The hypothesis that phytophagous insects regulate ecosystem processes is one of the most important and controversial concepts to emerge from research on insect ecology. The extent to which ecosystems are random assemblages of species that simply affect ecosystem processes or are tightly co-evolved groups of species that stabilize ecosystem function has important implications for management of ecosystem resources and "pests." Although this hypothesis is not contingent on natural selection at the supraorganismal level, concepts of group selection have developed from and contributed to this hypothesis.
Debate on the issue of group selection has solidified consensus on the dominance of direct selection for individual attributes. However, individual attributes affect other organisms and environmental conditions and generate feedback on individual fitness. Such feedback selection contributes to the inclusive fitness of an individual. The intensity of this feedback is proportional to the relatedness of interacting individuals. The greatest feedback selection is between near kin (kin selection). The frequency of interaction and the intensity of feedback selection declines as interacting individuals become less related. However, frequent interspecific interaction can lead to negative feedback (e.g., competition and predation) and reciprocal cooperation (mutualism), based on the tradeoff between gain or loss to each individual from such interaction.
Homeostasis at supraorganismal levels depends only in part on selection for attributes that benefit assemblages of organisms (i.e., group selection). The critical issue is the tradeoff required to balance individual sacrifice, if any, and inclusive fitness accruing from traits that benefit the group. Stabilization of environmental conditions through species interactions favor survival and reproduction of the constituent individuals. Therefore, feedback selection over evolutionary time scales should select for species interactions that contribute to ecosystem stability and mutually assured survival.
Major challenges for ecologists include defining stability (i.e., which ecosystem properties are stabilized, what range of deviation is tolerated, and what temporal and spatial scales are appropriate levels for measurement of stability) and evaluating the effect of mechanisms, such as biodiversity and herbivory, that contribute to stability. Traditionally, stability has been viewed as constancy or recovery of species composition over narrow ranges of time and space. Alternative views include reliability of NPP and biomass structure, which affect the stability of internal climate and soil conditions, and biogeochemical pools and fluxes over larger ranges of time and space. Stability may be achieved, not at the patch scale, but at the landscape scale where conditional stability is achieved through relatively constant proportions of various ecosystem types.
The relationship of stability to diversity has been a major topic of debate. Some species are known to control ecosystem properties, and their loss or gain can severely affect ecosystem structure or function. Furthermore, effects of different species often are complementary, such that diverse assemblages should be better buffered against changes in ecosystem properties in heterogenous environments. A few experimental manipulations of plant species diversity have shown that more diverse communities can have lower variability in primary production than do less diverse communities.
Phytophagous insects have been identified as potentially important regulators of primary production, hence of ecosystem properties determined by primary production. Phytophagous insects possess the key criteria of cybernetic regulators (i.e., small biomass, rapid amplification of effect at the ecosystem level, sensitivity to airborne or waterborne cues indicating ecosystem conditions, and stabilizing feedback on primary production and other processes). Low intensity of herbivory, under conditions of low densities or optimal condition of hosts, tends to stimulate primary production, whereas higher intensities, under conditions of high density or stressed condition of hosts, tend to reduce primary production. Clearly, this aspect of insect ecology has significant implications for our approaches to managing ecosystem resources and "pests."
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