<P> Biological diversity is dependent on natural disturbance . The success of a wide range of species from all taxonomic groups is closely tied to natural disturbance events such as fire, flooding, and windstorm . As an example, many shade - intolerant plant species rely on disturbances for successful establishment and to limit competition . Without this perpetual thinning, diversity of forest flora can decline, affecting animals dependent on those plants as well . </P> <P> A good example of this role of disturbance is in ponderosa pine (Pinus ponderosa) forests in the western United States, where surface fires frequently thin existing vegetation allowing for new growth . If fire is suppressed, douglas fir (Pesudotsuga menziesii), a shade tolerant species, eventually replaces the pines . Douglas firs, having dense crowns, severely limit the amount of sunlight reaching the forest floor . Without sufficient light new growth is severely limited . As the diversity of surface plants decreases, animal species that rely on them diminish as well . Fire, in this case, is important not only to the species directly affected but also to many other organisms whose survival depends on those key plants . </P> <P> Diversity is low in harsh environments because of the intolerance of all but opportunistic and highly resistant species to such conditions . The interplay between disturbance and these biological processes seems to account for a major portion of the organization and spatial patterning of natural communities . Disturbance variability and species diversity are heavily linked, and as a result require adaptations that help increase plant fitness necessary for survival . </P>

How does equilibrium play a role in the environment