Back to the drawing board: re‐thinking growth–defense tradeoffs

Abstract

Well‐established ecological equations such as logistic growth may carry implicit assumptions that are not immediately obvious. When such equations are adapted for a different context, missing awareness of their hidden assumptions can confound our conclusions. We demonstrate how this has happened in predator–prey models with rapid adaptation of prey defense to variable predation risk, where costs of defense typically manifest as a growth‐defense tradeoff. Assuming logistic prey growth, this tradeoff is often implemented as a reduced intrinsic growth rate, while the carrying capacity is unaffected. We show that this assumption is not only extremely ecologically unrealistic, but also results in highly problematic model behavior and destroys any potential for eco‐evolutionary or biomass‐trait feedbacks. To reconcile the problematic results of this assumption with its ubiquity in existing models with rapid adaptation, we conducted a literature survey of models on prey adaptation or predator–prey co‐adaptation. This reveals that many models, despite describing a tradeoff affecting only the intrinsic growth rate, do incorporate an additional cost affecting the carrying capacity, but in a way that is implicit and not acknowledged. It is this cost that drives the famous feedbacks between defense adaptation and predator–prey dynamics. Finally, we discuss the implications for existing theory on eco‐evolutionary dynamics, and provide guidelines for implementing growth‐defense tradeoffs in a more realistic way.