Room for one more? Evidence for invasibility and saturation in ecological communities

Abstract

Identifying the contributions of local and regional processes in shaping biological communities is key to understanding global patterns of species richness (Cornell 1999, Gaston 2000). Ecologists have typically fallen into two traditions to explain variation in diversity, one emphasizing local processes such as competition, predation, mutualism and the interaction between organisms and their environment, and the other focused on speciation, extinction and dispersal over broad regional scales (Terborgh and Faaborg 1980; Cornell 1985, 1993, 1999; Cornell and Lawton 1992; Ricklefs 1987, 2004). The local school of thought posits that species richness in communities is determined by processes that influence demographic rates within habitat patches (Chase and Leibold 2003). The regional approach argues that dispersal, speciation, extinction and the history of community assembly are of primary importance in shaping biological communities (Ricklefs 1987, 2004, Caswell and Cohen 1993, Gaston 2000, Gaston 2003, Mora et al. 2003, Fukami 2004, Smith et al. 2004, Smith and Bermingham submitted). The critical distinction between the two approaches hinges on whether local species richness is saturated, or whether communities have the intrinsic capacity to support more species than they actually contain. If the supply of species through colonization or speciation is much greater than the number that can coexist due to local ecological constraints, then communities are said to be saturated and under strong local control (Cornell and Lawton 1992). Alternatively, if speciation and dispersal provide fewer species than sites can support, then regional control over communities is dominant and local richness is unsaturated. The question of whether communities are near or far from saturation is critical to our understanding of patterns of the distribution and diversity of organisms. © 2006 Springer. All Rights Reserved.