When to defend: antipredator defenses and the predation sequence

Some authors have suggested that prey species stand to benefit most by defending as early as possible during predator-prey encounters, but species in nature employ antipredator defenses at various stages of interactions with their predators. Whether it is generally most advantageous to defend early or late during such encounters is an open theoretical question. We model conditions under which a prey species might evolve early or late defenses in response to predation. Adapting a two-prey, one-predator Rosenzweig-MacArthur system of differential equations, we analyze the effects of modified antipredator defenses (and their associated costs) on the ability of a new prey type to invade the one-prey, one-predator limiting system at equilibrium. We show that the outcome, in terms of invasion potential, is crucially dependent on the ratio of the prey's proportional population growth rate to the cost of predator encounters.