Migration and niche partitioning simultaneously increase species richness and rarity

To understand species abundance distributions effectively, it is essential to consider the forces which create the overwhelmingly long 'tail' of rare species. Both neutral and niche theory have been invoked to explain the rarity of certain species in ecological assemblages: niche theory stresses the importance of the resource division process, while neutral theory focuses on stochastic drift in densities of competitively identical species. Here, we show how migration and niche partitioning can act simultaneously to increase both the number of rare species and the total species richness. By simulating the dynamics of a metacommunity using an interconnected network of local communities, we incorporate niche partitioning into the otherwise per capita equivalent processes of birth and death. Niche partitioning is defined by the species niche fitness function, which is measured by the species optimal environmental conditions, the fundamental niche breadths of each species, and by the breadth of the environmental gradient of each local community. For a given niche breadth at the local community scale, unimodal relationships appear between the rate of migration, the number of rare species, and the total species richness. At the metacommunity scale, species richness was at first unaffected by increasing migration, but then crashed. At wide niche breadth, both the number of rare species and the total number of species started to decline at a low rate of migration. In contrast, when niche breadth was narrow, neither the number of rare species nor the total number of species started to decline until migration rates were much higher. This is because, for a given migration rate, niche breadth can affect the proportions of individuals coming from adjacent local communities. The integration of niche partitioning and migration therefore provides a rational explanation for the widespread rarity of species in ecological communities.