Insect responses to environmental changes caused by anthropogenic activity remain largely unknown. A number of studies have documented insect responses to elevated temperature; to increased atmospheric or aqueous concentrations of CO2 or various pollutants, including pesticides; and to habitat disturbance and fragmentation (e.g., Alstad et al. 1982, Arnone et al. 1994, Bezemer and Jones 1998, Heliovaara and Vaisanen 1986,1993, Kinney et al. 1997, Lincoln et al. 1993, Marks and Lincoln 1996). As described earlier, the indirect effects of these changes may be greater than the direct effects.
Changing land-use patterns and ecosystem fragmentation alter abiotic variables and have the most dramatic effects on survival or movement of various insects (e.g., J. Chen et al. 1995, Franklin et al. 1992, Roland 1993, Rubenstein 1992). Kimberling et al. (2001) reported that physical disturbance related to construction or waste disposal had relatively less effect on invertebrate communities than did conversion of shrub-steppe to agricultural use in eastern Washington in the United States. Invasive species often are favored by habitat disturbance and subsequently change patterns of community interaction. For example, invasive red imported fire ants, Solenopsis invicta, are most abundant in disturbed habitats (Stiles and Jones 1998). Where abundant, they displace native ants, and negatively affect many ground nesting birds, small mammals, and herpetofauna, through aggressive foraging behavior, high reproductive rates, and lack of predators (C. Allen et al. 2004). A. Suarez et al. (2001) found that habitat fragmentation also favored the invasive Argentine ant, Linepithema humile, at the expense of native ant species. Summerville and Crist (2001) found that rare species were disproportionately affected by habitat fragmentation.
However, humans are changing environmental conditions in many ways simultaneously, through fossil fuel combustion, industrial effluents, water impoundment and diversion, pesticide application, and land-use practices. Large areas have been planted to genetically modified crops or occupied by invasive exotic species. Global atmospheric concentrations of CO2 and other greenhouse gases are clearly increasing, and global climate has shown a distinct warming trend (e.g., Beedlow et al. 2004, Keeling et al. 1995). Acidic precipitation has greatly reduced the pH of many aquatic ecosystems in northern temperate countries, with more dramatic effects. Nitrogen subsidies resulting from increased atmospheric NOx may provide a short-term fertilization effect in N-limited ecosystems until pH-buffering capacity of the soil is depleted. Deforestation, desertification, and other changes in regional landscapes are fragmenting habitats and altering habitat suitability for organisms around the globe (J. Foley et al. 2005).
The interactions between environmental factors are poorly understood but often synergistic. For example, deforestation changes surface albedo and leads to regional warming and drying in tropical biomes and to cooling in temperate and boreal biomes (J. Foley et al. 2003, 2005). Furthermore, studies in the Amazon basin indicate that smoke from fires that accompany forest conversion to agricultural or urban land use reduce cloud cover (from 38% in clean air to 0% in heavy smoke), reduce droplet size, and increase the altitude at which water condenses, leading to more violent thunderstorms and hail, rather than warm rain (Ackerman et al. 2000, Andreae et al. 2004, Koren et al. 2004). Altered drainage patterns affect temperature and chemical conditions of aquatic ecosystems and opportunities for organisms to disperse upstream or downstream (Pringle et al. 2000).
Few studies have measured insect responses to multiple changes in ecosystem conditions. However, given insect sensitivity to environmental changes, as described earlier, any change will alter insect abundance and distribution and may increase the incidence of crop pests and vectors of human and animal diseases (Stapp et al. 2004, Summerville and Crist 2001, D. Willliams and Liebhold 2002, Zhou et al. 2002). Chapin et al. (1987) addressed plant responses to multiple stressors and concluded that multiple factors can have additive or syn-ergistic effects. We should expect some insects to disappear as habitat conditions exceed their tolerance ranges or their host plants disappear. Others will become more abundant and facilitate host plant decline by exploiting stressed and poorly defended hosts (see Chapter 3). Clearly, studies are needed on insect responses to multiple natural and anthropogenic changes to improve prediction of effects of environmental changes.
Insects are affected by abiotic conditions that reflect latitudinal gradients in temperature and moisture, as modified by circulation patterns and mountain ranges. At the global scale, latitudinal patterns of temperature and precipitation produce bands of tropical rainforests along the equatorial convergence zone (where warming air rises and condenses moisture), deserts centered at 30°N and S latitudes (where cooled, dried air descends), and moist boreal forests centered at 60°N and S latitudes (where converging air masses rise and condense moisture). Mountains affect the movement of air masses across continents, forcing air to rise and condense on the windward side and dried air to descend on the leeward side. The combination of mountain ranges and latitudinal gradients in climatic conditions creates a template of regional ecosystem types known as biomes, characterized by distinctive vegetation (e.g., tundra, desert, grassland, forest). Aquatic biomes are distinguished by size, depth, flow rate, and marine influence (e.g., ponds, lakes, streams, rivers, estuaries).
Environmental conditions are not static but vary seasonally and annually. In addition, environment conditions change over longer periods as a result of global processes and anthropogenic activities. Acute events (disturbances), such as storm or fire, can dramatically alter habitat conditions and resource availability for various organisms. Hence, insects must be able to avoid or adjust to changing conditions.
The inherent problems of maintaining body heat and water content and avoiding adverse chemical conditions by small, heterothermic organisms have led to an astounding variety of physiological and behavioral mechanisms by which insects adjust to and interact with environmental conditions. Research on genetic control of physiological processes is improving our understanding of the mechanisms of adaptation. Mechanisms for tolerating or mitigating effects of variation in abiotic factors determine the seasonal, latitudinal, and elevational distributions of insect species.
Many insects have a largely unappreciated physiological capacity to cope with the extreme temperatures and relative humidities found in the harshest ecosystems on the planet. However, even insects in more favorable environments must cope with variation in abiotic conditions through diapause, color change, evaporative cooling, supercooling, voiding of the gut, control of respiratory water loss, etc. Many species exhibit at least limited homeostatic ability (i.e., ability to regulate internal temperature and water content).
Behavior represents the active means by which animals respond to their environment. Insects are sensitive to a variety of environmental cues, and most insects are able to modify their behavior in response to environmental gradients or changes. Insects, especially those that can fly, move within gradients of temperature, moisture, chemicals, or other abiotic factors to escape adverse condi tions. Many species are able to regulate body heat or water content by using rapid muscle contraction, elevating the body above hot surfaces, seeking shade, or burrowing. Social insects appear to be particularly flexible in the use of colony activity and nest construction to facilitate thermoregulation.
Many insects are capable of flying long distances, but dispersal entails considerable risk, and many individuals do not reach suitable habitats. The probability that an insect will discover a suitable patch is a function of the tendency to disperse (as affected by life history strategy and crowding), endurance (determined by nutritional condition), patch size, distance, and the mechanism of dispersal (whether random, phoretic, or oriented toward specific habitat cues).
Environmental changes resulting from anthropogenic activities are occurring at an unprecedented rate. The effects of these changes on insects are difficult to predict because few studies have addressed the effects of multiple interacting changes on insects.
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