Much remains to be learned about responses by pest insect populations to anticipated climate shifts (Lawton, 1995; Lindroth, 1996b; Davis et al., 1998; Harrington et al., 1999). Insect pest populations can rapidly and unexpectedly build to large numbers in response to environmental changes, including changes in weather (Cappuccino and Price, 1995; Dempster and McLean, 1998), resulting in significant plant damage (Hewitt and Onsager, 1983; Barbosa and Schultz, 1987). Potential impacts of climate change on these insect populations will reflect the combined, integrated response to the direct effects from altered physical environments that influence physiological processes, coupled with the indirect consequences of altered food plant quality and availability resulting from both the direct impact of CO2 levels acting on plants and accompanying drought stress resulting from climatic shifts (Lansberg and Smith, 1992). The combined direct and indirect effects from climate change will alter key demographic attributes, thus resulting in unanticipated population-level consequences.
Competitive interactions among taxa represent primary interactions in natural communities, forming the basis of much ecological research. When key resources such as food are limiting, species abundances are lowered and some species may not be able to coexist with others (competitive exclusion). Interspecific competition can be common among insect herbivores (Denno et al., 1995). In most agricultural systems, however, it is probably less important than in natural communities (Cornell et al., 1998), although there are important examples. How will competitive interactions be affected by direct and indirect affects of altered CO2 levels?
For poikilotherms such as arthropod pests, physical conditions of the environment are critical as they can readily reverse the outcome of competitive interactions. For example, the well-studied, stored-grain beetle Tribolium confusum outcompetes Tribolium castaneum when grown at a temperature of 34°C and low relative humidity (RH = 30%); when RH = 70% but temperature remains at 34°C, the outcome is reversed. T. castaneum usually wins when RH is high at high temperatures, but not when temperatures are more moderate (24°C). Similar results are observed for the grain beetles Rhizopertha and Callandra based on other niche axes (Figure 13.3b) (Birch, 1953; Maguire, 1973). Grain beetle performance varies according to internal niche structure in this case (Maguire, 1973), and reversals in the outcome of interspecific interactions such as competition due to physical conditions such as temperature and relative humidity greatly complicates the picture. Studies on a range of organisms indicate that the interaction between competitive interactions and physical conditions can be important (Tilman et al., 1981). Climate shifts will likely affect any number of plant-arthropod and arthropod-arthropod competitive interactions in agricultural settings.
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