Consequences of Altered Plant Nutrition

Environmental stresses have large effects on the nutritional quality of host plants to insect herbivores. The nutritional

Temperature

Figure 13.3 (opposite page) Performance niche responses. (A) Population growth responses by the grain beetles Calandra oryzae and Rhizopertha dominica for temperature and moisture content of wheat. (Data from Birch, L.C. 1953. Ecology, 34:698-711; figure from Maguire, B. Jr. 1973. Am. Naturalist, 107:213-246. With permission.) The dashed line indicates conditions that determine competitive outcomes when beetles co-occur. (B) Herbivore performance by two arthropod herbivores (soybean looper, Pseudoplusia includens pupal mass, left; two-spotted spider mite, Tetranychus urticae, right) on soybeans with fertilizer containing different proportions of key minerals (S, P, and N). (From Busch, J.W., and L. Phelan. 1999. Ecol. Entomol., 24:132-145. With permission.) Note that the two herbivores show very different responses to the treatments; good conditions for one may not be good for the other.

quality of host plants will likely vary in response to altered concentrations of atmospheric gases (e.g., ozone or CO2 levels), particularly because they can lead to altered C:N:P ratios of leaf material which affects feeding (Lincoln et al., 1984, 1986; Bazzaz et al., 1987; Lincoln and Couvet, 1989; Bazzaz, 1990; Ayres, 1993; Roth and Lindroth, 1995; Watt et al., 1995; Lindroth, 1996b). Altered plant quality affects feeding behavior and, when insect herbivores are food-limited demographic responses (Joern and Gaines, 1990; Slansky, 1993; Joern and Behmer, 1997, 1998). This can be incredibly important as herbivores must maintain homeostatically balanced elemental ratios (biological stoichiometry, such as C:N:P ratios), a goal made more difficult when elemental ratios of food differ greatly from those of consumers (Sterner and Elser, 2002; Logan et al., 2003a, 2003b). Cold-adapted poikilothermic organisms contain 30% to 50% more N, P, protein, and RNA than warm exposed conspecifics (Woods et al., 2003). The implications of these results for global climate change are not settled, but could be important.

Naturally occurring plants are very responsive to environmental changes and are often stressed, with the chemical makeup foliar contents in remarkable flux (Buwai and Trlica, 1977a, 1977b; Bokhari, 1978; Bokhari and Trent, 1985a, 1985b; Mole and Joern, 1993; Mole et al., 1994). In addition to obvious and routine stress from nutrient-poor soils in many grasslands and agricultural settings, drought and herbivory often add opportunities for stress (Williams, 1979; Marrs, 1983; Bokhari and Trent, 1985a; Fitter, 1986; Mooney et al., 1991; White, 1993). Global warming and increased variability in precipitation will likely result in increased drought stress. Multiple environmental stresses affect the physiological state of the host plant, and significantly alter patterns of resource allocation among plant tissues (Chapin, 1980; Gershenzon, 1984; Bloom et al., 1985; Coley et al., 1985; Bazzaz et al., 1987; Chapin et al., 1987; Pearcy et al., 1987; Mooney et al., 1991).

13.2.2.1 Plant Stress and Foliar Quality for Herbivores

Elevated atmospheric CO2 can alter the relevant nutrient content of plant tissues, usually increasing the C:N ratio (Cave et al., 1981; Lincoln et al., 1986; Osbrink et al., 1987; Fajer, 1989; Bazzaz, 1990; Johnson, 1990; Fajer et al., 1991; Johnson, 1991; Ayres, 1993) in response to reasonably well-understood resource allocation dynamics in plant metabolism (Ayres, 1993). Insect herbivores may alter feeding behavior and demographic attributes in response to these nutritional changes (Osbrink et al., 1987; Akey, 1989; Fajer, 1989; Johnson, 1990; Fajer et al., 1991; Johnson, 1991; Behmer and Joern, 1993; 1994; Yang and Joern, 1994a). Results to date indicate that altered plant quality from increased CO2 can affect insect feeding, but responses are highly variable among insect herbivore taxa and can be small or equivocal (Watt et al., 1995).

13.2.2.2 Temperature, Food Quality, and Diet Processing

Tb in ectotherms influences food acquisition and processing capabilities (Huey and Stevenson, 1979; Crowder, 1983; Kara-sov, 1984; Zimmerman and Tracy, 1989; Yang and Joern, 1994a, 1994b). For example, more energy is obtained by a desert spider from selection of favorable environments than from sites with more prey because proper temperatures facilitate optimal digestion and use of nutrients (Riechert and Tracy, 1975). In herbivorous insects, temperature clearly affects foraging, digestion, and performance (Scriber and Led-erhouse, 1983; Stamp, 1990; Stamp and Bowers, 1990a, 1990b; Casey, 1993; Stamp, 1993; Yang and Joern, 1994b; Harrison and Fewell, 1995; Lactin and Johnson, 1995). Tiger swallowtail caterpillars exhibit increased consumption as temperatures increase, resulting in increased growth until respiratory expenditures became too high; digestibility was unaffected (Scriber and Lederhouse, 1983). Lower food quality often results in increased consumption by insect herbivores as well (Yang and Joern, 1994a, 1994b), but even this is limited compared to the anticipated effects.

13.2.2.3 Climate-Based Insect Population Dynamics and Range Shifts

Conventional wisdom predicts that strong correlations between weather and insect densities will exist, and that these correlations explain population fluctuations (Capinera, 1987). For example, statistically significant correlations between population change and abiotic factors have been identified for grasshoppers (Gage, 1977; Hardman, 1982; Logan and Hilbert, 1983; Johnson and Worobec, 1988; Capinera and Horton, 1989; Fielding and Brusven, 1990), although responses vary according to geographic location. Variation in grasshopper population densities from arid grasslands is positively correlated with winter and spring precipitation (Nerney and Hamilton, 1969; Capinera and Horton, 1989; Fielding and Brusven, 1990), while temperature has a greater influence on northern populations (Johnson and Worobec, 1988; Capinera and Horton, 1989).

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