The importance of social aggregation in connecting aboveground and belowground processes in a wet tropical forest

Background/Question/Methods

Social behavior of consumers can create spatial heterogeneity in ecosystem processes, including grouping effects on resource exploitation and species interactions. The impact of social behavior on ecosystem heterogeneity could be enhanced or dampened depending on the form of the relationship between aggregation effects and ecosystem processes. One useful system for addressing this hypothesis is social insects with stationary, long-lived nests in forest canopies. Previous studies have shown that the canopy nesting ant, Azteca trigona, links aboveground and belowground processes in a lowland wet forest in Panama because the refuse that falls from nests to the forest floor substantially amplifies decomposition rates and creates spatial heterogeneity in ecosystem processes. Here, we use this system to test whether social aggregation may be necessary in order for this amplification to take place. We predicted that increasing the level of refuse addition would more than linearly increase decomposition rates due to effects of other abiotic and biotic drivers of decomposition in this system. Such a non-linear response would suggest that increased numbers of nestmates (i.e., greater social aggregation) can influence consumer-driven links between aboveground and belowground processes.

Results/Conclusions

We found support for our prediction using an experimental study in which we added refuse in increasing amounts to randomly selected forest plots over the course of six weeks; we measured the effect of these treatments on decomposition of artificial substrates. As predicted, we found that the effect of refuse addition on decomposition followed a more-than-linear pattern for cellulose (but not for wood) over a 3-week period. These results suggest that the effects of A. trigona on belowground processes only emerge with moderate levels of refuse addition that are associated with larger nests. These findings suggest that social aggregation may sometimes be necessary for effects of consumer activities to be substantial enough to influence ecosystem processes in high-productivity systems like wet tropical forests.