Global Change and Terrestrial Ecosystems (1996) 13.1 INTRODUCTION
On average, Earth's climate has warmed significantly (~0.6°C) over the last century, and additional rapid changes are anticipated (Houghton et al., 2001). General circulation models for assessing future climate, coupled to direct measurements, predict that atmospheric CO2 levels will continue to increase (Schneider et al., 1992; Vitousek, 1994). More locally, significant regional climate changes in the primary agricultural regions of North America are anticipated (Rind et al., 1990; Schneider et al., 1992; Schneider, 1993; Reddy and Hodges, 2000), including increased summer temperatures (4°C to 8°C), decreased precipitation, and a significant drop in available soil water (ca. 40% to 50%), each associated with a significant change in seasonality. Some analyses indicate that both environmental changes and system responses can be sudden rather than gradual, further exacerbating the impact (Rietkerk and van de Koppel, 1997; van de Koppel et al., 1998; Alley et al., 2003; Gu et al., 2003). Human activity has greatly affected atmospheric concentrations of patterns of deposition of gaseous pollutants (e.g., SOx, NOx) that could affect insects in agro-ecosystems (Brown, 1995). In this context, important questions remain about responses of important natural and agricultural systems, including the issue of how agricultural pests and resulting food security will be affected. Diverse arthropod pests and plant pathogens significantly reduce agricultural production at present (Pimentel, 1991). How pests and their interactions with other organisms and the environment will respond to these changes, and whether such changes can be predicted is a big concern — one that requires consideration of regional and local climate changes that transcend global averages (Cammell and Knight, 1992; Lansberg and Smith, 1992; Harrington and Stork, 1995; Walther et al., 2002). The problem is complex and there are many possible ways that climate change from increased CO2 can affect the outbreak potential of insect herbivores in agricultural systems. In general, if CO2 continues to rise, average temperatures will also increase and precipitation will become more variable, suggesting that effects from temperature-dependent processes and plant responses to environmental stresses are the keys for understanding insect pests. One must also be cognizant of altered land use in the face of climate change as farmers and ranchers are likely to adapt and plant the most appropriate crops for new environmental and economic conditions.
Altered temperature and variable food quality (e.g., from changed CO2 levels and other atmospheric pollutants) will likely be primary but not exclusive drivers affecting agricultural pests (Fajer, 1989; Fajer et al., 1991; Ayres, 1993; Lindroth, 1996a, 1996b). Because insect populations often respond to plant foliar quality, plant communities, and vegetation structure, which in turn are expected to change with CO2 concentrations, effects of climate change on insect pests may initially be best understood as responses by plants that are then tracked by insects. However, we must also recognize that insect herbivores are attacked by a number of natural enemies, and these important interspecific interactions may also be vulnerable to environmental changes. Here, we examine a range of possible ecological responses that must be addressed to assess responses to agricultural insect pests to increased CO2 and climate change. The task will be very challenging if predictive insights are required because so little is currently known. Even though insect pests are poorly studied in the context of global climate change, scientists are beginning to amass many examples. Because "pests" are really nothing more than species living where they are not wanted, capable of reaching high densities with economic impact (Nothnagle and Schultz, 1987), no loss of insight results from our approach of relying on other, better-studied taxa to develop understanding.
Ample evidence documents that organisms are responding to global climate change (Harrington and Stork, 1995; Dukes and Mooney, 1999; Harrington et al., 1999; Hughes, 2000; Walther et al., 2001, 2002; Warren et al., 2001; Parmesan and Yohe, 2003; Root et al., 2003). Responses to changing environments include more variable population dynamics, altered phenologies, shifts in biogeographic distributions, and disrupted species interactions because of changes affecting one or more participants. Much remains to be learned about multispecies responses, especially those that affect species interactions and food web dynamics and which have important implications for pest responses. To predict responses by insects, we believe that this challenge must include mechanistic approaches applied carefully to the problem (Lawton, 1991, 1995; Hassell et al., 1993; Kareiva et al., 1993; Gutierrez et al., 1994; Gutierrez, 1996, 2000; Bezener and Jones, 1998; Davis et al., 1998). This approach should incorporate the impact of direct and indirect effects of environmental changes (e.g., temperature, atmospheric gases, and resulting land use) to food quality, insect physiological processes, and interactions among species. We also examine briefly the impact of altered temperature and food quality on food web dynamics, often called tritrophic interactions (Rosenheim, 1998), with each link that may be impacted directly and indirectly by anticipated changes in climate. To provide opportunities for predicting responses, we focus when possible on impacts of climate change to the interactions between pests, their food plants, and their predators using temperature-dependent physiological and population-based models (Gutierrez et al., 1994; Gutierrez, 2000) coupled to field and laboratory observations and experiments.
A big question with regard to food security concerns the likelihood that insect herbivore populations will remove critical amounts of plant tissue and adversely affect food, fiber, and forage production in the face of increased atmospheric CO2 levels. Focus on plant effects rather than just insect herbivore population responses adds one more level in the assessment process, but indicates that emphasis on tracking insect responses may be misleading. Other insect pests are more important as vectors of disease, and environmental changes that increase levels of transmission may be more important than direct tissue loss (Lines, 1995). In all cases, however, population and community dynamics of insects are driven by multiple abiotic and biotic ecological factors that act simultaneously and are often strongly impacted by food quality and temperature (Joern and Gaines, 1990; Belovsky and Joern, 1995; Harrington et al., 1999). Key life history responses of survivorship, growth, development, and reproduction are significantly affected by variable host plant nutritional quality (Joern and Behmer, 1997, 1998); habitat thermal characteristics and the capability for thermoregulation (Casey, 1993; Coxwell and Bock, 1995; Harrison and Fewell, 1995; Lactin et al., 1995); and interactions with other herbivores and predators (Belovsky and Joern, 1995; Cornell and Hawkins, 1995; Chase, 1996; Rosenheim, 1998; Harrington et al., 1999; Oedekoven and Joern, 2000). Each of these factors provides a useful temperature and/or CO2-dependent link to the direct impacts of climate change on individual responses at different levels in the food chain, and provides a way to mechanistically link climate change to population and important host plant-herbivore and predator-prey interactions.
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