Insects are particularly vulnerable to changes in temperature, water availability, and air or water chemistry because of their relatively large ratios of surface area to volume. However, many insects can live within suitable microsites that buffer exposure to environmental changes. Insects in aquatic environments or deep in soil or woody habitats may be relatively protected from large changes in air temperature and relative humidity (e.g., Curry 1994, Seastedt and Crossley 1981a). High moisture content of soil can mitigate heat penetration and protect soil fauna.
Most insects are subject to environmental variability that includes periods of potentially lethal or stressful abiotic conditions. Therefore, maintaining optimal body temperature, water content, and chemical processes is a challenge for survival in variable environments. Insects possess a remarkable variety of physiological and behavioral mechanisms for surviving in variable environments.
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Sample (Julian) date, I983
| Changes in abundance of a root bark beetle, Hylastes nigrinus, in undisturbed, 12-yr-old plantations (black squares) of Douglas fir and plantations thinned in September 1982 (asterisks), January 1983 (black circles), or May 1983 (white circles) in western Oregon. Arrow indicates time of thinning in May 1983. From Witcosky et al. (1986), courtesy of the Research Council of Canada.
Adaptive physiological responses can mitigate exposure to suboptimal conditions. For example, diapause is a general physiological mechanism for surviving seasonally adverse conditions, usually in a resistant stage, such as the pupa of holometabolous insects. Our understanding of the genetic and molecular basis for physiological processes has increased dramatically in the past 20 years. Diapause induction and termination are controlled by cues such as photoperiod and degree-day accumulation (daily degrees above a threshold temperature x number of days), which induce chemical signals from the brain (Denlinger 2002, Giebultowicz 2000, Giebultowicz and Denlinger 1986). In particular, pho-toreceptors that distinguish day from night trigger expression of genes that measure and accumulate information on day or night length, or both, and produce proteins that induce diapause (Hardie 2001). Denlinger (2002) and Giebultowicz (2000) reported that photoperiod affects patterns of expression, whereas temperature affects the amount, of several clock messenger ribonucleic acids (mRNAs; cryptochrome, cry; clock, clk; period, per; and timeless, tim), which also regulate circadian rhythms. The relative amounts of these mRNAs show distinct trends from long, warm days to shorter, cooler days, but their precise role in triggering the onset of diapause remains unknown (Denlinger 2002, Goto and Denlinger 2002). Various antibiotic proteins also are produced only during diapause, apparently to prevent infection during this vulnerable period, perhaps from tissue exposure to gut microorganisms while gut tissues are being reorgan ized (Dunn et al. 1994, Lee et al. 2002). Diapause termination often requires a minimum duration of freezing temperatures, or other factors, that maximize synchronization of development with seasonally suitable conditions (Ruberson et al. 1998). Beaver et al. (2002) reported that Drosophila males with mutated genes governing circadian rhythm produced fewer offspring than did wild flies, demonstrating the importance of the genes controlling periodicity. Nevertheless, exposed insects often are killed by sudden or unexpected changes in temperature, moisture, or chemical conditions of the habitat. Even diapausing insects suffer high mortality as a result of a combination of temperature, disease, predation, or other factors (Ruberson et al. 1998).
Behavior represents a more flexible means of responding to environmental variation, compared to physiology, because an animal can respond actively to sensory information to avoid or mitigate lethal conditions. Mobile insects have an advantage over sessile species in avoiding or mitigating exposure to extreme temperatures, water availability, or chemical conditions. Limited mobility often is sufficient within steep environmental gradients. Many small, flightless litter species need move vertically only a few millimeters within the soil profile to avoid lethal temperatures and desiccation at the surface following fire or canopy opening (Seastedt and Crossley 1981a). Some species choose protected habitats prior to entering diapause to reduce their vulnerability to potential disturbances. K. Miller and Wagner (1984) reported that pandora moth, Coloradia pandora, pupae in a ponderosa pine, Pinus ponderosa, forest were significantly more abundant on the forest floor in areas with open canopy and sparse litter than in areas with closed canopy and deeper litter. Although other factors also differ between these microhabitats, avoidance of accumulated litter may represent an adaptation to survive frequent ground fires in this ecosystem. In addition, mobile insects may be able to escape disturbed patches and often can detect and colonize suitable patches within variable environments (D. Johnson 2004).
Although small body size limits ability to regulate body temperature and water content, many insects are capable of at least limited homeostasis through physiological or behavioral mechanisms, or both. Some insects also must deal with variability in chemical or other abiotic conditions.
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