Predation-competition interactions for seasonally recruiting species.

We investigate the interacting effects of predation and competition on species coexistence in a model of seasonally recruiting species in a constant environment. For these species, life-history parameters, such as maximum productivity and survival, have important roles in fluctuation-dependent species coexistence in that they introduce nonlinearities into population growth rates and cause endogenous population fluctuations, which can activate the coexistence mechanism termed "relative nonlinearity." Under this mechanism, different species must differ in the nonlinearities of their growth rates and must make different contributions to fluctuations in competition and predation. Both of these features can result from life-history trade-offs associated with seasonal recruitment. Coexistence by relative nonlinearity can occur with or without predation. However, predation can undermine coexistence. It does this by reducing variance contrasts between species. However, when competition is not sufficient to cause endogenous population fluctuations, predation can enable fluctuation-dependent coexistence by destabilizing the equilibrium. This model also reproduces the classic finding that coexistence can occur with selective predation provided that it causes a trade-off between competition and predation. Our model is formulated for competition between annual plant species subject to seed predation, but it also applies to perennial communities where competition and predation limit recruitment to the adult population.