Predicting and mitigating the impacts of non-native taxa on native communities is the fundamental goal of invasion biology. The comparative functional response (CFR) framework has been recently proposed as a unifying approach to address this challenge. This approach uses short-term experiments to quantify functional responses of invaders and functionally similar native taxa on a common prey or resource and posits that invaders with "higher" functional responses relative to native species will have larger impacts on the recipient ecological community. This approach appears to be gaining traction among some invasion biologists; however, the underlying logic has not been connected to basic ecological theory. Here we examine the conceptual foundations of the CFR framework and recent derivatives [e.g., Relative Impact Potential (RIP), Relative Control Potential (RCP) indices] within the context of basic consumer-resource theory. Specifically, we use Lotka-Volterra consumer-resource and resource-ratio competition theory to examine the sensitivity of resource equilibrium, consumer-resource stability, and consumer competition to having a higher Type II functional response (i.e., higher attack rate, lower handling time, or combination thereof) relative to changes in other model parameters (i.e., conversion efficiency, consumer background mortality).
Results/Conclusions
Increasing attack rates or decreasing handling times reduces resource equilibrium abundance and decreases stability. Further, assuming exploitative competition for a single limiting resource, R* theory predicts that the consumer with the lowest resource equilibrium will displace other competitors. Thus, consumers with higher functional responses have greater impacts on resources over dynamical time scales, reduce system stability, and are expected to be competitively dominant. Viewed from within this simplified framework, CFR predictions can be linked to classic ecological theory. However, the emphasis of the CFR approach on the functional response may be misplaced. Our results demonstrate that resource equilibrium and the outcome of competitive interactions are more sensitive to changes in conversion efficiency and background mortality than changes in functional response parameters. These findings highlight the benefits of evaluating the assertions of the CFR approach in the context of basic ecological theory and the weaknesses of focusing narrowly on the functional response when trying to understanding the impacts of invaders.