Conceptualizing and Evaluating Non-Native Species

Non-native ("exotic" or "alien") species are typically viewed as human-introduced, invasive species that harm local ecosystems and economies. A paradigmatic example is Kudzu (Pueraria lobata), a prolific Japanese vine that can grow a foot a day. Introduced as a porch plant and then planted to reduce soil erosion, it now smothers millions of acres of trees in the American south. Or consider Zebra mussels (Dreissena polymorpha), which traveled from southeast Russia in ship ballast water and entered the Great Lakes of the United States. Within a dozen years they had spread to most aquatic ecosystems in the eastern U.S., clogging water intake pipes and causing annual damage in the hundreds of millions of dollars (Pimentel 2005). The assumption of most environmentalists and managers of natural areas is that non-natives should be controlled or eradicated to protect native ecosystems, species, and human interests.

There is also a diametrically opposed attitude toward non-native species, perhaps provoked by the ironic sight of environmentalists uprooting beautiful vegetation (such as purple loosestrife, Lythrum salicaria), poisoning fish, and even shooting charismatic mammals (such as mountain goats, Oreamnos americanus) from cliffs in an attempt at exotic removal. From this perspective, the antipathy toward non-natives species is based on scientific misinformation and is reinforced by the common prejudice against things foreign (Brown & Sax 2004, Davis et al. 2011). Some argue that nativeness is a confused and deceptive concept that confers no special value on a species, and that attempts to remove exotics and restore natives are generally misguided (Chew & Hamilton 2011).

Before we can decide how to evaluate non-native species, we must sort through competing ideas about what they are. The geographical-evolutionary view identifies non-natives as species that did not evolve in a particular area. This idea is reflected in the thought that native Coloradans, for example, are only those born and raised in the U.S. state of Colorado. This characterization ignores the extent to which species move around. Species evolve in one locale, then frequently migrate or expand their range to other places, and thrive for thousands of years well-adapted in their new homes. In some regions, more than half the native taxa resulted form historic colonization by invaders (Brown & Sax 2005). The evolutionary origin idea of nativity implausibly suggests that only humans living in Africa are native and that virtually all species on volcano-formed oceanic islands (originally barren of life) are exotics.

Non-native species are frequently identified as human-introduced species. While it is likely that the vast majority of the non-native species of concern today have been introduced by humans, this identification ignores that species move into radically different habitats on their own. For example, vagrant birds frequently travel between North America and Europe (Cassey et al. 2005). These species are foreign to their new habitats despite not being human-introduced. Furthermore, it seems implausible to consider that a seed arriving in a new habitat stuck to a human boot produces an exotic plant, while if it hitchhiked on the foot of a bird, it would be a native. The human-introduced criterion also has trouble explaining why human restoration of a species counts as native restoration rather than exotic introduction. When humans introduced wolves (Canis lupus) back into Yellowstone National Park in the 1990s the wolves were natives to that habitat, despite the fact humans trucked them in 500 miles from Canada. Non-native species are often assumed to be damaging invasive species that outcompete natives, unbalance local ecosystems, and threaten biodiversity. According to one study, 42% of threatened or endangered species on the U.S. Endangered Species lists are at risk primarily because of alien invasive species (Pimentel 2005). Others demure from the view that non-natives typically threaten biodiversity: "Recent analyses suggest that invaders do not represent a major extinction threat to most species in most environments — predators and pathogens on islands and in lakes being the main exception" (Davis et al. 2011). Some argue that exotics generally increase local and regional diversity while decreasing global diversity through homogenization (Brown & Sax 2005). There is little doubt that some exotics have caused massive amounts of damage both to human interests (e.g., Zebra mussels) and to nature itself. For example, Brown tree snakes (Boiga irregularis) introduced to Guam caused the extinction of many native bird species (Pimentel 2005).

Clearly we must not conflate the terms exotics and harmful invasive species. The "tens rule," an often cited generalization from invasion biology, suggests that only one in ten non-native species establish themselves in the wild, and that only one in ten of these become pests (Bright 1998). Non-natives can be beneficial by, for example, providing habitat for endangered native species (Schlaepfer et al. 2010). One commentator has argued, "It's hard to imagine a New England roadside without its tawny day lilies and Queen Anne's lace, yet both these species are aliens marked for elimination. . . Could it be these plants have actually improved the New England landscape, adding to its diversity and beauty?" (Pollan 1994). We should also note the vast benefits to humans of the imported, non-native species used in agriculture. Moreover, identifying non-natives with damaging invasives ignores that native species can also be damaging and invasive. For example, the native Mountain pine beetle (Dendroctonus ponderosae) "is currently suspected to be killing more trees than any other [insect] in North America" (Davis et al. 2011). While it is true that we know what to expect with native species in their native habitat and do not know what to expect with alien species in new habitats (Gould 1997), the equation of non-native species with damaging invaders is overly simplistic.I suggest we understand native species as species that have significantly adapted in response to resident species and the local abiotic environment. By adapted I do not mean "positively fit in" — aggressively competing is as much adapting as is establishing symbiotic relationships. Natives will have forged ecological links with other natives (via predation, commensuralism, and other forms of interaction) and perhaps responded evolutionarily as well. Non-natives are those species that are foreign to an ecological assemblage in the sense that they have not significantly adapted with the biota constituting that assemblage, or to the local abiotic conditions, and the local biota have not significantly adapted to them. Species are thus more or less native or exotic. Species movement (whether caused by humans or not) involves exotic arrival only when the movement is ecological and not simply geographical. The traveling species must find itself in an ecological assemblage to which it has not in fact adapted. Species migrating into new ranges are not exotics if the animals and plants they encounter are sufficiently similar to those in the home range. For example, Bison (Bison bison) migrating out of Yellowstone Park onto nearby forest lands on which they did not previous exist are not exotic because the same species are present as in the park. Human caused presence of wolves in Yellowstone is not exotic arrival, because wolves and the species in Yellowstone (though not the particular individuals) have adapted to each other. In contrast, Asian snow leopards (Panthera uncia) introduced into Yellowstone would be exotics — even if they played the same functional roles as wolves — because the leopards and the local species have never interacted and adapted to each other. This understanding of native species allows exotic species to naturalize over time, becoming native as they adapt with the local biota and abiota. Note that there is no implication here that native species are optimally suited for the local habitat — they need not be (Gould 1997).

Those charged with managing non-native species must confront a variety of values, including the value of individual organisms, species, ecosystems, biodiversity, wildness, and varied human interests (e.g., economic, recreational, and aesthetic interests of present and future generations). Prioritizing such values will not be easy as there is no simple formula to follow. Managers can make responsible decision if they take all these values into consideration and justify their decisions based on a clearly articulated weighing of their relative importance.

Many will argue that human interests should trump values associated with nonhumans (a view often labeled ‘anthropocentrism'). Fisherman may enjoy catching exotic species (e.g., species of Asian Carp in the Great Lakes) and object to their removal even though they threaten native species of fish. "Nonanthropocentrists" will reject the idea that only humans count morally or that human interests always trump values associated with nonhumans. Practically speaking these two types of values will often (though not always) converge on similar policies, especially when human interests are conceived broadly (to include non-economic values and the interests of future humans) (Norton 1991, Minteer 2009).

Some of the most controversial management disputes concerning non-native species arise when the exotic is a vertebrate animal species, especially a mammal. Ongoing controversies in U.S. National Parks concerning managing (and frequently removing) non-native mountain goats allegedly endangering species of plants, pits serious values against each other — the value of individual sentient animals versus the value (at the species level) of plants, and perhaps the value of ecosystems as well. Defenders of animals strongly object to the only management tool that is practically feasible — shooting the goats (Throop 2000).

Protection of biodiversity by removal of exotics may also pit the value of biodiversity against wildness value (i.e., the value of natural systems in virtue of their being relatively uninfluenced by humans). Removal of an invasive plant species that is well established is likely to involve the use of poison and bulldozers, and this type of manipulation can significantly increase human influence on nature. Wildness value can also support the removal of exotics when they are human-introduced. Lessening human impacts on natural systems manifests the virtue of humility and respect for the autonomy of nature. Much of the opposition to human-introduced exotics is likely based on such values. Some argue that human impacts on nature are so ancient and pervasive that attempts to minimize the humanization of nature are futile. They argue that we must reconcile ourselves to — and take responsibility for — our human modified world, rather than pine after a long gone wild nature (Sandler 2012). For them, exotics, whether human introduced or not, can play useful roles in the novel ecosystems emerging as a result of human-induced global change (Davis et al. 2011).

Perhaps the most serious ethical question about non-natives is whether the widespread opposition to them reflects and reinforces xenophobia. Jonah Peretti argues that "nativist trends in Conservation Biology have made environmentalists biased against alien species," and he wants to "protect modern environmentalists from reproducing the xenophobic and racist attitudes that have plagued nativist biology in the past" (Peretti 1998). William O'Brien argues that the rhetoric used in opposition to exotic species parallels the language used against human immigrants, and that the "foreigner-as-threat imagery" is powerful in ecology because of its resonance in the human realm (O'Brien 2006). Pollan argues that antipathy towards exotics results from biased stereotyping: "The current attack on alien species usually cites a few notorious examples of imported plants that have behaved badly, such as kudzu, Japanese honeysuckle, multiflora rose and purple loosestrife. These demon species are then used to tar the entire class of aliens with guilt by association" (Pollan 1994). Others think this comparison "Deserves ridicule: The . . . analogy, between stereotyping alien species and stigmatizing human races is . . . far fetched. While pejorative generalizations about human races are demonstrably untrue, it is a simple matter to show that gypsy moths, Kudzu vines, and Argentine ants are destructive precisely because they are alien species in new environments . . . There are more than enough cases in which exotic species have been extremely harmful to justify using the stereotype" (Ehrenfeld 1999). But if we accept the "tens rule" and only one in one hundred exotics are damaging, then stereotyping alien species is no better on statistical grounds than are the wrongful stereotypes of humans by race and sex.

The analogy between "biological nativists" (or purists) who favor native species over exotics" and cultural nativists (or purists) who worry about the mixing of races and cultures is both instructive and misleading. It is misleading because those who oppose exotic species do not believe in the inferiority of foreign species, while cultural purists who object to "biological pollution" from other races/peoples often are motivated by a belief in the inferiority of the foreign. Nevertheless, not all cultural purisms are morally problematic, and a defense of biological nativism can be constructed for reasons analogous to a justification for morally praiseworthy forms of cultural purism. Consider Jewish parents who want their children to marry other Jews. In so far as such preferences are grounded in a desire to perpetuate Jewish culture, they are laudable. Or consider a small community's attempt to preserve its local economy and culture by opposing the arrival of chain retailers and big box stores that threaten to drive local businesses extinct. Both these examples of cultural purism are grounded in a praiseworthy desire to preserve cultural diversity. Similarly, biological nativists' objection to exotic species can be defended as a way to protect the diversity of the planet's ecosystems from the homogenizing forces of globalization. It is true that species mixing — like the mixing of human races and cultures — can produce valuable diversity gains. And the introduction of an exotic does increase — by one — the local species count. But given that humans are wantonly mixing species around the globe, the trend is toward a globalization of flora and fauna that threatens to homogenize the world's ecological assemblages into a few mongrel ecologies. The end point of such species mixing is that habitats around the world that have sufficiently similar climatic and soil conditions will have the same species compositions, resulting in terrible biological impoverishment. Keeping a dandelion out of Yellowstone is much like keeping Wal-Mart out of a small New England town, or McDonald's out of India. Kudzu in the American South is like T.V. in Nepal, a threat to the diversity of the planet's communities and ways of life. The antipathy toward exotics can be justified as a way of supporting the diversity of communities of life on the planet (Hettinger 2001).