The argument that introduced species should not be judged on their geographic history ignores biological reasons why non-native organisms pose a particular threat. Unlike native species, non-natives arrive in their new homes with no shared evolutionary history with most other organisms already there. They leave their predators, parasites, and competitors behind, and they often attain huge population sizes, occupy large areas, and heavily impact native species and ecosystems.
Examples abound. Non-native Brazilian pepper changed south Florida’s landscape, shifting prairie to forest. The non-native fire tree altered nitrogen cycling in the Hawaiian Islands. Non-native cheatgrass modified fire regimes in the western United States by fueling more frequent and intense fires that increase its own populations while suppressing native grasses further. The non-native brown tree snake extirpated the forest birds of Guam, and the non-native emerald ash borer threatens to eliminate eastern North America’s ash trees. Invasion by non-native acacia and pine in the South African fynbos, a treeless ecosystem, not only threatens a biome with one of the highest endemism levels on Earth, but also the water supply for irrigation and domestic use—left unchecked, these invasive trees would cause an average loss of 30 percent of the water supply after 100 years.
In their opinion published earlier this month in The Scientist, Matthew K. Chew of Arizona State University and Scott P. Carroll of the University of California, Davis, suggest “excluding potentially harmful pests” is a valid goal, but that, in time, some non-natives will co-evolve to ‘fit in’ as new—and potentially beneficial—inhabitants. Our research suggests otherwise: surveying records in the United States, we found six times more non-native plant species than native species became ecologically problematic. Clearly, species origin is relevant to conservation. By biological default, non-native species should be viewed as “potentially harmful pests,” and it is prudent to employ the precautionary principle—both before and after non-native species have established populations. Well-documented phenomena such as lag times (where non-native species are long present in low numbers until some mechanism promotes their rapid expansion) and subtle, important impacts such as changed nutrient regimes argue against a “wait and see” approach. Non-natives often arrive with distinct advantages over natives, free from their hometown natural enemies, and are likely to evolve more competitive characteristics, becoming more aggressive invaders. One of the worst possible strategies would be to decrease our vigilance or discount origin in judging threat. Conservationists should beware of non-native species, particularly those with a bad rap for being invasive elsewhere.
Though Chew and Carroll approve of “programs aimed at preventing introductions or eradicating populations of introduced species when it can be done in a dependable, highly targeted manner,” they assert that many efforts against established non-natives are “permanent hopeless wars.” We would point instead to dramatic successes in cases that naysayers initially deemed unwinnable. Consider rats—adaptable, fecund critters that have hitchhiked on ships for centuries and were introduced to Pacific islands as food. Rats are legendary for devastating island biota worldwide. With little experience avoiding predators, naïve island vertebrates such as threatened birds are easy prey for the rodents. The threat was so dire that a few intrepid managers undertook the “impossible” beginning in the 1960s.
Results were unspectacular at first, but a simple revolution in how baits were deployed produced many successes. Conservationists can now boast successful rat eradications on 284 islands worldwide, including several islands around 10,000 hectares in size. Many studies document how native species depressed by rat predation have dramatically rebounded, and in some instances refugia provided by rat-free islands are the only hope for preventing native species extinction.
Eradications of increasing scope do not end with rats. With new technologies and well-planned campaigns, destructive feral goats have been removed from half of the Galapagos Islands, and feral pigs from Santiago, a rugged 585-square-kilometer island in the archipelago. Such feats would have been considered pipe dreams only a decade ago.
Many other invasive species, though not eradicated, have long been controlled at low abundances, thanks to campaigns once discouraged as quixotic wastes of resources. In California, massive European beachgrass stands, replacing native dune plants in a Nature Conservancy reserve, were viewed as an intractable problem because of the Conservancy’s then-policy against using herbicides. A persistent land manager cleared the invasion manually at low cost using convict labor and state public works employees. With most beachgrass eliminated, native vegetation flourishes. In many Kentucky nature preserves, what had seemed an unmanageable invasion of musk thistle has been well controlled for a decade by a land manager who defied skeptics and attacked the problem with the help of volunteer DUI offenders.
Despite these successes, invasion biologists rarely prescribe intensive management for most introduced species, in contrast to Chew and Carroll’s efforts to caricature researchers as agitators for destruction of our study organisms. This is readily apparent from the 336 research articles comprising the 2010 volume of the journal Biological Invasions: 87 percent (n=293) of these papers focus on the biology of non-native species while only 13 percent (n=43) concerned management. Of the 293 papers on biology, 25 percent (n=73) did promote management, but the remaining 75 percent did not address whether management was needed.
Let us be clear, invasion biologists have greatly advanced understanding of the invasion process and ramifications of allowing invaders to persist. Conservationists have shown that we can achieve monumental successes in eradicating invaders and mitigating their impacts. Recent attacks on invasion biology in high-profile academic journals and popular science media will deter continuation of this progress in the study of invasions, as well as our ability to manage them. One of the few areas where all would agree, put best in Chew and Carroll’s own words, is “ecologists, policy makers and conservationists should work to exclude potentially harmful pests.” This goal is achieved by in-depth research on when, how, and why non-native species become detrimental—as invasion biologists are currently doing—and not by raising specious arguments.
Sara Kuebbing is a doctoral candidate at the University of Tennessee where she studies plant invasions. Previously she coordinated an invasive plant outreach and management program for the Vermont Chapter of The Nature Conservancy. Dan Simberloff is an ecologist at the University of Tennessee who began studying invasions of insects, plants, and vertebrates in the 1970s. He is editor-in-chief of Biological Invasions and senior editor of the Encyclopedia of Biological Invasions. Julie Lockwood is an ecologist at Rutgers University. She is interested in species invasions and extinctions, and how these two forces interact to reshape biodiversity.
Other authors in this article include the following University of Tennessee researchers: M. Noelia Barrios-Garcia, Emmi Felker-Quinn, Martin A. Nuñez, Mariano A. Rodriguez-Cabal, Lara Souza, and Rafael D. Zenni.