Conditional mutualism in a membracid-ant association: temporal, age- specific, and density-dependent effects

Examined the impact of a tending ant, Formica altipetens, on the population dynamics of a membracid, Publilia modesta. Ants had a significant positive impact on membracid abundance in 1985 and 1987, but not in 1986. In 1985 and 1987, only membracid nymphs benefit directly from ant tending; experiments did not detect a direct positive impact of ants on the survival of membracid adults in any year. In 1985 and 1987, nymphs in large aggregations benefited from ant tending more than nymphs in small aggregations. Protection from a predatory salticid spider, Pellenes sp., may be at least one mechanism by which ants benefit membracid nymphs. In 1985 and 1987, when the association was mutualistic, spiders were significantly more abundant on membracid-infested plants without ants than on infested plants with ants; this was not the case in 1986. The age-specific benefits in this system may result because nymphs are especially susceptible to predation by spiders whereas the more agile and sclerotized adults are not. -from Authors