Humans have accomplished an unprecedented redistribution of the earth's living things. Both incidentally and deliberately, through migration, transport, and commerce, humans are continuing to disperse an ever-increasing array of species across previously insurmountable environmental barriers such as oceans, mountain ranges, rivers, and inhospitable climate zones. Among the most far-reaching consequences of this reshuffling is a sharp increase in biotic invaders - species that establish new ranges in which they proliferate, spread, and persist to the detriment of native species and ecosystems. In a world without borders, few if any areas remain sheltered from these wholesale immigrations, and for some areas such as oceanic islands, the trend has become an onslaught.
Biotic invasions can occur when organisms are transported to new, often distant, ranges where their descendants proliferate, spread, and persist. In a strict sense, invasions are neither novel nor exclusively human-driven phenomena. But the geographic scope, frequency, and the number of species involved have grown enormously as a direct consequence of expanding transport and commerce in the past 500 years, and especially in the past 200 years. Few habitats on earth remain free of species introduced by humans; far fewer can be considered immune from this dispersal. The species involved represent an array of taxonomic categories and geographic origins that defy any ready classification (Crawley, 1998).
The adverse consequences of biotic invasions are diverse and inter-connected. Invaders can alter fundamental ecological properties such as the dominant species in a community and an ecosystem's physical features, nutrient cycling, and plant productivity. The aggregate effects of human-caused invasions threaten efforts to conserve biodiversity, maintain productive agricultural systems, sustain functioning natural ecosystems, and also protect human health. We outline below the epidemiology of invasions, hypotheses on the causes of invasions, the environmental and economic toll they take, and tools and strategies for reducing this toll (D'Antonio, 1992).
