The role of space and behavior in an ant-membracid mutualism

I used a modeling framework to examine conditionality in the mutualism between the treehopper Publilia concava and ants in the genus Formica. Results show that treehoppers benefit from ant attendance by protection from predators, but also in other ways; tended treehoppers outperform untended treehoppers even with predators excluded. A possible untested benefit of ant tending is increased feeding rates. Benefit to treehoppers was density-dependent, with treehoppers in small aggregations receiving more benefit from ant tending than treehoppers in large aggregations. A model of mutualism based on consumer-resource theory was developed and applied to this interaction. The model predicts maximum benefit at low density for monotonic saturating recruitment (type II), and maximum benefit at intermediate density for sigmoidal saturating recruitment (type III), assuming that the benefit provided by each ant is independent of treehopper density. Experimental data provide support for these predictions: treehopper survivorship is proportional to the per-capita density of ants, and ant recruitment is described by a type II recruitment response. ^ A spatially explicit extension of this model was applied in a test of two possible mechanisms to explain a commonly reported distribution of treehoppers centered around ant nests. This distribution can result from differential survivorship (ant recruitment and therefore treehopper survivorship decreases as distance from the nest increases), or from the host-choice behavior of females. Results show that females are twice as likely to oviposit on plants with access to ants relative to plants with ants excluded. However, oviposition behavior is an unlikely explanation for spatial pattern formation in this interaction: eggs were laid on all ant accessible host-plants on which treehoppers were observed, regardless of spatial location. In contrast, the spatial distribution of treehoppers generated by the processes of differential survival and localized dispersal is consistent with the observed spatial pattern, as indicated by model simulations. These results suggest that pattern formation is generated passively at the scale of the patch, and support the use of host-visitor models of mutualism as a theoretical framework for understanding conditionality in ant-homopteran, and other host-visitor mutualisms.