Dispersal can sharpen parapatric boundaries on a spatially varying environment

Parapatry describes a geographic pattern in which the ranges of two species have separate but contiguous distributions without any physical barriers b etween them. We present results from a study of ecological mechanisms to ex plain parapatry in closely related species. These include competition, spat ially varying performances, and dispersal that depends on the densities of both species. We use a model consisting of two coupled nonlinear reaction-d iffusion equations with density-dependent diffusion terms and space-depende nt Lotka-Volterra-like competing interaction terms. The model is analyzed b y using a mixture of phase-plane analysis and numerical simulations. Results show that competition and dispersal can lead to completely segregat ed species ranges. Spatial variation favored and provided stability to para patric distribution. Parapatry occurred under several conditions, including when both species were identical in dispersal, intrinsic rate of growth, a nd competition but differed in their spatial performances. Results indicate that overlapping distributions and parapatry are equally expected for clos e species. Moreover, similar species in parapatry tend to exhibit equivalen t range sizes. This model explains how species can coexist regionally while maintaining spatial exclusion. It also describes how a species that is rar e in distribution can invade the range of a similar and widespread species. We discuss the limitations of using present species distributions for reco gnizing modes of speciation, and we suggest studying more extensively the r elationship between density-dependent dispersal and interspecific competiti on. We show that density-dependent dispersal can favor segregation.