Community Ecology

SPECIES CO-OCCURRING AT A SITE INTERACT TO VARIOUS degrees, both directly and indirectly, in ways that have intrigued ecologists since earliest times. These interactions represent mechanisms that control population dynamics, hence community structure, and also control rates of energy and tgp matter fluxes, hence ecosystem function. Some organisms engage ^gt' in close, direct interactions, as consumers and their hosts, whereas others interact more loosely and indirectly. For example, predation on mimics depends on the presence of their models, and herbivores are affected by their host's chemical or other responses to other herbivores. Direct interactions (i.e., competition, predation, and symbioses) have been the focus of research on factors controlling community structure and dynamics, but indirect interactions also control community organization. Species interactions are the focus of Chapter 8.

A community is composed of the plant, animal, and microbial species occupying a site. Some of these organisms are integral and characteristic components of the community and help define the community type, whereas others occur by chance as a result of movement across a landscape or through a watershed. For example, certain combinations of species (e.g., ruderal, competitive, or stress-tolerant) distinguish desert, grassland, or forest communities. Different species assemblages are found in turbulent water (stream) versus standing water (lake) or eutrophic versus oligotrophic systems. The number of species and their relative abundances define species diversity, a community attribute that is the focus of a number of ecological issues. Chapter 9 addresses the various approaches to describing community structure and factors determining geographic patterns of community structure.

Communities change through time as populations respond differently to changing environmental conditions, especially to disturbances. Just as population dynamics reflect the net effects of individual natality, mortality, and dispersal interacting with the environment, community dynamics reflect the net effects of species population dynamics interacting with the environment. Severe disturbance or environmental changes can lead to drastic changes in community structure. Changes in community structure through time are the subject of a vast literature summarized in Chapter 10.

Community structure largely determines the biotic environment affecting individuals (Section I) and populations (Section II). The community modifies the environmental conditions of a site. Vegetation cover reduces albedo (reflectance of solar energy), reduces soil erosion, modifies temperature and humidity within the boundary layer, and alters energy and biogeochemical fluxes, compared to nonvegetated sites. Species interactions, including those involving insects, modify vegetation cover and affect these processes, as discussed in Section IV. Different community structures affect these processes in different ways.

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