An important aspect of this functional hierarchy is the "emergence" of properties that are not easily predictable by simply adding the contributions of constitutive components. Emergent properties include feedback processes at each level of the hierarchy. For example, individual organisms acquire and allocate energy and biochemical resources, affecting resource availability and population structure in ways that change the environment and determine future options for acquisition and allocation of these resources. Regulation of density and resource use emerges at the population level through negative feedback (from declining resource availability and increasing predation at larger population sizes), which functions to prevent overexploitation, or through positive feedback, which prevents extinction. Similarly, species populations acquire and transport resources, but regulation of energy flow and biogeochemical cycling emerge at the ecosystem level. Potential regulation of atmospheric and oceanic pools of carbon and nutrients at the global level reflects integration of biogeochemical cycling and energy fluxes among the Earth's ecosystems.
Information flow and feedback processes are the mechanisms of regulation. Although much research has addressed energy and material flow through food webs, relatively little research has quantified the importance of indirect interactions or information flow. Indirect interactions and feedbacks are common features of ecosystems. For example, herbivores feeding above-ground alter the availability of resources for root-feeding organisms (Gehring and Whitham 1991, 1995, Masters et al. 1993); early-season herbivory can affect plant suitability for later-season herbivores (Harrison and Karban 1986, M. Hunter 1987). Information can be transmitted as volatile compounds that advertise the location and physiological condition of prey, the proximity of potential mates, and the population status of predators. Such information exchange is critical to discovery of suitable hosts, attraction of mates, regulation of population density, and defense against predators by many (if not all) insects.
This ecosystem information network among the members of the community, along with resource supply and demand relationships, provides the basis for regulation of ecosystem processes. Levels of herbivory and predation are sensitive to resource availability. If environmental conditions increase resource abundance at any trophic level, communication to, and response by, the next trophic level provides negative feedback that reduces resource abundance. Negative feedback is a primary mechanism for stabilizing population sizes, species interactions, and process rates in ecosystems. Some interactions provide positive feedback, such as cooperation or mutualism. Although positive feedback is potentially destabilizing, it may reduce the probability of population decline to extinction. The apparent ability of many ecosystems to reduce variation in structure and function suggests that ecosystems are self-regulating (i.e., they behave like cybernetic systems; e.g., E. Odum 1969, Patten and Odum 1981). Insects could be viewed as important mechanisms of regulation because their normally small biomass requires relatively little energy or matter to maintain, and their rapid and dramatic population response to environmental changes constitutes an effective and efficient means for reducing deviation in nominal ecosystem structure and function. This developing concept of ecosystem self-regulation has major implications for ecosystem responses to anthropogenic change in environmental conditions and for our approaches to managing insects and ecosystem resources.
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