Ecological Communities
44
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 non-
serpentine 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 transforme
