Environmental Change And Disturbance

Environmental changes across temporal and spatial gradients are critical components of an ecosystem approach to insect ecology. Insects are highly responsive to environmental changes, including those resulting from anthropogenic activity. Many insects have considerable capacity for long distance dispersal, enabling them to find and colonize isolated resources as these appear. Other insects are flightless and vulnerable to environmental change or habitat fragmentation. Because of their small size, short life spans, and high reproductive rates, abun dances of many species can change several orders of magnitude on a seasonal or annual time scale, minimizing time lags between environmental changes and population adjustment to new conditions. Such changes are easily detectable and make insects more useful as indicators of environmental changes than are larger or longer-lived organisms. In turn, insect responses to environmental change can affect ecosystem patterns and processes dramatically. Some phytophagous species are well-known for their ability, at high population levels, to reduce host plant density and productivity greatly over large areas. Effects of other species may be more subtle but equally significant from the standpoint of long-term ecosystem structure and function.

Environmental change operates on a continuum of spatial and temporal scales. Although strict definitions of environmental change and disturbance have proved problematic, environmental change generally occurs over a longer term, whereas disturbances are short-term events (Walker and Willig 1999, P. White and Pickett 1985). Chronic changes in temperature or precipitation patterns, such as following the last glaciation, occur on a scale of 103-105 years and may be barely detectable on human time scales. Long-term changes may be difficult to distinguish from cycles operating over decades or centuries, leading to disagreements over whether measured changes represent a fluctuation or a long-term trend. Acute events, such as fires or storms, are more recognizable as disturbances that have dramatic effects on time scales of seconds to hours. However, the duration at which a severe drought, for example, is considered a climate change, rather than a disturbance, has not been determined. The combination of climate and geologic patterns, disturbances, and environmental changes creates a constantly shifting landscape mosaic of various habitat and resource patches that determine where and how insects and other organisms find suitable conditions and resources.

Insect outbreaks traditionally have been viewed as disturbances (P. White and Pickett 1985,Walker and Willig 1999). P.White and Pickett (1985) proposed that disturbance be defined as any relatively discrete event in time that causes measurable change in population, community, or ecosystem structure or function. This definition clearly incorporates insect outbreaks. Similarly, human activities have become increasingly prominent agents of disturbance and environmental change.

Insect outbreaks are comparable to physical disturbances in terms of severity, frequency, and scale. Insects can defoliate or kill most host plants over large areas, up to 103-106ha (e.g., Furniss and Carolin 1977). For example, 39% of a montane forest landscape in Colorado has been affected by insect outbreaks (spruce beetle, Dendroctonus rufipennis) since about 1633, compared to 59% by fire and 9% by snow avalanches (Veblen et al. 1994), with an average return interval of 117 years, compared to 202 years for fire. Frequent, especially cyclic, outbreaks of herbivorous insects probably have been important in selection for plant defenses.

However, unlike abiotic disturbances, insect outbreaks are biotic responses to a change in environmental conditions. Recent outbreaks most commonly reflect anthropogenic redistribution of resources, especially increased density of commercially valuable (often exotic) plant species. Outbreaks usually develop in dense patches of host plants and function to reduce host density, increase vegetation diversity, and increase water and nutrient availability (Schowalter et al. 1986). Management responses to insect outbreaks often are more damaging to ecosystem conditions than is the insect outbreak. For example, insecticides, such as arsenicals and chlorinated hydrocarbons, had long-term, nonselective effects on nontarget organisms. Removing dead or dying host plants, and even living plants, in advance of insect colonization has caused serious soil disturbance and erosion, as well as change in community structure. Principles of integrated pest management (IPM) improved approaches to managing insects by emphasizing adherence to ecological principles. Consideration of insects as integral components of potentially self-maintaining ecosystems could further improve our management of insects and ecosystem resources, within the context of global change.

Currently, human alteration of Earth's ecosystems is substantial and accelerating (J. Thomas et al. 2004, Vitousek et al. 1997). Anthropogenic changes to the global environment affect insects in various ways. Combustion of fossil fuels has elevated atmospheric concentrations of CO2 (Beedlow et al. 2004, Keeling et al. 1995), methane, ozone, nitrous oxides, and sulfur dioxide, leading to increasingly acidic precipitation and prospects of global warming. Some insect species show high mortality as a direct result of atmospheric toxins, whereas other species are affected indirectly by changes in resource conditions induced by atmospheric change (Alstad et al. 1982,Arnone et al. 1995, Heliovaara 1986, Kinney et al. 1997, Lincoln et al. 1993,W. Smith 1981).A thinning ozone layer at higher altitudes and toxic ozone levels at lower altitudes have similar effects (Alstad et al. 1982). However, the anthropogenic changes with the most immediate effects are land-use patterns and redistribution of exotic species, including plants, insects, and livestock. These activities are altering and isolating natural communities at an unprecedented rate, leading to outbreaks of insect "pests" in crop monocultures and fragmented ecosystems (Roland 1993) and potentially threatening species incapable of surviving in increasingly inhospitable landscapes (Samways et al. 1996, Shure and Phillips 1991,A. Suarez et al. 1998). J. Thomas et al. (2004) compared species losses of British butterflies, birds, and plants and found that loss of butterfly species has been greater than that of birds and plants; current rates of species disappearance represent the sixth major extinction event through time. Predicting and mitigating species losses or pest outbreaks depends strongly on our understanding of insect ecology within the context of ecosystem structure and function.

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