Biotic Homogenization

Humanity’s migrations across, and subsequent modification of, the landscape have had innumerable effects on the many organisms in which we share this world. Mounting evidence suggests that the dual processes of humanmediated extirpation of native species and the introduction of nonnative species have resulted in significant changes in biological diversity at different spatial scales. Global species diversity has decreased over time as a result of native species extinctions, but at regional and local scales species diversity has typically increased because the establishment of nonnative species has outpaced the loss of native species. Furthermore, species gains are frequently the result of the widespread invasion of ubiquitous nonnative species into areas containing rare, and often unique, native species. As a result, increases in local or alpha-diversity typically occur at the expense of decreased beta-diversity or increased community similarity among regions. The process by which species invasions and extinctions increase the genetic, taxonomic or functional similarity of two or more locations over a specified time interval (i.e. beta-diversity decreases over time) is called biotic homogenization (McKinney and Lockwood, 1999). Another outcome of the combined effects of invasions and extinctions is a decrease in biological similarity over time (i.e. increased betadiversity), referred to as biotic differentiation (Olden and Poff, 2003). See also: Biodiversity – Threats; Conservation of Biodiversity; Conservation Biology and Biodiversity; Species Richness: Small Scale Biotic homogenization is now considered a distinct facet of the broader biodiversity crisis having significant ecological, evolutionary and social consequences (Olden et al., 2004). Some critics, however, argue that biotic homogenization is not a new phenomenon in the earth’s history. Indeed, the palaeontological record is replete with examples of episodic mixing of biotas causing homogenization when physical barriers to movement are removed. For example, the opening of the transpolar corridor between the Pacific and Atlantic oceans and the formation of the Panamanian land bridge between North and South America during the Great American Interchange permitted the massive flux of species between formerly isolated regions. However, in sharp contrast to these episodic events, recent times have witnessed the breaching of natural biogeographical barriers from human activities at rates and distances far exceeding those observed during the history of the earth. Charles Elton (1958) was perhaps the first to recognize this phenomenon when he discussed the breakdown of Wallace’s Faunal Realms by global commerce during European settlement. In more recent decades, human activities have greatly hastened themixing process by increasing the frequency and the spatial extent of species introductions across the globe through ballastwater discharge from international shipping, bait-bucket releases associated with recreational fishing, the global pet trade, intentional translocations of wildlife for recreation purposes, biological control and inadvertent releases from aquaculture and horticulture activities.See also: Dispersal: Biogeography; Macroecology