Conditional Mutualism in a Membracid‐Ant Association: Temporal, Age‐Specific, and Density‐Dependent Effects

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Abstract
We examined the impact of a tending ant, Formica altipetens, on the population dynamics of a membracid, Publilia modesta. Controlled ant—exclusion experiments revealed three ways in which the strength and occurrence of this mutalism was conditional. First, we detected yearly variation in the impact of ants on membracids. Ants had a significant positive impact on membracid abundance in 1985 and 1987, but not in 1986. Second, we found age—specific effects of ants of membracids. In 1985 and 1987 our experiments revealed that only membracid nymphs benefit directly from and tending; we did not detect a direct positive impact of ants on the survival of membracid adults in any year. Third, we documented a density—dependent effect of ants on membracids. In 1985 and 1987, nymphs in large aggregations benefited from ant tending more than nymphs in small aggregations. Observations in 1985—1987 suggest that 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, whereas this was not the case in 1986. We hypothesize that 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.