Ecologists have long observed that predation can maintain the diversity of competing species. Much work has been directed towards identifying mechanisms to explain such observations, and one necessary condition has emerged from these studies—species’ competitive ability must be negatively correlated with sensitivity to predation. Direct evidence for this competition–defense tradeoff is limited, so we performed a meta-analysis of 36 studies to systematically evaluate the available evidence for the tradeoff and its influence on communities. Using a novel approach, we inferred competitive ability and sensitivity to predation by quantifying species’ responses to experimental resource addition and consumer removal, respectively, and then combined the correlation between these responses across studies. We also asked if this correlation is related to the diversity of natural communities and if it can predict how communities respond to fluctuations in resource availability and consumer pressure.

Results/Conclusions

There was little evidence for a competition–defense tradeoff at the community level; instead we found that competitive ability and sensitivity to predation were positively correlated overall. Allocation-based tradeoffs, such as the competition–defense tradeoff, require that all individuals divide an equal quantity of resources among multiple tasks, but in different ways. This assumption of equal resources may not be met when individuals are of different species. However, the correlation between competition and defense can still influence species diversity and modify communities’ responses to fluctuating abiotic or biotic conditions. Evenness was higher in those communities with a tradeoff, and resource addition and consumer removal reduced diversity because some species were no longer held in balance by their limiting factors. In contrast, in communities with a positive competition–defense correlation, evenness was lower and resource addition and consumer removal enhanced diversity by differentially benefiting species that are both poor resource competitors and poor at withstanding predators. In addition to revealing these general patterns, this study serves to remind ecologists that it is important to critically evaluate how organismal traits scale up to emergent community-level properties because species-level phenomena do not always extend to higher levels of organization.