Aquatic insects have positive indirect effects on terrestrial prey

Two or more prey species are often fed on by a shared predator, forming an indirect connection between their populations. Changes in the density of one prey species may alter predator numbers or behavior leading to population increases or decreases in the other prey species. Sometimes one of the prey species is mobile and has arrived from a neighboring ecosystem, as in the case of aquatic insects that are fed on by terrestrial predators. In response to high numbers of aquatic insects spider density on land may increase, causing an increase in predation on terrestrial prey. Simultaneously the presence of aquatic insects could alter spider behavior, reducing their feeding upon terrestrial prey. We conducted experiments in the laboratory and field to examine the indirect effect of aquatic midges on terrestrial prey consumed by spider predators. To test the behavioral effect of midge alternative prey on spiders we presented a range of midge densities (5-100+ ind.) to spiders in mesocosm containers with naturally-occurring and sentinel terrestrial prey. Midges at all densities decreased rates of predation on both prey species. To test the effect of midge alternative prey at different densities of predators we manipulated wolf spider density and the presence of midges in field enclosures while measuring rates of predation with sentinel prey. Midge presence decreased rates of predation on sentinel prey at low and high wolf spider density, with the effect significantly greater where spiders were most dense. This short-term positive indirect effect on terrestrial prey by aquatic insects is caused by predator behavior and persists even when predator numbers are elevated. Such apparent mutualisms may be present at many ecosystem interfaces and over large areas.