The climax community can propagate itself continuously and prevent the establishment of additional taxa. These are called species diversity indices. In general, there is a gradient of increasing species density from the poles to the equator and from high elevations to low elevations. Several factors are responsible for the origins of species diversity gradients. Others are responsible for maintaining these gradients.
Islands generally have fewer species.
Long time periods permit the origin of more species. Under some situations, more productive habitats favor diversity. Usually some element of disturbance or competition is involved in maintaining high density gradients. At the same time, overall environmental stability is important in maintaining diverse communities. Other important factors are spatial heterogeneity, or environmental patchiness, endemism and genetic isolation.