The relative importance of spatial vs. temporal variability in generating a conditional mutualism

We explored the relative importance of temporal vs. spatial variability to the conditionality of a mutualism between the treehopper Publilia modesta and the ant Formica obscuripes. The effect of the ants on the membracids varied considerably among years. When the effect of the ants on the membracids was estimated in five sites spread over 5 km there was mixed evidence for spatial conditionality in the mutualism. Using repeated surveys of nymph number we found that the effect of the ants on nymph number varied among sites through time. When total new adult production was examined, however, no such interaction between ants and location was evident. Aggregations displayed strong negative density-dependent adult production. Thus, while in some sites ants had a greater positive effect on nymph survivorship, aggregations at those sites produced proportionately fewer adults because of negative density dependence. Density dependence reduced or eliminated spatial differences in the effect of the ants-on the membracids. Differences among sites did not explain spatial variation in the effect-of ants on nymphs or new adult production. We conclude that temporal variability is much more important than spatial variability in generating conditionality in the mutualism.