Concept 45.1 Communities Contain Species That Colonize and Persist

• A community is a group of species that coexist and interact with one another within a defined geographic area.
   
• A community is characterized by its species composition: the particular mix of species it contains and the abundances of those species.
   
• A community contains those species that are able to colonize an area and persist there.
   

Concept 45.2 Communities Change over Space and Time

• Variation in space in environmental conditions and habitat structure is associated with predictable turnover in species composition. Review Figure 45.2
   
• Even in a constant environment, ongoing extinction and colonization result in a steady turnover in the species composition of communities through time.
   
• After a sudden disturbance, species sometimes replace one another in a more or less predictable sequence called succession. Succession following a disturbance that removes the original community often results in a community that resembles the original one. Review Figure 45.4 and ANIMATED TUTORIAL 45.1
   
• Some types of disturbance lead to an ecological transition, in which an original community is replaced by a different type of community.
   
• Climate change can cause the species composition of a community to change over time. Review Figure 45.5
   

Concept 45.3 Trophic Interactions Determine How Energy and Materials Move through Communities

• Each species has a unique niche—a set of environmental tolerances and pattern of interactions with other species.
   
• The trophic interactions by which a species obtains energy are a critical aspect of its niche. These interactions determine how energy and materials flow through communities.
   
• Energy enters communities through primary producers, which use photosynthesis to synthesize organic compounds that can be used by the rest of the community.
   
• Species that feed on primary producers are called primary consumers, or herbivores. Those that consume primary consumers are secondary consumers (or primary carnivores), and so on. These feeding positions are called trophic levels. Review Table 45.1 and WEB ACTIVITY 45.1
   
• Trophic interactions can be described in a diagram called a food web. Review Figure 45.6
   
• The total amount of energy that primary producers capture and convert into chemical energy during some period of time is called gross primary productivity (GPP). The portion of GPP that becomes available to consumers during that time is called net primary productivity (NPP). Review WEB ACTIVITY 45.2
   
• The ecological efficiency of energy transfer from one trophic level to the next is only about 10 percent. Review Figure 45.7
   
• Changes in species composition or species densities at any trophic level may cause a trophic cascade of changes at other trophic levels. Review Figure 45.8
   

Concept 45.4 Species Diversity Affects Community Function

• Species diversity has two primary components: the number of species in the community, called species richness, and the distribution of those species' abundances, called species evenness. Review Figure 45.9
   
• Ecologists often quantify diversity by using a mathematical diversity index that incorporates both species richness and species evenness.
   
• The outputs of the interactions between species in a community and between species and their physical environment are measures of community function. Community outputs increase with increasing species diversity of the community. For example, NPP is greater, and more stable over time, in diverse communities. Review Figure 45.10
   

Concept 45.5 Diversity Patterns Provide Clues to Determinants of Diversity

• Species richness is greatest in the tropics and decreases with increasing latitude. Possible causes of tropical species diversity include greater tropical climate stability, greater energy input, and greater habitat complexity. Review Figures 45.11 and 45.12
   
• Species richness is greater on large islands than on small islands and greater on islands closer to a source of colonists than on more distant islands. Review Figure 45.13
   
• The theory of island biogeography proposes that community diversity on islands represents a balance between the rate at which new species colonize and the rate at which resident species go extinct. Review Figures 45.14, 45.15, ANIMATED TUTORIAL 45.2, and WORKING WITH DATA 45.1
   

Concept 45.6 Community Ecology Suggests Strategies for Conserving Community Function

• Ecological systems provide humans with a variety goods and services. These ecosystem services have economic value. Review Table 45.2
   
• Production of ecosystem services is threatened by human activities, including the fragmentation of natural habitats. The detrimental effects of habitat fragmentation can be mitigated by land-use management that minimizes extinction in each habitat “island” and maximizes dispersal among them. Review Figure 45.17 and ANIMATED TUTORIAL 45.3
   
• Ecosystem function often can be preserved or restored by focusing on particular species that play especially important roles in the community, and by maintaining or restoring overall species diversity. Review Figure 45.18