Competitive exclusion and coexistence of species with complex life cycles.

Complex life cycles are life histories in which abrupt ontogenetic transformations and niche shifts occur at the transition between stages. The effects of this niche differentiation between stages on coexistence between species are investigated using a simple discrete model of two-stage populations. The model incorporates exploitation competition for limiting resources within stages, between stages, and between species. While species with simple life cycles can never coexist at equilibrium, stable coexistence is shown to be possible between species with complex life cycles provided that (1) one species is more efficient in resource utilization at low resource abundance in the larval stage while the other is more efficient at low resource abundance in the adult stage; and (2) each species is mainly limited by that stage which is less efficient at low resource abundance. Stable coexistence is somewhat easier between a species with a simple life cycle and one with a complex life cycle. It requires that (1) the species with the simple life cycle should not be decidedly more efficient than that with the complex life cycle in utilizing the resource on which it lives; and (2) the main resource limitation for the species with a complex life cycle should occur in that stage which escapes competition with the species with a simple life cycle. Lastly, a complex life cycle can offer a decisive competitive superiority over a simple life cycle in interspecific competition, which suggests that competition can be a driving force of the evolution of complex life cycles.