Disturbance and Diversity in Tropical Rain Forests: The Density Effect

A null model for the effects of treefall gaps on tree species diversity of tropical rain forests is based on the well‐known relationship between abundance and diversity: habitats supporting larger numbers of individuals can support more populations and more species than habitats supporting small numbers of individuals. Because seedling establishment and sapling density increase following canopy opening, gaps are often also sites of high species diversity. Both tree density and diversity are likely to be greater in areas of high stand‐turnover rates. Thinning and non‐catastrophic mortality reduce both density and diversity in areas of low gap frequency. Nevertheless, natural and anthropogenic disturbances, such as logging, have been hypothesized to both enhance and limit species diversity through changes in habitat heterogeneity, shifts in competitive balances among species, and creation of otherwise‐rare habitats. Species‐accumulation curves can be used to evaluate these alternatives by comparing observed species‐abundance patterns with expected changes in diversity due to density effects alone. Topographic variation in density and diversity of shrubs and small saplings at the La Selva (Costa Rica) Biological Station preserve was analyzed as an example. The total number of species for each of 18 sites was estimated by fitting a two‐parameter hyperbola (similar to the Michaelis‐Menten equation) to sample data using a maximum‐likelihood method. Although both density and diversity were higher on slopes than on ridges, total species richness was nevertheless higher on slopes than is predictable from density effects alone. High stand‐turnover rates on slopes may enhance tree species diversity through increases in tree density and in habitat heterogeneity.