Introduced predators catch the public’s eye when they devastate native species. The Burmese python in South Florida, for example, has caused greater than a 90 percent decline in prey species such as raccoons and opossums (Dorcas et al. 2012), and the brown tree snake has eliminated 15 of 16 native bird species on Guam (Lockwood et al. 2007). In North America, one of the most damaging invasives is the common carp, introduced as a sport fish in 1887. Carp uproot vegetation, releasing great amounts of sediment (Sorensen and Bergstedt 2011), which transforms clear lakes and wetlands into turbid messes, devastating native fishes and thousands of acres of waterfowl habitat.
More often it is plants that change entire habitats. The toxic “killer alga” Caulerpa taxifolia and its congener C. racemosa have transformed more than 15,000 hectares of seagrass meadows off the coasts of Spain, France, Italy, Croatia, and Tunisia into sterile monocultures, devastating local fisheries (Meinesz 2001, Klein and Verlaque 2008). Likewise, in Florida, forests of Brazilian pepper and Australian paperbark trees have replaced some 200,000 hectares of native sawgrass and muhly grass prairies (Schmitz et al. 1997).
Some introduced species appear to benefit conservation by providing habitat or food for rare species (Schlaepfer et al. 2011). However, what looks benign can often turn out to be harmful. In Spain, for example, the Louisiana red swamp crayfish was lauded for enhancing populations of kites and other predatory birds (Tablado et al. 2010), yet roosts of these birds have now killed hundreds of ancient oaks (García et al. 2011).
Native species occasionally cause problems, but this happens far less frequently. In the United States, an introduced plant species is 40 times more likely to become problematic than a native species, and the few natives that do become harmful are almost always associated with some human activity, such as overgrazing or fire suppression (Simberloff et al. 2012). To paraphrase Aldo Leopold, eons of evolution adapt native species to their environment, while newly introduced species lacking this coevolutionary history are likely to disrupt the ecosystem (Leopold 1942).
Troubling Time Bombs
The long presence of an apparently innocuous introduced species gives no assurance that it is risk free. Many non-native populations remain small and restricted for decades or longer before exploding across the landscape with devastating impact (Crooks 2005). Introduced plants in Europe, for example, take 150 to 400 years to reach a stable geographical limit (Williamson et al. 2009, Gassó et al. 2010). Such time lags can generate an “invasion debt,” a burden of damage that introduced species may cause far into the future (Essl et al. 2011).
In addition, some formerly innocuous non-natives become invasive because of subtle environmental changes (e.g., in climate or hydrology), or upon introduction of another species that facilitates the invader’s spread — the phenomenon of invasional meltdown (Simberloff 2006). In South Florida, for example, the long-harmless Chinese banyan (Ficus microcarpa) became invasive after introduction of its host-specific pollinating wasp (Parapristina verticillata) (Kauffman et al. 1991). Recent molecular genetic research shows that several invaders, such as common reed and reed canarygrass, were innocuous for long periods, only to become highly damaging upon subsequent introduction of new genotypes (Saltonstall 2002, Lavergne and Molofsky 2007).
Biologists who study non-native invasions have also detected many ecosystem-wide impacts that are subtle and gradual but nonetheless harmful. For example, some invasive plants increase concentrations of nitrogen (Yelenik et al. 2004) or phosphorus (Turner et al. 2008) in soils, impacting belowground species such as microorganisms and small invertebrates (Loo 2009), and fostering invasion by other introduced species previously restricted by nutrient-poor soil (Vitousek 1986).
In sum, any new introduction warrants concern. Proposals for deliberate introductions should be intensely scrutinized, and strong action should be taken to stem newly detected invasions. Success is certainly possible. Dramatic new technologies are leading the way, such as pheromones to control carp (Fine and Sorensen 2008) and microbeads to manage zebra mussels (Aldridge et al. 2006). There is every reason to expect more successes if we continue to take the threat seriously. The precautionary principle mandates no less.
Daniel Simberloff is the Gore Hunger Professor of Environmental Science, Ecology, and Evolutionary Biology at the University of Tennessee in Knoxville.