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Do
invasive species threaten the environment? April
4, 2005 By
Mark Sagoff, Ph.D.
This essay acknowledges that governmental agencies may properly target pathogens and pests (whatever their origin) that pose risks to human health, agricultural production, or to any other well-defined economic interest. The idea of “harm to the environment,” in contrast, is far too inchoate and amorphous a concept to constrain agency powers or to direct agency actions. Legislation that targets non-native species that may “threaten” the environment or “harm” ecosystems will only bloat agency budgets and boost agency powers in a futile, pointless, and limitless effort to control all non-native species, since any exotic organism may change the environment and thus may be deemed or defined to “threaten” it. Absent a testable, operational, and justiciable concept of “harm to the environment” or “ecological harm,” agencies should direct their efforts to organisms that pose risks to health, welfare, or property – rather than to the “natural environment.” Harm
to the Environment U.S.
Executive Order 13112 www.invasivespecies.gov
defines an “invasive species” as “an alien species whose
introduction does or is likely to cause economic or environmental harm
or harm to human health.” With respect to human health, to
agriculture, and to other well-defined economic interests, agencies
are able 1) to apply criteria for identifying harm (such as illness,
mortality, or crop loss) and 2) to assess the risk an introduced
organism may cause that harm. For example, the Centers for Disease
Control (CDC) and the Animal and Plant Health Inspection Service
(APHIS) do not target all non-native species, but just those pathogens
and pests likely to threaten well-defined interests, such as human
health and agricultural production. Agencies
like APHIS and the CDC can generally identify potential pests and
pathogens and predict how they will affect human beings and crops. In
contrast, ecologists concede they cannot predict how an introduced
organism will behave in the natural environment -- if and when it will
spread, how it may evolve to become competitive, and what impacts it
may have. Simberloff (1999, 329) has written, “Virtually every
specialist in invasion biology who has examined the matter concludes
that aspects of the ecological impact of a non-indigenous species are
inherently unpredictable.” Schmitz
and Simberloff (1997) add, “The effects of introduced species are so
poorly understood and the record of predicting which ones will cause
problems is so bad that one can question how much credence to place in
a risk assessment.” Wagner (1993) agrees, “There is so much
contingency involved among organisms that we regard as invasive, that
their study has to be essentially a case by case analysis. . . No two
situations are alike.” Some
conservationists consider as “harm” any significant change if
caused by a nonnative species
to a “natural” environment. If so, every non-native species that
can survive in and thus affect a natural area is potentially harmful
and would be targeted for elimination. In the absence of criteria for
defining “environmental harm” and of methods for predicting the
behavior of introduced species, no non-native organism can be
“proven innocent” (Ruesink et al. 1995). Accordingly, legislation
that enables agencies to prevent “harm to the environment” or to
“ecosystems” creates carte blanche powers to exclude or
eliminate all non-natives species – a worthless, futile, endless,
and budget-busting task. Invasive Species and Extinction Non-native
species (other than predators in lakes and other small island-like
environments), especially plants, are generally not more significant
contributors to extinction than are native species, off-road vehicles,
hunting, weather, fire, contingent events, pesticides, pollution, and
many other factors. Nonnative predators have been a significant cause
of extinction in a few lakes and other small island-like environments.
Vermeij (1996, 6) has written, “The evidence so far points to the
conclusion that invaders often cause extinction on oceanic islands and
in lakes, but rarely in the sea or on large land masses” It
is hard to find a single example of an extinction anywhere caused
principally by introduced plants. “In fact, there are surprisingly
few instances in which extinctions of resident species can be
attributed to competition from new species” (Davis 2003, 481). Davis
(p.481) adds that “there is no evidence that even a single long-term
resident species has been driven to extinction, or even extirpated
within a single US state, due to competition from an introduced plant
species.” Those
who argue that non-native species are a significant cause of
extinction cite as their authority an article by Wilcove et al.
(1998). These authors considered five kinds of stressors --“habitat
destruction, the spread of alien species, overharvest, pollution
(including siltation), and disease” – as possible causes of
extinction. These authors did not “try to distinguish between major
and minor threats to each species because such information was not
consistently available.” Since Wilcove at al. do not distinguish
major from minor threats, they concede they did not assess “the
relative importance” or “the extent to which each of these factors
… is responsible for endangering species.” Wilcove
et al. fail to provide any empirical study to support the view, for
which their paper is often cited, that invasive species constitute a
major factor in species extinction. The article refers to only two
reports that identify “leading” or “primary” threats to
imperiled species. The first, Richter et al. (1997), described a
subjective opinion poll in which many respondents chose exotics among
other causes from a list of “leading” sources of stress. Richter
et al. caution, “Our results must be interpreted in light of their
resting in expert opinions rather than on published reports.” In the
context a widely-shared animus against non-native species and the
funding opportunities available for combating them, expert opinions
are no substitute for empirical studies. In
the second study, Schemske et al. (1994), after sifting through US
Fish and Wildlife Service data, identified the primary causes of
endangerment for 98 plant species protected under the Endangered
Species Act. These authors reported that invasive species posed no
more of a threat than off-road vehicles to these 98 endangered plants.
It is unlikely that empirical studies, if undertaken, could show that
in randomly selected ecosystems, non-native species, especially
plants, are more important factors in extinction than are native
species and many other contributing causes and conditions. No
Biological Difference Exists Between Native and Non-Native Species
Executive
Order 13112 defines a “native species” as one that “other than
as a result of an introduction, historically occurred or currently
occurs in that ecosystem.” An alien species is usually defined as
one that arrived at a site as a result of human activity (Webb 1985;
Richardson et al. 2000, 98). No scientific experiment – no study
based on randomly chosen samples rather than pre-selected cases –
attempts to show that the travel arrangements a species made correlate
with its effects on the environment. An historical fact –
colonization after a certain time or with human assistance –
distinguishes native from non-native species. This historical
difference correlates with no biological difference. Scientific
(randomized) studies have yet to discern any biological difference
that distinguishes non-native from native species and thus explains
why non-native are more likely in general to be “harmful” to the
environment. Native
and alien species are equally “natural” in the sense that the same
laws, rules, processes and mechanisms of nature apply to both and that
no biological principle or property distinguishes between them.
Ecologists cannot tell by examining the current state of an ecosystem
-- they must undertake paleoecological and other historical research
– whether the system is relatively full of non-native species or
free of them and which species are native and which exotic. Invaded
and non-invaded systems would seem function about as well – and
non-native species act much like native ones – if biologists working
with scientific (random) samples cannot tell which is which except by
historical inquiry or discern, in general, any tell-tale biological
difference between them. Non-native
species affect ecosystems primarily by increasing the number of
species (species richness). In Hawaii, “the native flora consists of
about 1100 species – and an additional 4600 exotic plants have been
identified there . . . .” (Vitousek 1990, 8; cf. Moulton and Pimm
1986). Davis (2003) has written that “more than 4000 plant species
introduced into North America north of Mexico during the past 400
years are naturalized (established to various degrees), and these new
species now represent nearly 20% of the continent’s vascular plant
species.” Huston
(1994, 318), has written, “With regard to biological diversity,
invasions potentially lead to an increase in species richness, as
invading species are added to the species gene pool.” Theory
and observation suggest that “invasions may actually increase total
species richness” (Parker et al. 1999, 8). Many
ecologists have proposed a connection between species richness and
ecosystem functioning and productivity. “Recent experiments have
shown increasing net primary productivity (NPP) and nutrient retention
in ecosystems as the number of plant species increases” (Hooper and
Vitousek 1997, 1312; cf. Waide et al. 1999). If
nonnative species generally increase species richness, as they plainly
do, and if species richness
contributes to ecosystem functioning, productivity, etc., then
non-native species generally
contribute to ecosystem functioning, productivity, and the like. Only
if one excludes non-native species by definition from biodiversity or
species richness – or makes their presence a per se or stipulated
criterion of ecosystem decline – can one “prove” that non-native
species in general or in most places damage “biodiversity” or
“ecosystems.” The
“Enemy Release” Hypothesis
The
“enemy release hypothesis: contends that if “an organism
introduced into a new region leaves behind its natural predators,
competitors, and parasites, its chances of reproductive success
increase” relative to native species (Withgott, 2004). This
hypothesis has been disconfirmed by case studies and scientific tests.
Clay
(1995) found that non-native grasses in the United States have, on
average, more pathogen species than co-occurring native grasses.
Vermeij (1996) adds, “Evidence from marine as well a terrestrial
invasions implies that invaders quickly establish interactions with
new hosts and parasites, which may impose new population controls and
selective regimes on the invaders themselves.” According
to a literature review, “community studies imply that non-indigenous
species (NIS) are no less affected by enemies than native species in
the invaded community” (Colautti et al., 2004, p. 721). One
can look for places – Guam is often cited – in which an introduced
predator has decreased local biodiversity. Sites selected on neutral
grounds or at random – and thus that support a scientific argument
– reveal, on the contrary, positive relationships between native and
exotic species richness (Londsdale, 1999; Stohlgren et al., 1999;
Levine, 2000). According
to Houlahan and Findlay (2004, p. 1132), in a scientific sample of
wetlands, ‘‘Exotic species were no more likely to dominate a
wetland than native species, and the proportion of dominant exotic
species that had a significant negative effect on the native plant
community was the same as the proportion of native species with a
significant negative effect.’’ In addition, “There was no
evidence to support the hypothesis that exotic species are more able
to dominate invaded communities because they have fewer natural
enemies than native plants” (1135). Conclusion
Governmental
agencies properly target pests and pathogens known or thought to
threaten human health, welfare, crops, or other definable economic
interests. To
expand agency powers and budgets to combat non-native species thought
by some to threaten the environment, in contrast, is to delegate to
these agencies discretion to govern in the name of a concept of
“harm” that is far too amorphous, unconstrained, and equivocal to
be constitutional and far too normative or value-laden to be
scientific. Mark
Sagoff is Senior Research Scholar at the Institute for Philosophy and
Public Policy at the School of Public Policy at the University of
Maryland, College Park, Maryland 20742. His e-mail is: msagoff@umd.edu.
Sagoff’s most recent book is Price, Principle, and the
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