THE ECOSYSTEM LEVEL OF ORGANIZATION INTEGRATES species interactions and community structure with their responses to, and effects on, the abiotic environment. Interactions among organisms are the mechanisms governing energy and nutrient fluxes through ecosystems. The rates and spatial patterns in which individual organisms and populations acquire and allocate energy and nutrients determine the rate and direction of these fluxes (see Chapters 4 and 8).

Communities vary in their ability to modify their abiotic environment. The relative abundance of various nutrient resources affects the efficiency with which they are cycled and retained within the ecosystem. Increasing biomass confers increased storage capacity and buffering against changes in resource availability. Community structure also can modify climatic conditions by controlling albedo and hydric fluxes, buffering individuals against changing environmental conditions.

A major issue at the ecosystem level is the extent to which communities are organized to maintain optimal conditions for the persistence of the community. Species interactions and community structures may represent adaptive attributes at the supraorganismal level that stabilize ecosystem properties near optimal levels for the various species. If so, anthropogenic interference with community organization (e.g., species redistribution, pest control, overgrazing, deforestation) may disrupt stabilizing mechanisms and contribute to ecosystem degradation.

Insects affect virtually all ecosystem properties, especially through their effects on vegetation, detritus, and soils. Insects clearly affect primary productivity, hence the capture and flux of energy and nutrients. In fact, insects are the dominant pathway for energy and nutrient flow in many aquatic and terrestrial ecosystems.

Energy Flow Ecosystems

They affect vegetation density and porosity, hence albedo and the penetration of light, wind, and precipitation. They affect accumulation and decomposition of litter and mixing and porosity of soil and litter, thereby affecting soil fertility and water flux. They often determine disturbance frequency, succession, and associated changes in efficiency of ecosystem processes over time. Their small size and rapid and dramatic responses to environmental changes are ideal attributes for regulators of ecosystem processes, through positive and negative feedback mechanisms. Ironically, effects of detritivores (largely ignored by insect ecologists) on decomposition have been addressed by ecosystem ecologists, whereas effects of herbivorous insects (the focus of insect ecologists) on ecosystem processes have been all but ignored by ecosystem ecologists until recently.

Chapter 11 summarizes key aspects of ecosystem structure and function, including energy flow, biogeochemical cycling, and climate modification. Chapters 12-14 cover the variety of ways in which insects affect ecosystem structure and function. The varied effects of herbivores are addressed in Chapter 12. Although not often viewed from an ecosystem perspective, pollination and seed predation affect patterns of plant recruitment and primary production as described in Chapter 13. The important effects of detritivores on organic matter turnover and soil development are the focus of Chapter 14. Finally, the potential roles of these organisms as regulators of ecosystem processes are explored in Chapter 15.

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