Long-term effects of inducible defense

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Abstract
In laboratory populations and in a model of prey population growth, the freshwater ciliate prey Euplotes octocarinatus (Carter) showed long-term advantages from predator-induced defense, whereas the fitness of the turbellarian predator Stenostomum sphagnetorum (Luther) was diminished. The population growth rate (r) of E. octocarinatus stayed positive (r = 0.2/day) despite the abundant presence of the predator until other factors limited further growth, but r = 0.6/day without predators. Inducible defense may contribute to destabilizing the prey’s numerical population dynamics in ponds. Stenostomum was able to feed and to grow on E. octocarinatus as the only prey, but this prey was considerably less available to the defense-inducing predator than a non-defended prey. Stenostomum has the ability to feed intensely on Euplotes for a few hours until the ciliate develops a defense (Kusch, 1995). During 40 days of cultivation, the predator’s population growth rate was 0.05-0.08/day with a surplus of Euplotes as the only prey, compared to a growth rate of r = 0.09-0.14/day with the non-defended prey Colpidium campylum (Stokes). The predator therefore has a disadvantage from induced defense in the prey, whereas competitors that can feed on larger prey are favored. The evolution of inducible defense should cause a coevolution of predators, who can either suppress recognition or overwhelm the defenses of the prey.