Conservation Biology, Pages 3137Volume 17, No. 1, February 2003
Nonindigenous Species: Ecological Explanation, Environmental Ethics, and Public Policy
DAVID M. LODGE* AND KRISTIN SHRADER-FRECHETTE
*Department of Biological Sciences, University of Notre Dame, Notre Dame, IN 46556, U.S.A.,email email@example.comDepartment of Philosophy, University of Notre Dame, Notre Dame, IN 46556, U.S.A.
The public is getting a mixed message from ecologists, other scholars, and journalists on the topicof nonindigenous species. Misunderstandings and tension exist regarding the science, values, environmentalethics, and public policy relevant to invasive species, which are the subset of nonindigenous species that causeeconomic or environmental damage. Although there is a natural background rate at which species invasionsoccur, it is much lower than the current human-induced rates at which species are being moved around theglobe. Contrary to some recently voiced opinions , the fact that some species invasions occur without humanassistance does not confer acceptability on all species invasions. Also, despite claims to the contrary, the re-ductions of native biodiversity caused by nonindigenous species are large and well documented. Even if thatwere not true, an emphasis on species numbers alone as a metric for the impact of nonindigenous speciesdoes not adequately incorporate the high value many humans place on the uniqueness of regional biota. Be-cause regional biota are being homogenized by species invasions, it has become an appropriate and officialpublic policy goal in the United States to reduce the harm done by invasive species. The goal is not, however,a reduction of numbers of nonindigenous species per se, as recently claimed by some authors, but a reductionin the damage caused by invasive species, including many sorts of environmental and economic damage. Amajor challenge remaining for ecology, environmental ethics, and public policy is therefore the developmentof widely applicable risk-assessment protocols that are acceptable to diverse constituencies. Despite apparentdisagreements among scholars, little real disagreement exists about the occurrence, effects, or public-policyimplications of nonindigenous species.
Especies No Nativas: Explicacin Ecolgica, tica Ambiental y Poltica Pblica
El pblico est recibiendo un mensaje confuso de ecologistas, otros acadmicos y periodistassobre el tema de especies no nativas. Existen malos entendidos y tensin en relacin con la ciencia, los va-lores, la tica ambiental y las polticas pblicas relevantes a las especies invasoras, que son un subconjuntode las especies no nativas que causan daos econmicos o ambientales. Aunque existe una tasa natural a laque ocurren invasiones, es mucho ms baja que las actuales tasas, inducidas por humanos, a las que espe-
cies son movidas alrededor del mundo. Al contrario de algunos autores recientes, el hecho de que algunasinvasiones de especies ocurren sin asistencia humana no le confiere aceptabilidad moral sobre todas las in-vasiones de especies. Tambin, a pesar de recientes afirmaciones de lo contrario, las reducciones de biodiver-sidad nativa debido a especies no nativas son notables y estn bien documentadas. An si no fuera verdad,el nfasis slo en el nmero de especies como una medida del impacto de especies no nativas no incorporaadecuadamente el alto valor que muchos humanos reconocen en la singularidad de la biota regional. De-bido a que la biota regional est siendo homogeneizada por invasiones de especies, la reduccin del daocausado por especies invasoras se ha convertido en una poltica pblica apropiada y oficial en los EstadosUnidos. Sin embargo, la meta no es la reduccin de especies no nativas, en si, como afirman algunos autoresrecientes, sino una reduccin de los impactos dainos de las especies invasoras, incluyendo muchos tipos dedao econmico y ambiental. Por lo tanto, un reto mayor para la ecologa, la tica ambiental y la poltica
Paper submitted August 18, 2002; revised manuscript accepted September 13, 2002.
Nonindigenous Species Lodge & Shrader-Frechette
Conservation BiologyVolume 17, No. 1, February 2003
pblica es el desarrollo de protocolos de evaluacin de riesgos ampliamente aplicables que sean aceptablespara electores diversos. A pesar de aparentes desacuerdos entre acadmicos, existe poco desacuerdo real
acerca de la ocurrencia, el impacto o las implicancias en poltica pblica de las especies no nativas.
Most ecologists and farmers need little convincing thatmany nonindigenous species cause major changes inecosystems and crop production. Farmers and ranchers,for example, lose about $13 billion per year to invasiveplants, many of which are nonindigenous (Westbrooks1998). To ecologists and others, nonindigenous speciesin general are also known as alien or exotic speciesspecies that did not previously exist in a given region.Nonindigenous species that spread and cause ecologicalor economic harm are called weeds by farmers and inva-sive species by ecologists (for other definitions seeDavis & Thompson 2000; Richardson et al. 2000). A re-cent, crude estimate of the annual cost imposed by inva-sive species on the United States is $138 billion (Pimen-tel et al. 2000); because the study considered only asubset of invasive species and excluded many indirectand non-market costs, it is certainly an underestimate. Ithas thus become a shared goal of many constituencies inthe United States and many other countries to reducethe occurrence and impact of invasive species (Mack etal. 2000; National Invasive Species Council 2001).
Considerable misunderstanding about this issue hasbeen promulgated in the popular press, which tends tomagnify misunderstandings (or differing emphases)among scholars. For example, a recent issue of the
had an article entitled Alien Species OftenFit in Fine, Some Scientists Contend (Derr 2001), whichfollowed by a few pages an article documenting the dev-astating impact on California forests of nonindigenousoak blight fungus (Woodsen 2001). The public is gettinga mixed message, and some ecologists have contributedto the confusion.
Confusion and tension about nonindigenous species isunderstandable because science, conflicting value sys-tems, environmental ethics, and public policy have be-gun to intersect strongly on this issue. Value judgmentsare made about whether the invasive speciesinducedchanges described by ecologists are good or bad. Some-times value judgments are made and reported by scien-tists themselves, with no distinction made between thechanges in the natural world that they have documentedand the judgments they make about the acceptability ofsuch changes. As citizens, scientists are just as entitledas anyone else to make such judgments, but not underthe banner of scientific credibility. When ecologists or
others confuse normative judgments with descriptionsof environmental change, the role of science in publicpolicy development is compromised. Different people,of course, will make such judgments differently, or at leastweigh them differently against competing goals. What isharm for one person may be good for another. The his-tory of fisheries management, for example, is repletewith examples of now-regretted species introductionsand conflation of descriptions of fish stocks with judg-ments about how humans should manage fisheries (Ra-hel 1997). In general, the pathways that move speciesamong biogeographic regions within continents andthat transport species among continents are often asso-ciated with economic activity and trade globalizationthat benefit millions worldwide.
Thus, two strongly supported and often competinggoalsincreasing economic activity and protecting theenvironment from invasive speciesshould be balancedby public policy. Given the critical expertise that ecolo-gists possess on the environmental impacts of invasivespecies, they must be more careful both in communicat-ing scientific knowledge and in making clear whenvalue judgments are passed on scientific results. Ecolo-gists are in a unique position to contribute to risk analy-ses of nonindigenous species. But ecologists must recog-nize that such risk analyses willand shouldincludevalues, inferences, and goals in addition to largely scien-tific claims about protecting ecosystems from invasivespecies.
Here we address some of the major sources of confu-sion surrounding nonindigenous species. We select top-ics that have been highlighted by recent technical andpopular publications. The topics are ordered roughlyfrom those that are more purely scientific to those thatexplicitly integrate scientific information into publicpolicy. We hope our treatment of these topics will illu-minate some of the central issues surrounding nonindig-enous species and make clear that existing misunder-standings are tangential to the important public policychallenges about which there is little real disagreementamong scientists and policymakers.
The Naturalness of Species Invasions
Flannery (2001:345347) suggests that lions and ele-phants could be reintroduced into North America to re-
Conservation BiologyVolume 17, No. 1, February 2003
Lodge & Shrader-Frechette Nonindigenous Species
place those that disappeared 13,000 years ago. AncientNorth American lions were apparently the same speciesthat exist on the plains of Africa today. The elephantswould replace the ecologically similar mammoths andmastodons that once were part of a speciose herbivo-rous megafauna. Without these herbivores, with whichNorth American native plants co-evolved, some nativeplants can barely reproduce and cannot thrive (Barlow2001). Turning away from those species that became ex-tinct and instead thinking about species that arrived inNorth America, Flannery (2001:141) notes that had thecreosote bush arrived [from Argentina] last centuryrather than 10,000 years ago, it would doubtless be pro-claimed the most noxious weed ever to have invadedNorth America.
The perspective that emerges from these and manyother observations about the biogeographic history oflife on Earth is that the supposed balance of nature ismuch more complicated than previously thought (Pick-ett et al. 1992) and that . . . biological invasions are nat-ural and, more important, necessary for the persistenceof life (Botkin 2001). Even if the arrival of humans wasresponsible for the extinction of the North Americanmegafauna in the last few thousands of years, extinc-tions and invasions of biota characterized Earth long be-fore humans existed (Flannery 2001). Even on the scaleof years and decades, species ranges change (Lodge1993 ). And as Botkin (2001) points out, invasions ofnew habitats allow the long-term persistence of species,as populations in old habitats are extirpated in the faceof environmental change. Thus, it is true that species in-vasions are natural and that the very definition of non-indigenous sometimes hinges on what time frame is be-ing considered.
However, at least three important qualifications mustbe added to any claims about the naturalness of speciesinvasions or the time frames involved. The first qualifica-tion is scientific, the second a consideration of publicpolicy, and the third ethical. First, although humans are,of course, as natural as any other species, in recent cen-turies human influence has increased far more dramati-cally than that of any other species. The human-inducedrate not only of species extinction but also of species in-vasion has increased exponentially, in concert with theexponential growth of the human population over thelast few hundred years. In addition, in more recent de-cades, global human travel and commerce have in-creased disproportionately relative to the increase in thesheer number of humans. Combined, these factors haveproduced burgeoning rates of nonindigenous species inevery ecosystem that has been monitored (e.g., Cohen &Carlton 1998). Although species invasions are natural,both the rate of their occurrence and the distances tra-versed by species now exceed by orders of magnitudethose of only a few hundred years ago (Cohen & Carlton1998; Williamson 1996).
Second, rational disagreement exists about the tempo-ral benchmarks for ecological conservation or restora-tion. The U.S. National Park Services pre-Europeanbenchmark is not as arbitrary as Botkin (2001) sug-gests, however. The arrival of Europeans in North Amer-ica marked an ecologically significant time of rapid in-crease in human population, travel, and commerce. It wasthe beginning of an enormous increase in the rate of ar-rival of nonindigenous species. Clearly, any such bench-marks would differ for other continents, and whetherthe National Park Services benchmark is appropriate forother U.S. agencies and applications should be the topicof scientifically informed public-policy discussions.
Third, some writers, such as Sagoff (1999, 2000), pre-suppose that whatever is natural (e.g., species invasions)is morally acceptable, an example of the naturalistic fal-lacy (Moore 1951). This presupposition is obviously false,as murder, auto accidents, and species extinctions are allnatural or normal, but clearly they are neither moral noracceptable. Moore (1951) reasoned that what is thecase never provides grounds for what ought to be thecase: descriptive claims never provide sufficient groundsfor normative claims. Yet Sagoff (2000) assumes that theundesirable effects of both native and invasive species aremorally acceptable when he writes that no one hasshown that exotics are more likely than natives to beharmful and when he claims that native cousins [of ex-otics] . . . are as bad or worse. Even if Sagoff (2000) isright (as he surely is) that some native species are asharmful to human goals as some nonindigenous species,this fact does not provide grounds for condoning in-creased unevaluated introductions of nonindigenous spe-cies. Sagoff (2000) uses these statements to arrive at the il-logical conclusion that it is ethically acceptable to allowinvasive species to add to the harmful effects alreadycaused by some natives. Instead, an ethically defensibleconclusion is that both native species and nonindigenousspecies should be managed with respect to what is bothhumanly and ecologically desirable.
Overall, then, the fact that some biological invasionshave occurred throughout the history of life does notmean that contemporary invasions are an inappropriatetarget for management. Rather, the dramatically acceler-ating rate of invasion in recent centuries and decades re-quires management toward explicitly articulated goals.
Effects of Nonindigenous Species
For both prehistorical periods (Wilson 1992; Flannery2001) and recent periods (Mack et al. 2000; Sala et al.2000), the dramatic effects of species invasions on otherspecies and ecosystems are extremely well documented.Thus, Sagoff (2000) is wrong in asserting that little evi-dence exists for invasive speciescaused extinctions ofnative spec...