Abiotic Conditions

Relatively few studies have addressed the effects of abiotic conditions on species interactions. J. Chase (1996) experimentally manipulated temperature and solar radiation in experimental plots containing grasshoppers and wolf spiders in a grassland. When temperature and solar radiation were reduced by shading during the morning, grasshopper activity was reduced, but spider activity was unaffected, and spiders reduced grasshopper density. In contrast, grasshopper activity remained high in unshaded plots, and spiders did not reduce grasshopper density. Stamp and Bowers (1990) also noted that temperature affects the interactions between plants, herbivores, and predators.

Hart (1992) studied the relationship between crayfish, their caddisfly (Trichoptera) prey, and the algal food base in a stream ecosystem. He found that crayfish foraging activity was impaired at high flow rates, limiting predation on the caddisfly grazers and altering the algae-herbivore interaction. Kelly et al.

(2003) reported that exposure of stream communities to UV radiation reduced aquatic grazing and led to increased algal biomass.

Abiotic conditions that affect host growth or defensive capability influence predation or parasitism. Increased exposure to sunlight can increase plant production of defensive compounds and reduce herbivory (Dudt and Shure 1994, Niesenbaum 1992). Light availability to plants may affect their relative investment in toxic compounds versus extrafloral nectaries and domatia to facilitate defense by ants (Davidson and Fisher 1991). Fox et al. (1999) reported that drought stress did not affect growth of St. John's wort, Hypericum perforatum, in the United Kingdom directly, but it increased plant vulnerability to herbivores. Stamp et al. (1997) found that the defensive chemicals sequestered by caterpillars had greater negative effects on a predator at higher temperatures.

Altered atmospheric conditions (e.g., CO2 enrichment or pollutants) affect interactions (Alstad et al. 1982, Arnone et al. 1995, V. C. Brown 1995, Heliovaara and Vaisanen 1986,1993, Kinney et al. 1997, Roth and Lindroth 1994, Salt et al. 1996). For example, Hughes and Bazzaz (1997) reported that elevated CO2 significantly increased C to N ratio and decreased percentage nitrogen in milkweed, Asclepias syriaca, tissues, resulting in lower densities but greater per capita leaf damage by the western flower thrips, Frankliniella occidentalis. However, increased plant growth at elevated CO2 levels more than compensated for leaf damage. Yet Salt et al. (1996) reported that elevated CO2 did not affect the competitive interaction between shoot- and root-feeding aphids. Mondor et al.

(2004) found that the aphid, Chaitophorus stevensis, showed reduced predator-escape behavior in enriched CO2 atmosphere, but greater escape behavior in enriched O3 atmosphere, compared to ambient atmospheric conditions. Couteaux et al. (1991) found that elevated CO2 affected litter quality and decomposer food-web interactions. Ozone, but not nitrogen dioxide or sulfur dioxide, interfered with searching behavior and host discovery by a braconid parasitoid, Asobara tabida.

Disturbances affect species interactions in several ways. First, disturbances act like predators for intolerant species and reduce their population sizes, thereby affecting their interactions with other species. Second, disturbances contribute to landscape heterogeneity, thereby providing potential refuges from negative interactions (e.g., Denslow 1985). For example, disturbances often reduce abundances of predators, perhaps facilitating population growth of prey populations in disturbed patches (Kruess and Tscharntke 1994, Schowalter and Ganio 1999).

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