Insect ecology addresses an astounding variety of interactions between insects and their environment. However, key aspects of insect ecology involve feedback between insect responses to changes in environmental conditions, especially resource supply, and their capacity to modify, and potentially stabilize, energy and nutrient fluxes. As shown throughout this text, each level of hierarchical organization can be described in terms of characteristic structure, function, and feedback regulation. Feedback integration among hierarchical levels occurs primarily through responses to, and modification of, variation in environmental conditions (see Fig. 1.2). Insect behavioral and physiological attributes that affect their interactions with the environment are under genetic control. Evolution represents feedback on individual attributes that affect higher levels of organization.
The importance of environmental change and disturbance as a central theme in insect ecology has been recognized only recently. Disturbance, in particular, provides a context for understanding and predicting individual adaptations, population strategies, organization and succession of community types, and rates and regulation of ecosystem processes. Environmental changes or disturbances kill individuals or affect their activity and reproduction. Some populations are reduced to local extinction, but others exploit the altered conditions. Population strategies and interactions with other species also affect ecosystem properties in ways that increase the probability of disturbance (or other changes) or that mitigate environmental changes and favor persistence of species less tolerant to change. Insects contribute greatly to feedback between ecosystem properties and environmental variation. This aspect of insect ecology has important consequences for ecosystem responses to global changes resulting from anthropogenic activities.
Energy and biogeochemical fluxes integrate individuals, populations, and communities with their abiotic environment. Energy flow and biogeochemical cycling processes determine rates and spatial patterns of resource availability. Many, perhaps most, species attributes can be shown to represent tradeoffs between maximizing resource acquisition and optimizing resource allocation among metabolic pathways (e.g., foraging activity, defensive strategies, growth, and reproduction). The patterns of energy and nutrient acquisition and allocation by individuals determine the patterns of storage and fluxes among populations; fluxes among species at the community level; and storage and flux at the ecosystem level that, in turn, determine resource availability for individuals, populations, and communities. Resource availability is fundamental to ecosystem productivity and diversity. Resource limitation, including reduced availability resulting from inhibition of water and nutrient fluxes, is a key factor affecting species interactions. Herbivore and predator populations grow when increasing numbers of hosts or prey are available or incapable of escape or defense because of insufficient resource acquisition or poor food quality.
Regulatory mechanisms emerge at all levels of the ecological hierarchy. Negative feedback and reciprocal cooperation are apparent at population, community, and ecosystem levels. Cooperation benefits individuals by improving ability to acquire limiting resources. This positive feedback balances the negative feedbacks that limit population density, growth, and ecological processes. At the population level, positive and negative feedbacks maintain density within narrower ranges than occur when populations are released from regulatory mechanisms. The responsiveness of insect herbivores to changes in plant density and condition, especially resulting from crop management, introduction into new habitats, and land use, bring some species into conflict with human interests. However, insect outbreaks in natural ecosystems appear to be restricted in time and space and function to (1) maintain net primary production (NPP) within relatively narrow ranges imposed by the carrying capacity of the ecosystem and (2) facilitate replacement of plant species that are poorly adapted to current conditions by species that are better adapted to these conditions. Regulatory capacity appears to reflect selection for recognition of cues that signal changes in host density or condition that affect long-term carrying capacity of the ecosystem.
The issue of ecosystem self-regulation is a key concept that significantly broadens the scope of insect ecology. Although this idea remains controversial, accumulating evidence supports a view that insect outbreaks function to reduce long-term deviation in NPP, at least in some ecosystems. Although outbreaks appear to increase short-term variation in some ecosystem parameters, reversal of unsustainable increases in NPP could reduce long-term variation in ecosystem conditions.
Models of group selection predict that stabilizing interactions are most likely in ecosystems where pairs of organisms interact consistently. Hence, selection for stabilizing interactions might be least likely in ecosystems where such interactions are inconsistent, such as in harsh or frequently disturbed environments. However, selection for stabilizing interactions also might be less direct in productive, highly diverse ecosystems with little variation in abiotic conditions or resource availability, such as tropical rainforest ecosystems. Stabilizing interactions are most likely in ecosystems where selection would favor interactions that reduce moderate levels of variation in abiotic conditions or resource availability.
Insects play key roles in regulation of primary and secondary production. Their large numbers, rapid reproduction, and mobility may maximize their interactions with other organisms and the rate at which they evolve reciprocal cooperation.
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