Ecology in Action

The tremendous biological diversity of some plant communities may be a reflection of the variety of direct and indirect interactions that plants have with predators, competitors, and mutualists. Ecologists have several ways of measuring biological diversity; some diversity indices, such as the Shannon index, integrate species richness and evenness. Alpha diversity measures species richness within an area, beta diversity measures species turnover, while gamma diversity is the combined species richness of all communities under consideration. Biotic and abiotic factors can influence community diversity directly and indirectly. For example, in southwest Finland, host plant abundance directly and positively influenced lepidopteran species abundance. In the African savanna, herbivorous mammals indirectly and negatively affected bird diversity by consuming trees and reducing the abundance of insects that served as food for the birds. Abiotic factors influencing community diversity include the type of habitat, geological heterogeneity, nutrient levels, and the type and intensity of disturbance. Ecologists predict that diverse communities will be more stable than less diverse communities, but that the populations of species in diverse communities will be less stable.