Cost-benefit model for the induction of an antipredator defense.

The production of neck spines by Daphnia pulex in response to the presence of predatory Chaoborus larvae entails a demographic cost as well as a benefit in reducing predation. I develop a model that quantitatively analyzes the costs and benefits of defensive spine formation in D. pulex by modifying life tables of both the spined (SM) and typical (TM) morphs of this prey to account for the effects of different levels of Chaoborus predation on population growth rate. At low Chaoborus densities the population growth rate (and thus fitness) of TM exceeds that of SM and spine formation is therefore disadvantageous in the population. Above a critical Chaoborus density, however, the opposite is true and spine formation is favored. The exact value for this critical Chaoborus density is influenced by both food availability for Daphnia and the degree of spatial overlap between predator and prey. The model predicts that spine induction is more advantageous under relatively poor food conditions, which suggests that the cost of this antipredator defense may not be an energy loss, but merely a lengthening of the developmental process in spined instars. The model also predicts that any predator or prey behavior that reduces spatial overlap between the two species in nature will make the formation of defensive spines less advantageous.