Ecosystems are complex systems with structure, represented by abiotic resources and a diverse assemblage of component species and their products (such as organic detritus and tunnels) and function, represented by fluxes of energy and matter among biotic and abiotic components (see Fig. 1.3). This complexity extends to the spatial delineation of an ecosystem. Ecosystems can be identified at microcosm and mesocosm scales (e.g., decomposing logs or treehole pools), patch scale (area encompassing a particular community on the landscape), landscape scale (the mosaic of patch types representing different edaphic conditions or successional stages that compose a broader ecosystem type), and the regional or biome scale.
Addressing taxonomic, temporal, and spatial complexity has proved to be a daunting challenge to ecologists, who must decide how much complexity can be ignored safely (Gutierrez 1996, Polis 1991a, b). Evolutionary and ecosystem ecol-ogists have taken contrasting approaches to dealing with this complexity in ecological studies. The evolutionary approach emphasizes adaptive aspects of life histories, population dynamics, and species interactions. This approach restricts complexity to interactions among one or a few species and their hosts, competitors, predators, or other biotic and abiotic environmental factors and often ignores the complex feedbacks at the ecosystem level. In contrast, the ecosystem approach emphasizes rates and directions of energy and matter fluxes. This approach restricts complexity to fluxes among functional groups and often ignores the contributions of individual species. Either approach, by itself, limits our ability to understand feedbacks among individual, population, community, and ecosystem parameters and to predict effects of a changing global environment on these feedbacks.
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