Consumer coexistence through the gleaner–opportunist trade-off in the light of sigmoidal functional responses

Abstract Species coexistence relies on ecological trade-offs, such as the gleaner–opportunist trade-off, which enables two consumers to coexist on a single shared resource through temporal resource variation and differences in their resource-dependent growth rates. The shapes of the consumers’ functional responses play a central role in this mechanism as they determine the consumers’ effect on the stability of the consumer–resource dynamics and their response to resource variation. While recent studies suggest that sigmoidal functional responses may be more common in nature than previously assumed, their impact on coexistence in the gleaner–opportunist model has not been systematically analyzed. In this study, we compared population growth models of two consumers with one or both exhibiting a sigmoidal functional response to the original gleaner–opportunist model with one consumer exhibiting a linear and the other a saturating functional response. Comparing the size of the parameter space allowing coexistence, we found that coexistence is most likely when consumers exhibit contrasting effects on and responses to resource variation, such as one consumer exhibiting a stabilizing sigmoidal and the other a saturating functional response, or one exhibiting a destabilizing sigmoidal and the other a linear functional response. Through this stabilizer–destabilizer trade-off, coexistence can also arise between two consumers that exhibit sigmoidal functional responses. Overall, sigmoidal functional responses provided the opportunity for coexistence that was of similar magnitude as in the original gleaner–opportunist model. Critically, sigmoidal functional responses allowed coexistence for less extreme handling times than the original model, emphasizing their importance when fitting models to empirical data.