The Ecological Consequences of a Fragmentation Mediated Invasion: The Argentine Ant, Linepithema humile, in Southern California

Habitat fragmentation can facilitate species loss through a number of processes. The remaining habitat may only sample a subset of the existing fauna (Preston 1962).Alternatively, species may become locally extinct within the remaining habitat after fragmentation (Bolger et al 1991). Small populations associated with the reduction in total area may become sensitive to stochastic processes, both demographic (Shaffer 1981; Gilpin and Soulé 1986; Goodman 1987) and environmental (Shaffer 1981). Fragmentation may reduce colonization, or immigration into the habitat remnants, especially by less mobile species (Turner and Corlett 1996). Deterministic processes associated with detrimental edge effects (Wilcove 1985; Yahner 1988; Robinson et al. 1995) and the loss of landscape level processes, such as fire regimes (Leach and Givnish 1996), also cause local extinctions post fragmentation. Fragmentation may also facilitate the immigration or invasion of exotic species that may directly compete with, prey upon, parasitize or otherwise indirectly affect native species (Diamond and Case 1986). Some of these processes are correlated. For example, an increase in edge around a habitat fragment will facilitate the penetration of exotic species that may be prominent along those edges. This is particularly a problem in urban landscapes because many successful exotic species are associated with human-mediated disturbance (Elton 1958; Fox and Fox 1986; Orians 1986; Petren and Case 1996). Exotic ants have been inadvertently introduced worldwide (Williams 1994). Many species, such as the big headed ant (Pheidole megacephala), the little fire ant (Wasmannia auropunctata), the imported fire ant (Solenopsis invicta) and the Argentine ant (Linepithema humile, formerly Iridomyrmex humilis) have wreaked ecological havoc where they have become established (Williams 1994). Most of the impact of these species has been measured in loss of native ant diversity, which is greatly reduced or completely eliminated