Biological invasions: Prospects for slowing a major global change

One of the greatest ongoing ecological changes of the Anthropocene is the rearrangement of the earth’s biogeography, as people deliberately or inadvertently move species around. We have been doing this for a long time – the Lapita people from southeast Asia brought various plants and animals (including the Pacific rat) to many islands in the Pacific beginning about 3,000 years ago. However, the number of species introduced to new regions has massively increased in the last few decades as the forces that transport them have grown. From 1970 to 2010, the amount of cargo shipped by sea tripled, and it is on course to double once again by 2030. Amount of air cargo is growing even more rapidly, and passenger air travel is increasing at almost as high a rate. We could as well call the Anthropocene the Homogeocene. The array of impacts of introduced species is staggering. Most obvious are those of introduced predators on native prey (e.g., rats on many island seabird populations [Pascal, 2011]) and introduced herbivores on native island vegetation (e.g., Cronk, 1989). The spread of the Burmese python (Python molurus bivittatus) in Florida and the subsequent drastic decline of populations of prey species ranging in size from rabbits to bobcats (Dorcas et al., 2012) is a noteworthy recent example. Introduced species can compete with native species and also transmit new pathogens to them. The North American gray squirrel (Sciurus carolinensis) does both to the European red squirrel (S. vulgaris), outcompeting it for nuts and spreading the parapox virus, to which the gray squirrel is resistant and the red squirrel is highly susceptible (Rushton et al., 2006). The red squirrel population has declined drastically in Great Britain, and the gray squirrel is now spreading on the continent from an initial escape from captivity in Italy (Wauters, Tosi and Gurnell, 2002). Competition can be between unrelated species – in New Zealand, the introduced wasp Vespula vulgaris outcompetes threatened native birds for “honeydew” produced by scale insects (Beggs and Wardle, 2006). Introduced parasites and pathogens devastate native species. Examples include avian malaria, a major threat to many native Hawaiian birds (van Riper et al., 1986) after its introduction with Asian caged songbirds; crayfish plague, introduced to Europe with resistant North American crayfish and currently ravaging native crayfish (Lodge et al., 2012); and white-nose disease, a European fungus threatening several North American bat species (Turner, Reeder and Coleman, 2011). Some native species undergo a kind of “genetic extinction” from hybridization with related invaders. For instance, North American mallards (Anas platyrhynchos) introduced for hunting have extensively hybridized with native Hawaiian ducks (A. wyvilliana) and New Zealand grey ducks (A. superciliosa superciliosa)(Rhymer and Simberloff, 1996). In addition to these frequently occurring types of impacts, particular invasions can have idiosyncratic but nonetheless devastating effects. For example, the North American cane toad (Rhinella marina), brought to Australia in an ill-advised attempt at biological control of introduced beetles on cane, kills threatened native predators such as quolls when they attack the highly toxic toad (Smith and Phillips, 2006). Invasion biologists, especially recently, have explored ways in which particular invasions affect entire ecosystems (Vitousek and Walker, 1989; Ehrenfeld, 2011; Simberloff, 2011), for instance by changing nutrient cycles, fire or hydrological regimes, or physical structure. Such changes affect many species simultaneously. In Florida, invasion by Australian paperbark (Melaleuca quinquenervia) leads to more frequent, hotter lightninginduced fires, to which native plants such as sawgrass (Cladium jamaicense) and muhly grass (Muhlenbergia capillaris) are maladapted (Serbesoff-King, 2003). This invasion is thus transforming parts of the “river of grass” into patches of paperbark-dominated forest, with consequent changes in the animal community. Introduced plant pathogens that devastate the dominant native vegetation can dramatically modify an entire ecosystem. The Asian chestnut blight fungus (Cryphonectria parasitica) spread through eastern North America in the Domain Editor-in-Chief Donald R. Zak, University of Michigan