Populations of insects can fluctuate dramatically through time, with varying effect on community and ecosystem patterns and processes, as well as on the degree of crowding among members of the population. The amplitude and frequency of fluctuations distinguish irruptive populations, cyclic populations, and stable populations. Cyclic populations have stimulated the greatest interest among ecolo-gists. The various hypotheses to explain cyclic patterns of population fluctuation all include density-dependent regulation with a time lag that generates regular oscillations.
Disturbances are particularly important to population dynamics, triggering outbreaks of some species and locally exterminating others. Disturbances can affect insect populations directly by killing intolerant individuals or indirectly by affecting abundance and suitability of resources or abundance and activity of predators, parasites, and pathogens. The extent to which anthropogenic changes in environmental conditions affect insect populations depends on the degree of similarity between conditions produced by natural versus anthropogenic changes.
Population growth can be regulated (stabilized) to a large extent by density-dependent factors whose probability of effect on individuals increases as density increases and declines as density declines. Primary density-dependent factors are intraspecific competition and predation. Increasing competition for food (and other) resources as density increases leads to reduced natality and increased mortality and dispersal, eventually reducing density. Similarly, predation increases as prey density increases. Although the relative importance of these two factors has been debated extensively, both clearly are critical to population regulation. Regulation by bottom-up factors (resource limitation) may be relatively more important in systems where resources are defended or vary significantly in quality, whereas regulation by top-down factors (predation) may be more important where resources are relatively abundant and show little variation in quality. Inverse density dependence results from cooperation among individuals and represents a potentially destabilizing property of populations. However, this positive feedback may prevent population decline below an extinction threshold. Populations declining below their extinction threshold may be doomed to local extinction, whereas populations increasing above a critical number of individuals (release threshold) continue to increase during an outbreak period. These thresholds represent the minimum and maximum population sizes for species targeted for special management.
Development of population dynamics models has been particularly important for forecasting changes in insect abundance and effects on crop, range, and forest resources. General models include the logistic (Verhulst-Pearl) equation that incorporates initial population size; per capita natality, mortality, and dispersal (instantaneous rate of population change); and environmental carrying capacity. The logistic equation describes a sigmoid curve that reaches an asymptote at carrying capacity. This general model can be modified for particular species by adding parameters to account for nonlinear density-dependent factors, time lags, cooperation, extinction, competition, predation, etc. Models are necessarily simplifications of real systems and may represent effects of multiple interacting factors and chaotic processes poorly. Few models have been adequately validated and fewer have evaluated the effects of input quality on accuracy of predictions. Few population models have been developed to predict effects of insect population dynamics for ecosystem processes other than commodity production. Nevertheless, models represent powerful tools for synthesizing information, identifying priorities for future research, and simulating population responses to future environmental conditions.
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