The ecosystem is a fundamental unit of ecological organization, although its boundaries are not easily defined. An ecosystem generally is considered to represent the integration of a more or less discrete community of organisms and the abiotic conditions at a site (Fig. 1.3). However, research and environmental policy decisions are recognizing the importance of scale in ecosystem studies (i.e., extending research or extrapolating results to landscape, regional, and even global scales; e.g., Holling 1992, G. Turner 1989). Ecosystems are interconnected, just as the species within them are interconnected. Exports from one ecosystem become imports for others. Energy, water, organic matter, and nutrients from terrestrial ecosystems are major sources of these resources for many aquatic ecosystems. Organic matter and nutrients eroded by wind from arid ecosystems are filtered from the airstream by ecosystems downwind. Some ecosystems within a landscape or watershed are the sources of colonists for recently disturbed ecosystems. Insect outbreaks can spread from one ecosystem to another. Therefore, our perspective of the ecosystem needs to incorporate the concept of interactions among ecosystem types (patches) within the landscape or watershed.
Overlapping gradients in abiotic conditions establish the template that limits options for community development, but established communities can modify abiotic conditions to varying degrees. For example, minimum rates of water and nutrient supply are necessary for establishment of grasslands or forests, but once canopy cover and water and nutrient storage capacity in organic material have developed, the ecosystem is relatively buffered from changes in water and nutri-
ent supply (e.g., E. Odum 1969, Webster et al 1975). Although ecosystems usually are defined on the basis of the dominant vegetation (e.g., tundra, desert, marsh, grassland, forest) or type of water body (stream, pond, lake), characteristic insect assemblages also differ among ecosystems. For example, wood-boring insects (e.g., ambrosia beetles, wood wasps) are characteristic of communities in wooded ecosystems (shrub and forest ecosystems) but clearly could not survive in ecosystems lacking woody resources.
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