Ecological Communities Chapter 44 Hillis. Chapter 44 Ecological Communities Key Concepts 44.1 Communities Contain Species That Colonize and Persist 44.2

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.


• 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.


• 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.


• 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.


• 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


• 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.


• 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 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


• 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 Strategiesfor 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


• 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

Ecological Communities Chapter 44 Hillis. Chapter 44 Ecological Communities Key Concepts 44.1 Communities Contain Species That Colonize and Persist 44.2