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Ecological Communities
Chapter 44 Hillis
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
dung-producers.
• 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 speciesrich 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 shade-tolerant 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.