Neutral processes and reduced dispersal across Amazonian rivers may explain how rivers maintain species diversity after secondary contact

Abstract

Amazonian rivers are only partial barriers to the dispersal of most species, but they still form the limits between the distributions of many similar species. We show that two competitively-identical species may remain allopatric for hundreds of generations when a river only reduces the chance of a species crossing it. To illustrate this, we developed a two-dimensional cellular automata for two allopatric species under neutral-theory dynamics and recorded the time required for the first extinction of a species and the frequency with which it occurred across replicate simulations. Our results indicate that neutral processes associated with reduced dispersal across rivers can maintain competitively-identical species allopatric for hundreds of generations despite repeated river crossings. These cross-river incursions were rarely successful owing to the low likelihood of a rare invader outcompeting resident populations. This process provides a plausible mechanism for the maintenance of Amazonian biodiversity and may explain the spatial-distribution limits of species caused by large rivers in the Amazon that are not absolute barriers to dispersal.