Dr. Pangloss restrained by the Red Queen - steps towards a unified defencetheory

Animals and plants defend themselves against a variable community of biolog ical enemies. We argue that the effectiveness of allocation to defence (the success of defence per unit allocation) may be expected to decrease as the diversity of attack types increases, and asked how the optimal allocation to defence covaries with the effectiveness of defence. Variation in effecti veness links optimal defence to coevolutionary processes the prime characte ristic of coevolutionary interactions is that they promote and maintain gen etic variation in both hosts and their enemies, leading to variation in the effectiveness of defence. We present a simple model suggesting that as eff ectiveness decreases, the fitness benefit of defence disappears. In other w ords, when effectiveness is low, the optimal strategy is to tolerate damage . As effectiveness increases, the optimal allocation flips rapidly from no- defence (tolerance) to high allocation to defence, and then decreases at a decelerating pace as effectiveness increases. We conclude that diversifying coevolution; as it covaries with the effectiveness of defence, constrains the evolution of optimal defence strategies and may be a very important com ponent in determining the optimal allocation to defence and variation in th e success of defence as it is seen in the wild.