Species Richness and Resistance to Invasions

Abstract

Traditionally, tropical forests, and especially tropical rain forests, have been contrasted with extratropical communities in terms of their species diversity and stability (Elton 1958). Unfortunately, ecologists have used the word’ stability’ to mean several different things (Orians 1975; Harrison 1979; Pimm 1984): Resilience can be defined as a rate of return of population densities, community composition, or collective properties like biomass production, to conditions preceding a perturbation. Persistence usually means how long presence of individual populations or community composition last. Resistance means the degree to which a variable of interest remains unaltered following perturbation. Constancy usually means a lack of change (low variability) of variables of interest over time. Finally, systems are defined as stable in a narrow sense if, and only if, variables of interest return to their initial (equilibrium) values, following perturbation. Elton (1958) himself switched between different meanings of stability when he talked about absence of insect outbreaks in tropical forests (high population constancy) and about higher frequency of extinctions and invasions in simple communities (low persistence). Elton suggested that species rich communities like tropical rain forests possess “complex systems of checks and buffers” responsible for their stability. Causal positive connections between biotic diversity and low variability or high persistence of tropical comunities have been questioned many times since Elton’s influential book was published (Futuyma 1973; Farnworth and Golley 1974; Leigh 1975; Wolda 1978, 1983; Maury-Lechon et al. 1984). Elton, however, should be prized for drawing the long-lasting attention of ecologists to relationships between diversity and stability in ecological systems.