44.1 Communities Contain Species That · •Example: On Krakatau, tropical forests eventually came back. But return of the original community is not guaranteed. Concept 44.2 Communities

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.


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.


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.


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.


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


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.


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.


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.


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


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


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.


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.


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)


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.


• 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.


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.


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


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 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.


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.”


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


Change in the biomass (dry mass) of primary

producers per unit of time is an approximation

for NPP.


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.


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


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.


• 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.


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.


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


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.


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)


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


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


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



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.



• 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.



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)



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.



• 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



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


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



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.



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.



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.



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.



Humans are rapidly converting natural

communities into less diverse, human-

managed 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



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.



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.



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



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.



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)





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.

Documents

44.1 Communities Contain Species That · •Example: On Krakatau, tropical forests eventually came back. But return of the original community is not guaranteed. Concept 44.2 Communities