Non-Native Species

Ned Hettinger

For Ethics, Science, Technology, and Engineering, 2nd Edition, ed. by Britt Holbrook and Carl Mitcham (Macmillan Reference, October, 2014)


Introduction


Non-native (exotic) species are usually considered species introduced by humans into new habitats that typically cause harm. Consider Kudzu (Pueraria lobata), a vine brought from Japan that was planted to reduce soil erosion. With a growth rate of up to a foot a day, and spreading over 200 square miles a year, it now blankets millions of acres in the southeastern United States, killing trees and shrubs by heavy shading. Zebra mussels (Dreissena polymorpha), traveling in ship ballast water from Russia, entered the Great Lakes and within a dozen years had spread to most aquatic ecosystems in the eastern U.S., clogging water intake pipes and causing annual damage of hundreds of millions of dollars (Pimentel, 2005). The assumption of many conservationists is that non-natives are species out of place that should be eradicated to protect native species, ecosystems, and human interests.


Recently a less hostile attitude toward non-natives is being advanced. From this perspective, the general opposition to non-natives is scientifically unsupported and practically fruitless (Davis et al., 2011-a). Nativeness confers no special value on a species (Chew & Hamilton, 2011). The preference for natives is ethically problematic because it is based on–and reinforces–a common prejudice against things foreign (Peretti, 1998). We should stop objecting to non-natives because of their origin and, except in the rare cases where they are extremely harmful and workable control strategies are available, we should embrace them as “fellow inhabitants of planet earth” (Davis, 2013).


What is a Non-Native Species?


How to characterize non-native species is problematic. Some argue that any classification “is to some extent arbitrary” (Simberloff, 2012) and others dismiss nativeness as a “a confused and deceptive concept” (Chew & Hamilton, 2011). We should distinguish three ways of categorizing non-native species: Species that are human-introduced, species whose evolutionary origin was in some other location, and species that have not significantly adapted to the local ecological assemblage.


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 have been introduced by humans, species often move into radically different habitats on their own. Vagrant birds frequently travel between North America and Europe (Cassey et al., 2005). Ballooning spiders can travel hundreds of miles by putting out a strand of silk, catching the breeze, and hanging on. These species are likely foreign to their new habitats despite not being human-introduced.


The human-introduced criterion implausibly implies that a seed arriving in a new habitat stuck to a human boot would produce an exotic plant, while that same species would be native if it traveled on the feathers of a bird. This criterion also can’t explain why human introduction of a species as part of a restoration project 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 being introduced from Canada by humans.


Non-natives might be conceived as species that did not originally evolve in a region but arrived later. This fits with how the native/non-native distinction is applied to humans. To be a native “Charlestonian” one must be born and raised in Charleston, South Carolina; long time residence is not enough. This categorization once again fails to account for species mobility. 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 habitats. In some regions, more than half the native taxa resulted from historic colonization by invaders (Brown & Sax, 2005). Volcano-formed islands were originally barren of life and this evolutionary origin criterion would implausibly entail that virtually all the species on such islands are non-natives.                                       

We can solve these problems if we specify non-natives as those species that are foreign to an ecological assemblage because they have not significantly adapted with the local biota or abiota, and the local biota have not significantly adapted to them. Native species are those that have significantly adapted in response to resident species and the local abiotic environment. Natives have interacted with, forged ecological links with (e.g., predation), and perhaps even responded evolutionarily to other natives and the local abiotic environment.


Species movement involves non-native 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 organisms and abiotic conditions they encounter are sufficiently similar to those in their home range. Bison (Bison bison) expanding their range outside of Yellowstone Park onto nearby forest lands are not exotics because the same species are present as in the park. Recent 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, if Asian snow leopards (Panthera uncia) were introduced into Yellowstone, they would be exotics — even if they played the same functional roles as wolves — because the leopards and the local species have never adapted with each other.


Species can thus be more or less native, depending on the extent of their adaptation and the degree of difference between their old and new ecological assemblages. Non-native species can naturalize over time, becoming native as they adapt with the local biota and abiota. The longer a species persists in a new habitat, the more likely that it will have significantly co-interacted with locals and thus become native. Note that there is no implication that native species are optimally suited for the local habitat or that they have become harmonious ecological fits (Gould 1997).


Non-Natives as Harmful, Invasive Species


Non-native species are often assumed to be invasive, out-competing natives, unbalancing local ecosystems, and threatening biodiversity. Non-native species are less likely to have the competitors, predators, and parasites that limit their expansion in their home ranges and native species will not have developed responses to them. 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). There are reports that the economic damage caused by invasions is greater than that caused by natural disasters (Ricciardi et al., 2011).


There is ongoing scientific controversy about how serious a problem non-native species are in general and when compared with native species. It has been widely asserted that non-native species are the second leading cause of species extinction. This claim has been criticize as based on inadequate data and as gaining its status as fact by careless repetition (Davis, 2011-b). But even if non-native species are not a significant cause of species extinction, they still can dramatically reduce the population levels of native species and otherwise compromise biodiversity.


We should not assume that non-natives species are necessarily invasive (viz., spread and become pests). While the often cited “tens rule” has been criticized for suggesting that there is a uniform probability of invasiveness for all types of non-native species, it does put to rest the idea that non-natives always cause trouble. According to this rule, only one in ten non-native species that escape captivity establish themselves in the wild, and only one in ten of these spread and become pests (Davis, 2009). Non-natives can also provide benefits: Non-native tamarisks (Tamarix spp) in the American southwest are the preferred nesting habitat for an endangered flycatcher (Empidonax traillii extimus) (Schlaepfer et al., 2011). If we include non-native species used in agriculture (e.g., wheat), it is arguable that introduced, non-native species have been beneficial overall.


Moreover, native species can also be invasive: 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). Some invasion biologists argue that we should stop judging species on their origin (their non-native status) and focus solely on their likely environmental impact (Davis et al., 2011), while others demure pointing to studies indicating non-natives tend to be more invasive than natives (Paolucci, 2013).


Evaluating Non-Native Species


There are also serious value disagreement about non-natives. Anthropocentrists will argue that human interests should invariably trump those of non-humans. If fisherman enjoy catching Asian Carp, then we should not prevent them from entering the Great Lakes solely to protect native fish. Sentientists who believe all and only conscious animals should count morally are likely to object to shooting non-native mountain goats to protect endangered plant species. Biocentrists think all living individuals are morally considerable and might object to poisoning non-native vegetation. Holists who value ecosystems, species, and biodiversity generally will be much more supportive of controlling non-native individuals.


Responsible thinking about non-natives must consider other values as well. Given that aesthetics is an important reason to value nature, non-native species that are beautiful–e.g., purple loosestrife (Lythrum salicaria)–should be looked on more favorably. Naturalness (i.e., the extent to which something is relatively uninfluenced by humans) is another important value for many conservationists. Although humans have influenced nature since our emergence as a species, in recent centuries that impact has become so dramatic that relatively unhumanized nature has become increasing valuable. When a human-introduced, non-native species drastically changes natural systems (as with Kudzu), nature’s independence has been further compromised and humans have–once again–put their stamp on the natural world. A contrasting point of view is 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 and Thompson, 2009). Non-natives, 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).


Serious ethical questions about the antipathy toward non-natives arise because it parallels negative attitudes toward human immigrants. Widespread opposition to non-natives species is believed to reflects and reinforce an irrational fear or dislike of things foreign (O'Brien, 2006). The military rhetoric used about non-natives–“invasion biology,” “war on exotic species,” “the threat from alien invaders”–fits a xenophobic world view. The analogy between "biological nativists" (or purists) who favor native species over exotics and cultural nativists (or purists) who object to the mixing of races and cultures is instructive and worrisome. Nonetheless, it is unlikely that those who oppose non-natives species believe in the inferiority of these foreign immigrants as is typical with xenophobic, cultural purists afraid of "biological pollution" from “lesser” races. Moreover, not all forms of cultural purism are morally problematic. Consider Jewish parents who want their children to marry other Jews in order to perpetuate Jewish culture. Or consider a small community's attempt to preserve its local economy by opposing the arrival of chain retailers that threaten to drive local businesses extinct. Such desires to foster distinctive local cultures and preserve cultural diversity are praiseworthy. Similarly, biological nativists' objection to non-native species can be defended as a way to protect the diversity of the planet's ecosystems from the homogenizing forces of globalization. Replacing earth’s diverse biological communities with a planet of weedy, cosmopolitan species would be tragic (Quammen, 1998). 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 commercial T.V. in Nepal, a threat to the diversity of the planet's communities and ways of life (Hettinger, 2001).


Some believe that the mixing and blending of flora and fauna from around the world will not lead homogenization but instead to hybridization and new forms of diversity (Keurlatz and van der Weele, 2009). “Introductions of species can provide the long - term residents with new ecological and evolutionary prospects ” much like how “the influx of new minds and perspectives” into a business or academic field can enhance its vitality, diversity, and longevity (Davis, 2011-b). But not all increases in diversity are valuable and they can be gained at the sacrifice of native biodiversity and integrity. Would bringing back extinct sabertooth tigers and woolly mammoths to the American continent–as some have suggested (Soule)–be desirable diversity enhancement? The global commingling of species might also produce diversity gains that many would find problematic.



References


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