Mechanisms and density dependence of benefit in an ant-membracid mutualism

I examined mechanisms and patterns of benefit for the membracid (treehopper ) Publilia concava tended by the ant Formica obscuriventris to test two hyp otheses: that treehoppers benefit from ant attendance only by protection fr om predators, and that density-dependent benefit depends on the presence of predators. I used a factorial design, manipulating ants and predators in 1 996, and ants and removal of uncollected honeydew in 1997. Results showed t hat treehoppers benefit from ant attendance by protection from predators. A dditionally, results suggested that treehoppers benefit from ant attendance in ways other than protection from predators; tended treehoppers outperfor m untended treehoppers even with predators excluded. There was no support f or the hypothesis that a proximate benefit of ant-tending includes removal of uncollected honeydew. A possible benefit (untested) of ant-tending is in creased feeding rates. Treehoppers in small aggregations benefited more tha n treehoppers in large aggregations, indicating a density-dependent benefit in this mutualism, independent of predator level. Correspondingly, the num ber of ants per treehopper was highest for small aggregations. This study s uggests that individuals can benefit from mutualisms in complex ways. Addit ionally, it adds to a growing number of studies that support the hypotheses that mutualisms may be stabilized by density-dependent benefit and that de nsity-dependent benefit may be driven by the recruitment patterns of mutual ists.