Recurrent speciation rates on islands decline with species number

In an archipelagic system, species diversity is maintained and determined by the balance among speciation, extinction, and migration. As the number of species increases, the average population size of each species decreases, and the extinction likelihood of any given species grows. In contrast, the role of reduced population size in geographic speciation has received comparatively less research attention. Here, to study the rate of recurrent speciation, we adopted a simple multi-species two-island model and considered symmetric interspecific competition on each island. As the number of species increases on an island, the competition intensifies, and the size of the resident population decreases. In contrast, the number of migrants is likely to exhibit a weaker than a proportional relationship with the size of the source population due to rare oceanic dispersal. If this is the case, as the number of species on the recipient island increases, the impact of migration strengthens and decelerates the occurrence of further speciation events. According to our analyses, the number of species can be stabilised at a finite level, even in the absence of extinction.