Principles of Life
Hillis • Sadava • Heller • Price
Chapter 45 provides an overview of the principles of community ecology. There is an emphasis on species composition being a result of colonization and extinction events. Community composition along environmental gradients, succession following disturbance, and response to climate change are discussed. Trophic interactions, energy flow in communities, and the concept of trophic cascades are described. The importance of species diversity to community function is discussed, including how it impacts ecosystems services. Patterns of species diversity are described, as well as the theory of island biogeography. The chapter ends with examples of how knowledge of community ecology is used to maintain and restore diversity and community function.
45.1 Communities Contain Species That Colonize and Persist
Communities are assemblages of species that coexist and interact with one another within a defined area. A community contains those species that have colonized minus those that have gone locally extinct.
45.2 Communities Change over Space and Time
• Species composition varies along environmental gradients
•
Several processes cause communities to change over time
Species composition changes along environmental gradients, such as elevation or soil types. In any community there is ongoing colonization and extinction, and thus a steady turnover in species composition. Disturbance events wipe out some or all species, and as species recolonize, they replace one another in a predictable sequence called succession. The result is often a community that is similar to the original one; or there can be an ecological transition to a different community. As climate and physical conditions change over time, community composition also changes.
45.3 Trophic Interactions Determine How Energy and Materials Move through Communities
• Consumer–resource interactions determine an important property of communities
•
Energy is lost as it moves through a food web
•
Trophic interactions can change the species composition of communities
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Each species has a niche—defined by its environmental tolerances, how it obtains energy, how it interacts with other species, etc. Trophic interactions cause energy and materials to flow through a community. Autotrophs are the primary producers that convert solar energy into chemical energy. Heterotrophs get food from organic molecules made by others. Decomposers are heterotrophs that feed on waste products or dead bodies of organisms and are important in recycling materials.
Ecological efficiency is about 10% from one trophic level to the next. Species composition can be influenced by key species; removal of the species causes a trophic cascade of effects on other species.
45.4 Species Diversity Affects Community Function
•
The number of species and their relative abundances contribute to species diversity
• Species diversity affects community processes and outputs
Species diversity includes richness (number of species) and evenness (distribution of abundance), both of which can influence community function. Communities with high species diversity tend to have higher net primary productivity and be more stable.
45.5 Diversity Patterns Provide Clues to Determinants of Diversity
•
Species richness varies with latitude
•
Diversity represents a balance between colonization and extinction
Geographic patterns of species diversity have long been noted and studied. Species richness varies with latitude; the greatest diversity in most groups is in the tropics.
The reasons for this are an area of active research. On oceanic islands, diversity is lower than mainland areas, and small, isolated islands have fewer species than larger islands. These patterns are explained by the theory of island biogeography, which predicts the equilibrium number of species based on colonization and extinction rates. The theory has been tested in many experiments and field studies.
45.6 Community Ecology Suggests Strategies for Conserving
Community Function
•
Ecological communities provide humans with goods and services
•
Ecosystem services have economic value
•
Island biogeography suggests strategies for maintaining community diversity
•
Trophic cascades suggest the importance of conserving critical components of food webs
•
The relationship of diversity to community function suggests strategies for restoring degraded habitats
Ecological communities provide humans with many essential ecosystem services, which have often been taken for granted. But the economic value of these services can sometimes be calculated (e.g., for crop pollination or for natural water purification systems). Diversity is critical for maintaining these ecosystem functions, and conservation research seeks ways to preserve diverse communities. Habitat fragmentation results in species loss by creating habitat “islands” surrounded by
“seas” of human-modified landscape. Island biogeography predicts loss of species from these “islands,” and also suggests ways to reduce species losses, such as
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dispersal corridors. Conservation efforts may target a species with large influence on community structure, such as the wolves in Yellowstone. The field of restoration ecology uses principles of community to restore function to degraded ecosystems.
Chapter 45 Opening Question
Can we use principles of community ecology to improve methods of coffee cultivation?
45.1 Communities Contain Species That Colonize and Persist
Community
—a group of species that coexist and interact with one another within a defined area
Biologists may designate community boundaries based on natural boundaries (e.g., the edge of a pond) or arbitrarily.
They may restrict study to certain groups (e.g., the bird community).
Communities are characterized by species composition ; that is, which species they contain and the relative abundances of those species.
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.
Local extinctions can occur for many reasons:
•
Species unable 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 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 pioneering plant species that require high light levels disappeared from the island’s now-shady interior.
Species composition continues to change as new species colonize and others go extinct.
FIGURE 45.1 Vegetation Recolonized Krakatau
45.2 Communities Change over Space and Time
Ecologists have documented recurring patterns of species compositional change.
Species composition varies along environmental gradients, after disturbances, and with changing climate.
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Species composition changes along environmental gradients, such as elevation or soil types.
A transect is a straight line used for ecological surveys.
A transect along an environmental gradient will show the species turnover through space.
FIGURE 45.2 Species Turnover along an Environmental Gradient
Many animal species are associated with particular plant communities:
The plants they eat may be there; or because plants modify physical conditions and contribute to habitat structure.
Habitat structure determines the animal’s ability to get food or avoid predators.
FIGURE 45.3 Many Animals Associate with Habitats of a Particular Structure
In any community there is ongoing colonization and extinction, and thus a steady turnover in species composition.
Species turnover can result from disturbance events: volcanic eruptions, wildfires, hurricanes, landslides, human activities.
Some or all the species are wiped out, and environmental conditions are changed.
(ANIMATED TUTORIAL 45.1 Succession on a Glacial Moraine)
Species often replace one another in a predictable sequence called succession .
Example: A patch of elephant dung is colonized by a sequence of dung beetle species.
(VIDEO 45.1 A redwood forest and succession in a forest gap)
FIGURE 45.4 Dung Beetle Species Composition Changes over Time
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.
Examples:
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.
If the original community is not reestablished, there is an ecological transition to a different community.
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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 45.5 Species Composition Changes with Climate Change
45.3 Trophic Interactions Determine How Energy and Materials Move through Communities
Each species in a community has a unique niche .
This concept refers to the environmental tolerances of a species, which define where it can live.
Also refers to the ways a species obtains energy and materials and to patterns of interaction with other species in the community.
Consumer–resource, or trophic interactions cause energy and materials to flow through a community.
Trophic levels
—feeding positions
Primary producers , or autotrophs , convert solar energy into a form that can be used by the rest of the community.
Heterotrophs get energy by breaking apart organic compounds that were assembled by other organisms.
Primary consumers (herbivores) eat primary producers.
Secondary consumers (carnivores) eat herbivores.
Tertiary consumers eat secondary consumers.
Omnivores feed from multiple trophic levels.
Decomposers , or detritivores , feed on waste products or dead bodies of organisms.
Decomposers are responsible for recycling of materials; they break down organic matter into inorganic components that primary producers can absorb.
TABLE 45.1 The Major Trophic Levels
Trophic interactions are shown in diagrams called food webs .
Arrows indicate the flow of energy and materials—who eats whom.
FIGURE 45.6 A Food Web in the Yellowstone Grasslands
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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.
Change in biomass of primary producers (dry mass) 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.
FIGURE 45.7 Energy Flow through Ecological Communities
The per capita growth rate of a species is related to the sum of positive and negative contributions of species with which it interacts.
Succession can be driven by such interactions.
Examples: Late-colonizing dung beetles inhibit early colonizers by competing for nutrients.
Late-arriving plant species shade out pioneering species.
Consumer–resource interactions can have ripple effects across trophic levels, resulting in a trophic cascade .
In Yellowstone National Park, wolves were extirpated by hunting by 1926.
Elk were culled each year to prevent them from exceeding carrying capacity, until
1968. Elk population then rapidly increased.
The elk browsed aspen trees so heavily that no young aspens could get a start.
Elk also browsed streamside willows to the point that beavers (who depend on willows for food) were nearly exterminated.
Wolves were reintroduced in 1995 and preyed primarily on elk.
Aspen and willows grew again, and the beaver population increased.
FIGURE 45.8 Removing Wolves Initiated a Trophic Cascade
45.4 Species Diversity Affects Community Function
Species diversity has two components:
Species richness
—the number of species in the community.
Species evenness —the distribution of species’ abundances
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FIGURE 45.9 Species Richness and Species Evenness Contribute to Diversity
Both aspects of diversity affect community function .
A species’ influence in a community depends on its interactions, and also its abundance.
Communities with a few very abundant species are largely defined by them, rather than by the many rare ones.
Communities can be thought of as systems with inputs and outputs.
Important measures of community function are the total flow of energy into the community (GPP), and net energy available for consumption by heterotrophs
(NPP).
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 increased as species diversity increased.
FIGURE 45.10 Species and Functional Group Diversity Affect Grassland
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.
In the prairie plant experiment, the most species-rich plots also had the most plant functional groups :
Plant groups differing in traits such as ability to grow in warm versus cool seasons, associations with N-fixing bacteria, allocation to growth versus reproduction, etc.
FIGURE 45.10 Species and Functional Group Diversity Affect Grassland
Productivity (Part 2)
45.5 Diversity Patterns Provide Clues to Determinants of Diversity
Geographic patterns of species richness suggest factors that affect diversity.
Early explorer–naturalists noticed that species richness varies with latitude.
Greatest diversity of many plant and animal groups occurs in the tropics.
(VIDEO 45.2 Portrait of a rainforest ecosystem: Amazon of South America and other locations)
FIGURE 45.11 Species Richness Increases toward the Equator
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Why the tropics support more species is an unresolved question.
Possibilities:
•
Climatic conditions have been stable and not affected by the glacial cycles that caused massive shifts in geographic ranges in temperate regions.
• Tropics have abundant solar energy and high productivity. Greater energy flow through communities could facilitate coexistence of more species with narrow, specialized niches.
• Environmental heterogeneity: in general, diversity is higher in more structurally complex habitats.
(LINK Concepts 44.2 and 44.3 Some ways in which differences in resource use evolve and promote coexistence)
FIGURE 45.12 Structurally Complex Habitats Support Greater Diversity
Oceanic islands have fewer species than areas of comparable size on nearby mainlands.
Small islands contain fewer species than large islands, and isolated islands contain fewer species than comparable-size islands closer to a mainland.
These patterns could not be explained by productivity, habitat heterogeneity, or disturbance rate.
FIGURE 45.13 Area and Isolation Influence Species Richness on Islands
In 1963, MacArthur and Wilson formulated the theory of island biogeography :
Equilibrium species richness on islands depends on relative rates of colonization and extinction.
Small islands—small populations sizes, greater likelihood of extinctions
More isolated islands (farther from a colonizing source)—less likely that colonizers will reach it
(ANIMATED TUTORIAL 45.2 Biogeography Simulation)
FIGURE 45.14 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.
(APPLY THE CONCEPT Diversity patterns provide clues to determinants of diversity)
FIGURE 45.15 The Theory of Island Biogeography Can Be Tested
45.6 Community Ecology Suggests Strategies for Conserving
Community Function
Ecological communities provide humans with critical goods and services, which depend on community diversity.
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These ecosystem services include food, clean water, clean air, fiber, building materials, fuel, flood control, soil stabilization, pollination, and climate regulation.
TABLE 45.2 Some Major Ecosystem Goods and Services
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.
Ecosystem services have economic value.
Example: In the U.S. 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 and $300 million annually to run.
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, humanmanaged communities such as croplands, pastures, and urban areas.
Large areas of habitat are being fragmented. The fragments can be seen as habitat
“islands” surrounded by “seas” of human-modified habitat.
FIGURE 45.16 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.
(ANIMATED TUTORIAL 45.3 Edge Effects)
The theory of island biogeography suggests ways to minimize effects of fragmentation.
Enhance colonization: cluster habitat fragments together, connect fragments with dispersal corridors
Reduce extinctions: retain large patches of original habitat, maintain ability of the fragments to support healthy populations.
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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 45.17 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 45.18 Species Richness Can Enhance Wetland Restoration
Disturbance sometimes results 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
•
Attract wild bee pollinators by planting coffee close to intact forest patches and leave flowering weeds in place
FIGURE 45.19 Traditional Coffee Cultivation and Community Diversity
Answer to Opening Question
Traditional low-intensity cultivation may yield less per acre, but are profitable because they avoid costs of chemicals and labor.
It also avoids pollution and helps maintain natural communities and their species.
autotrophs community decomposers detritivores ecological efficiency ecological transition ecosystem services
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food web gross primary productivity (GPP) heterotrophs island biogeography net primary productivity (NPP) niche omnivores primary consumers primary producers secondary consumers species composition species evenness species richness succession tertiary consumers trophic trophic cascade trophic levels
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