Physiological Ecology And Nichebased Responses

Physiological responses often determine our ability to predict species (population) responses to changes in environmental conditions in a reasonably analytical fashion (Dunham, 1993; Gutierrez, 1996). For example, abiotic (e.g., temperature or humidity) and biotic conditions (e.g., protein in food) may combine to affect developmental rate, growth, survival, and reproduction. In a niche-based framework (Maguire, 1973; Chase and Leibold, 2003), knowing an organism's response within an environmental state-space allows one to predict individual fitness and ultimately population responses (Figure 13.1); these types of responses have been worked out in some detail for some taxa for certain niche axes (Birch, 1953; Clancy and King, 1993; Busch and Phelan, 1999). Except under unusual situations when the entire structure of the ecosystem shifts (e.g., complete defoliation), insect pests will not have much impact on overall environmental conditions, so changes external to these agro-ecosystems are expected to drive population responses in a niche-based view. If critical environmental conditions change, then populations change as well by increasing or decreasing in abundance, depending on whether conditions are more or less favorable (Figure 13.1). Basic studies document the power of the relationship between developmental rate and temperature (Figure 13.2) or food quality (Figure 13.3), and the effects on population dynamics.

To predict likely responses of insect pest populations to global environmental changes from the anticipated increased CO2 levels in the atmosphere, we examine the interactive effects of temperature and food quality on population processes as a model for developing predictions regarding insect pest responses to changing climates. In addition to effects from overall increases in crop productivity, the C:N content of food is predicted in response to increased atmospheric CO2, resulting in leaf material that is generally of lower primary nutritional quality (relative amounts of protein and carbohydrates) to many insect herbivores (Fajer 1989; Fajer et al.,

A Initial condition

B Increased population d o o d o o

A Initial condition

B Increased population d o o d o o

Temperature

Figure 13.1 (a) Individual or population performance based on two niche axes, temperature and food quality. Optimum combinations support highest levels of performance, such as survival, development, or fecundity, resulting in maximal population growth rates. Some combinations are unsuitable for individuals to persist in the habitat. (b through d) Possible effects of changing environments from initial conditions on performance. (b) Population does better under new conditions. (c) Population does about the same. (d) New conditions lead to local extinction.

Temperature

Temperature

Figure 13.1 (a) Individual or population performance based on two niche axes, temperature and food quality. Optimum combinations support highest levels of performance, such as survival, development, or fecundity, resulting in maximal population growth rates. Some combinations are unsuitable for individuals to persist in the habitat. (b through d) Possible effects of changing environments from initial conditions on performance. (b) Population does better under new conditions. (c) Population does about the same. (d) New conditions lead to local extinction.

1991). For many insect herbivores, however, changes in the amount and types of plant defenses through altered secondary chemistry may be as important as changes in primary nutritional quality (Lincoln et al., 1984, 1986; Lincoln and Couvet, 1989; Stamp, 1990; Stamp and Bowers, 1990a, 1990b; Stamp, 1993; Yang and Stamp, 1995).

Figure 13.2 Effect of temperature on critical life history rates (see Wermelinger and Seifert, 1999). Data show oviposition rates of the spruce bark beetle (Ips typographus) over a range of temperatures. Note that the nonlinear response at higher temperatures corresponds to models proposed by Logan et al. (1976) and Lactin et al. (1995).

Figure 13.2 Effect of temperature on critical life history rates (see Wermelinger and Seifert, 1999). Data show oviposition rates of the spruce bark beetle (Ips typographus) over a range of temperatures. Note that the nonlinear response at higher temperatures corresponds to models proposed by Logan et al. (1976) and Lactin et al. (1995).

Was this article helpful?

0 0

Post a comment