Niche diversity refers to the evolutionary process in which species diverge in order to take advantage of open niches in their environment. This can be seen in areas where there are many very similar species that have radiated from a common ancestor.
In areas with high niche diversity, the rate of evolution of new species tends to slow down as the niches in that environment are filled up. This is because niche diversity is driven by small populations of an ancestor species evolving to take full advantage of a very specific set of environmental conditions. The large number of cichlid fish species in African lakes are due to niche diversity as each species occupies only a very small and specific range within the lake.