The number and kinds of species within a community, its biodiversity, are affected by several factors. Besides the role of competition and predation, as noted earlier, the level of disturbance is important. The intermediate disturbance hypothesis states that diversity is highest with a moderate level of disturbance. For example, in a prairie community, fires that occur too frequently or too infrequently lead to lower species diversity. Fires that occur at an intermediate level, every 3-6 years, prevent the dominance of certain species that would reduce diversity. In other cases, communities with intermediate disturbance have higher diversity because they have a mix of both early and late successional species.