It is reasonable to expect that climate change will affect the temporal and spatial associations between species interacting at different trophic levels (Porter, 1995; Sutherst et al., 1995; Harrington et al., 1999; Gutierrez, 2000; Walther et al., 2002). What happens to our predictions as we increase the number of species that must be included in our analyses?
Understanding the impact of climate changes on herbivore-predator interactions may be as or more important than plant-herbivore interactions (Cornell et al., 1998). Based on life table analyses, natural enemies acting in a top-down fashion were found to be the most important source of mortality overall in exophytic insects in agro-ecosystems. Both natural enemies and plant factors were important sources of mortality for endophytic insect herbivore populations. As described below, networks of interactions provide many opportunities for compensatory responses (Rosenheim, 1998; Oedekoven and Joern, 2000) and thresholds (Belovsky and Joern, 1995), context-dependent processes that might be especially sensitive to changing climates and seemingly act suddenly, resulting in large changes with only small changes in environmental conditions. For example, host plant quality may alter the impact of predation (Oedekoven and Joern, 2000), or the presence of predators may alter foraging by insect herbivores (Lima, 1998; Losey and Denno, 1998; Eubanks and Denno, 2000b; Danner and Joern, 2003). In turn, changes in local climate may alter the magnitude or even the qualitative expression of these already complex interactions.
Spiders exert a significant limiting influence on grasshopper populations through size-selective predation (Belovsky and Slade, 1993; Oedekoven and Joern, 1998, 2000). Responses by each species are affected directly by temperature (Kemp, 1986; Li and Jackson, 1996; Lactin and Johnson, 1998b), with important implications for understanding seasonal coincidence (phenologies), individual growth characteristics, daily activity schedules, microhabitat use, and spider feeding characteristics (functional responses and size relationships). Population densities of both participants will greatly affect the success of these predator-prey interactions. Each of these attributes can alter effective predator control of prey populations as each is affected by temperature.
What happens to insect herbivore populations in the face of climate change when more than one interacting species is affected? At present, no answer to this basic question really exists. The advantage of a mechanistic approach (Dunham et al., 1989; Huey, 1991; Lawton, 1991; Dunham, 1993; Lawton, 1995; Gutierrez, 1996) is that it permits one to extrapolate results to other sites and years, unlike approaches that merely correlate climatic attributes with population responses. A modeling hierarchy is needed that links the influence of temperature variability to physiological processes underlying feeding, digestion, and resulting demographic consequences (growth, development, survival, and reproduction) on different quality food. Temperature-based seasonal phenology and daily activity cycles and, in turn, individual-based life history responses over a range of food quality in both grasshoppers and spiders can then be examined.
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