BIOTIC FACTORS
Biotic factors also affect species richness. An ecotone is a transitional zone where two distinct communities meet. It contains most of the ecological niches of each community plus its own unique niches. This edge effect results in increased species richness at the communities' margins.

If one species in a community dominates and outcompetes other species, this lowers species richness. Removal of the dominant species increases the diversity of the other species.

Geological history also affects species richness. Older, more stable habitats such as a topical rain forest enjoy greater species richness than a recently disturbed area. The time hypothesis explains that recently glaciated regions have lower species richness.