There's an argument that even the dichotomy between "native" and "non-native" is ultimately meaningless. Species have always migrated; to identify one as native is to draw an arbitrary line in time. Davis favors a continuum, using labels such as "long-term resident" and "recently arrived"—the idea being that these terms are both more accurate and less loaded.
A number of scientists and other scholars have wondered whether all the fretting about immigrant species somehow reflects xenophobia. There are some provocative parallels between attitudes toward human immigrants and attitudes toward their floral and faunal counterparts. Philosopher Mark Sagoff has noted the stereotypes—such as aggressiveness and unbridled fertility—that apply to human and nonhuman newcomers alike. Champions of this argument can also resort to a familiar trump card. You guessed it: The Nazis were eco-nativists. Under the Third Reich, policies to exterminate foreign plants echoed the more notorious policies against groups of people.
In the United States today, it's harder to see a connection. The Minutemen aren't about to join the battle against yellow star thistle. Then again, a few years ago, an upstart faction of the Sierra Club unsuccessfully tried to take over the board on an anti-immigration platform (on the grounds that new residents in the United States typically expand their carbon footprints). Still, liberal attitudes toward immigrants seem to coincide more often with environmentalism (which, in any case, is not necessarily synonymous with eco-nativism).
A better analogue may be resistance to globalization. As weedy species insinuate themselves throughout the globe, they are often likened to McDonald's. Conservationists want to preserve their local wildlife just as community activists want to save their mom-and-pop burger joints.
The debate inevitably leads to dorm-roomish head-scratchers. Typically, invasive species are considered those that migrated due to human agency, deliberate or accidental. So are people part of nature or separate from it? If we are part of it, isn't anything we do "natural"? (The same argument could be made about global warming.) If we're apart from nature, isn't anything we do unnatural, including eradicating the species we introduced? Come to think of it, aren't humans the most invasive species of all?
Not even the most heretical scientists say that we should just let things take their course with no attempts at management. Rather, they argue against reflexive assumptions that non-native species are bad. We should recognize their benefits as well as their drawbacks, acknowledge the trade-offs that come with trying to keep species out in a globalized world, and respect the difficulty of targeting just one element of nature.
Those caveats don't preclude conservation campaigns. The claims of traditional ecologists are compelling, precisely because they embody values that are widely shared. Most of us would be disturbed by radical changes to our national parks or neighborhood landscapes. We might decide, as a society, that in certain areas, it's worth the costs of trying to minimize such change. But let's see these efforts for what they are: expressions of human preferences rather than imperatives that flow directly from the science. Tammy whacking, anyone?