or matter that moves into
and out of the community per unit of time.
This flux (flow rate) reflects exchange between
the community and the environment.
The outputs affect the species in the community
and organisms in other ecosystems, including
humans. These outputs represent “goods” and
“services.”
Concept 44.3 Community Structure Affects Community Function
The flux of energy through communities:
Energy enters communities through primary
producers.
• 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
Figure 44.6 Energy Flow through Ecological Communities
Concept 44.3 Community Structure Affects Community Function
Change in the biomass (dry mass) of primary
producers per unit of time is an approximation
for NPP.
Concept 44.3 Community Structure Affects Community Function
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 44.3 Community Structure Affects Community Function
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; this
energy is converted to heat and is not
available to the next trophic level
Concept 44.3 Community Structure Affects Community Function
Each species has a unique niche that
determines its function in a community.
The concept of the niche has two meanings:
• The environment where we expect to find
the species, based on its tolerance to the
physical conditions (where the species has
a positive per capita growth rate)
 The biological environment is also
important—presence of predators,
competitors, etc.
Concept 44.3 Community Structure Affects Community Function
• Niche also refers to a species’ functional
role in the community.
 It is largely defined by how it affects
other species—what resources it uses
and makes unavailable to other species,
what it produces that other species can
use, whether it affects the physical
environment.
Concept 44.3 Community Structure Affects Community Function
Ecologists look for broad patterns in the
relationship between community structure and
function.
One aspect of community structure that
influences community function is species
diversity.
Concept 44.3 Community Structure Affects Community Function
Species diversity has two components:
Species richness—the number of species in
the community
Species evenness—the distribution of species’
abundances
A community of four equally abundant species
is more diverse than one in which 75% of the
individuals belong to one species and 25% are
spread among three other species.
Figure 44.7 Species Richness and Species Evenness Contribute to Diversity
Concept 44.3 Community Structure Affects Community Function
The properties of a community with a few
abundant species will be defined mostly by
those species, whereas the properties of a
community with equally abundant species will
reflect the influence of all of them.
Concept 44.3 Community Structure 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 (a measure
of NPP) increased as species diversity
increased.
Figure 44.8 Species Richness and Number of Functional Groups Affect Primary Productivity
(Part 1)
Concept 44.3 Community Structure 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 44.3 Community Structure Affects Community Function
Plant groups differ in traits such as ability to
grow in warm versus cool seasons,
associations with N-fixing bacteria, allocation
to growth versus reproduction, etc.
In the prairie plant experiment, the species-rich
plots with the most functional groups had
higher NPP, suggesting that niche
complementarity was important.
Figure 44.8 Species Richness and Number of Functional Groups Affect Primary Productivity
(Part 2)
Concept 44.4 Diversity Patterns Provide Clues to What
Determines Diversity
Geographic patterns of species diversity shed
light on the factors that affect diversity.
Early naturalists noticed that species richness
varies with latitude.
The greatest diversity of many plant and animal
groups occurs in the tropics.
Figure 44.9 Species RichnessIncreases toward the Equator
Concept 44.4 Diversity Patterns Provide Clues to What
Determines Diversity
Why do the tropics support more species?
• Climatic conditions in the tropics have been
stable over long time periods and were not
affected by the glacial cycles that caused
massive shifts in geographic ranges of
temperate species.
• Absence of disturbance at large spatial
scales may have allowed these
communities to retain more of their species.
Concept 44.4 Diversity Patterns Provide Clues to What
Determines Diversity
• Tropics have abundant solar energy and
high productivity. Greater energy flow
through communities could facilitate
coexistence of more species with narrow,
specialized niches.
• Variation in habitat structure: in general,
diversity is higher in more structurally
complex habitats.
Concept 44.4 Diversity Patterns Provide Clues to What
Determines Diversity
Species richness varies on islands:
Species richness is greater on large than on
small islands, and species richness is greater
on islands near a mainland than on more
distant islands.
These patterns could result from an equilibrium
between the rate at which new species
colonize an island and the rate at which
resident species go extinct.
Figure 44.10 Area and Isolation Influence Species Richness on Islands (Part 1)
Figure 44.10 Area and Isolation Influence Species Richness on Islands (Part 2)
Concept 44.4 Diversity Patterns Provide Clues to What
Determines Diversity
In 1963, MacArthur and Wilson formulated the
theory of island biogeography:
• An island gains species only if they
colonize from elsewhere (e.g., the
mainland).
• Colonization rate declines as the island fills
with species.
• Extinction rate increases as the island fills
with species.
Concept 44.4 Diversity Patterns Provide Clues to What
Determines Diversity
• The number of species reaches an
equilibrium when the colonization rate
equals the extinction rate.
• Population sizes decrease as island size
decreases. Small populations are more at
risk of extinction; thus equilibrium species
richness should be greater on large islands.
• Fewer colonizers find their way to distant
islands; thus equilibrium species richness
will be higher on close islands.
Figure 44.11 MacArthur and Wilson’s Theory of Island Biogeography
Concept 44.4 Diversity Patterns Provide Clues to What
Determines 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 44.12 The Theory of Island Biogeography Can Be Tested (Part 1)
Figure 44.12 The Theory of Island Biogeography Can Be Tested (Part 2)
Concept 44.5 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 44.1
Concept 44.5 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 44.5 Community Ecology Suggests Strategies
for Conserving Community Function
The problem was solved by introducing dung
beetle species from other parts of the world
that could process cattle dung.
The introductions were done carefully to ensure
that native dung beetles were not harmed.
Concept 44.5 Community Ecology Suggests Strategies
for Conserving Community Function
Ecosystem services have economic value.
• Example: In the United States, 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 44.5 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.
Instead, they invested $1.5 billion in land
protection and better sewage treatment in the
Catskills where the water reservoirs are
located.
Concept 44.5 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,
forming habitat “islands” in human-modified
landscapes.
Figure 44.13 Habitat Fragmentation in Tropical Forests
Concept 44.5 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 44.5 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 and connect fragments
with dispersal corridors.
• Reduce extinctions: retain some large
patches of original habitat, maintain ability
of the fragments to support healthy
populations.
Concept 44.5 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 44.14 A Large-Scale Study of Habitat Fragmentation
Concept 44.5 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 44.5 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 44.15 Species Richness Can Enhance Wetland Restoration
Figure 44.15 Species Richness Can Enhance Wetland Restoration (Part 1)
Figure 44.15 Species Richness Can Enhance Wetland Restoration (Part 2)
Figure 44.15 Species Richness Can Enhance Wetland Restoration (Part 3)
Concept 44.5 Community Ecology Suggests Strategies
for Conserving Community Function
Disturbances sometimes result 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—
intercropping—such as planting shadetolerant coffee under taller timber trees.
• Attract wild bee pollinators by planting
coffee close to intact forest patches and
leave flowering weeds in place.
Figure 44.16 Traditional Coffee Cultivation and Community Diversity
Answer to Opening Question
Traditional low-intensity coffee cultivation may
yield less per acre, but it is profitable because
it reduces the costs of chemicals and labor.
It also avoids pollution and helps maintain
natural communities and their species.