Changes in predator diversity via species invasions and extinctions drastically change ecological communities. However, it remains difficult to predict the ecosystem -level effects of these changes. One current limitation is that the effect of consumer diversity is highly dependent on predator and prey traits. Consequently, the functional diversity of consumers is often more important than species richness. However, current approaches for quantifying functional diversity are either descriptive (e.g., morphology) or idiosyncratic to systems (e.g., phylogenetic distance) and are not linked to relevant consumer-resource theory that broadens our ability to predict how changes in diversity will impact ecosystem function. Here, we propose a functional diversity metric that is broadly applicable and improves our predictive capacity. Specifically, we quantify functional diversity based on the parameters of predator functional responses because they define the strength of predator-prey interactions,. To test its effectiveness, we simulated the evolution of traits (e.g. body size and functional traits) and predation. We compared the relative predictive value of different metrics of functional diversity to determine when different metrics are most useful in predicting the effects of predator diversity.
Results/Conclusions
Our proposed metric, based on relevant consumer-resource models, generally provided better predictive power than metrics based on a trait such as body size or a phylogenetic distance metric. These differences were most pronounced in the absence of strong coevolution among traits, where only the metric tightly linked to consumption performed well. However, differences between metrics were negligible when the trait evolution was tightly correlated. When traits coevolve, the variation observed in a single trait, such as body size, provided nearly as much predictive power as our proposed functional diversity metric. Therefore while other metrics provide some insight, our proposed functional diversity metric regularly provides the highest amount of predictive power. Given the role of functional responses in linking predator and prey population dynamics, using an approach that explicitly uses these parameters as traits will enhance our ability to predict how the removal or addition of a predator will affect prey community structure and potentially how such changes in the predator assemblage will cascade down to affect lower trophic levels and ecosystem function.