Predator Prey Interactions

Climatic conditions (especially microclimate) also have a very big impact on arthropod predator-prey interactions. This interaction is important because natural predators and parasitoids in agricultural settings often limit abundance levels of insect herbivore pests on crops (Cornell et al., 1998; Rosenheim, 1998; Gutierrez, 2000), including successful bio-control efforts. The chain of events linking predators with prey species that affect predation success is large, and disruption at any level can significantly impact the likelihood that agricultural pests will increase.

13.3.2.1 Mediating Effects of Microclimate

An excellent example illustrating the importance of microclimatic conditions on spider mite predator-prey interactions is the interaction between the serious corn pest, Banks grass mite Oligonychus pratensis), and the predatory mite Neoseiu-lus fallacis, which can limit Banks grass mite in a standard predator-prey interaction (Perring et al., 1986; Berry et al., 1991a, 1991b). Temperature and humidity at the leaf surface defined the population dynamics of these two species, and greatly affect the ability of the predatory mite to hold the herbivorous Banks grass mite to low population levels. Outbreaks of Banks grass mite occurred during hotter years when corn was moderately drought stressed, in part because physical conditions determined whether the predatory mite population could increase sufficiently to exert control. Under cooler, wetter conditions, the predatory mite could control Banks grass mite and decrease economic losses. Banks grass mite populations are favored by warmer, dry conditions, whereas the predatory mite was favored by cooler, humid conditions. A short period (days) of hot, dry weather is sufficient for the Banks grass mite to escape the controlling influence of predation and cause significant damage to corn. Other studies further document the interaction between temperature and prey density in determining the predator's efficiency at capturing prey (functional response) (Hardman and Rogers, 1992). To transfer results to field situations, we emphasize that conclusions depend on conditions within the boundary layer at the leaf surface — the microclimate relevant to these very tiny mites — rather than the standard reference conditions measured by most weather stations and conditions predicted by GCC. While it is clear that microclimatic conditions drive this important predator-prey interaction, it is less clear how to scale these results in the context of predicted global climate change.

13.3.2.2 Context-Dependent Interactions between Predators and Prey

Food acquisition and activity is greatly affected by risk from predation (Lima, 1998), and such interactions can greatly alter performance outcomes of predator prey interactions. In choice tests, Plantago-feeding caterpillars of the butterfly Junionia coenia select plants with the lowest concentrations of iridoid glycocides, a plant defensive compound (Stamp and Bowers, 2000). Caterpillars fed equally on leaves with high and low iridoid glycoside levels in the presence of stinkbug predators (Podisus maculiventris), presumably spending less time feeding and more time in hiding or escape behaviors. Grasshoppers feed much less in the presence of wolf spiders than in predator-free environments, affecting survival, growth, developmental rate, and reproduction (Rothley et al., 1997; Danner and Joern, 2003). Eating higher-quality food mediates the predation risk effect such that grasshoppers with predation risk fed high-quality food have the same performance levels as individuals with lower-quality food but no predation risk (Danner and Joern, 2003). And, pea aphids in alfalfa exhibit a high propensity to drop from plants in the presence of the foliar foraging predaceous big-eyed bugs or ladybird beetles (Coccinella septempunctata) (Losey and Denno, 1998). The propensity by aphids to drop is also influenced by the quality of the resource to be abandoned and the risk of mortality in the new microhabitat. What these examples illustrate is the need to be aware of coexisting species in addition to physical features of an organism's environment to fully understand and predict responses. Of course, this makes it difficult to predict how changing environments will affect herbivores because different interactions may dominate in different settings, and the expression of these interactions is context dependent.

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