Multispecies interactions are highly complex. Species can simultaneously compete for space and enhance each other's food acquisition (mutualism), as described by Cardinale et al. (2002) for three caddisfly species that in combination increase substrate surface heterogeneity and near-surface velocity and turbulent flow that control food delivery (see later in this section). Two species with overlapping resource requirements could become "competitive mutualists" with respect to a third species that would compete more strongly for the shared resources (Pianka 1981).
The strength, and even type, of interaction can vary over time and space depending on biotic and abiotic conditions (e.g., B. Inouye 2001,Tilman 1978). Interactions can change during life history development or differ between sexes. For example, immature butterflies (caterpillars) are herbivores, but adult butterflies are pollinators. Insects with aquatic immatures are terrestrial as adults. Immature males of the strepsipteran family Myrmecolacidae parasitize ants, whereas immature females parasitize grasshoppers (de Carvalho and Kogan 1991). Herbivores and host plants often interact mutualistically at low herbivore population densities, with the herbivore benefiting from plant resources and the plant benefiting from limited pruning, but the interaction becomes increasingly predatory as herbivory increases and plant condition declines (see Chapter 12).
The strength of an interaction depends on the proximity of the two species, their ability to perceive each other, their relative densities, and their motivation to interact. These factors in turn are affected by abiotic conditions, resource availability, and indirect effects of other species. Modeling interaction strength for prediction of community dynamics has taken a variety of approaches that may be subject to unrecognized biases or to nonlinear or indirect effects (Abrams 2001, Berlow et al. 1999).
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