A METAPOPULATION MODEL OF SPECIES BOUNDARIES

The majority, if not all, species have a limited geographic range boun ded by a distribution edge. Violent ecotones such as sea coasts clearl y produce edges for many species; however such ecotones, while suffici ent for the formation of an edge, are not always necessary. We demonst rate this by simulation in discrete time of a spatially structured fin ite size metapopulation subjected to a spatial gradient in per-unit-ti me population extinction probability together with spatially structure d dispersal and recolonisation. We find that relatively sharp edges se parating a homeland or main geographical range from an outland or zone of relatively sparse and ephemeral colonisation can form in gradual e nvironmental gradients. The form and placing of the edge is an emergen t property of the metapopulation dynamics. The sharpness of the edge d eclines with increasing dispersal distance, and is dependent on the re lative scales of dispersal distance and gradient length. The space ove r which the edge develops is short relative to the potential species r ange. The edge is robust against changes in both the shape of the envi ronmental gradient and to a lesser extent to alterations in the kind o f dispersal operating. Persistence times in the absence of environment al gradients are virtually independent of the shape of the dispersal f unction describing migration. The common finding of bell shaped popula tion density distributions across geographic ranges may occur without the strict necessity of a niche mediated response to a spatially autoc orrelated environment.