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What's your Definition of Native?

By Jeremy Houser

The discussion about the controversy around invasive species in the last edition of the Almanac raised a related question: What exactly is a native species? It’s an important practical question if there’s any validity to the views of those who argue that many people are reflexively opposed to non-native species without first considering whether they are in fact harmful. If we’re spending millions of dollars and thousands of hours controlling species based on nothing more than “nativeness,” then it matters quite a lot if that definition is a little fuzzy. As I previously argued, I think these claims range from somewhat overstated to completely ridiculous, but the question of what is a native species remains interesting.

Most of the time, the definition rests on one crucial distinction: Were humans involved? A native plant or animal population could exist in a certain place because it evolved there (think Darwin’s finches on the Galapagos Islands), because it moved there on its own, or was carried there by wind, water, or an animal. The only things termed “non-native” or “exotic” would be those that got where they are today on the backs of humans (whether accidental or intentional). Simple, right? Well, there are a few challenges. . .

First, there is the question of time. While scientists would almost universally define “native” in terms of (lack of) human involvement, in common usage the word is often understood as how long something has been in a given place. A poster titled “American Wild Flowers” hangs on the wall in the Wakeman Center. It features several familiar flowers that, while introduced by humans long ago, are generally thought to belong here; they are important and recognizable parts of otherwise natural ecosystems. The time cut-off for “feeling native” can be personal and arbitrary (Kudzu is clearly not part of “the way things used to be”) or as universal and clear-cut as the voyage of Columbus, which began the process of remixing flora and fauna that had been separated by the ocean for millennia.

Second, there is the question of proof. Using 1492 as a dividing line is useful because it approximates the scientific definition that rests on human involvement. However, because it was so long ago, it adds to our uncertainty. Not many species can naturally disperse from Europe or Asia to North America, so we can be fairly certain that Old World species that turn up here are truly non-native. But over smaller distances, it can be hard to know whether a species moved on its own or was unwittingly transported by settlers or sailors hundreds of years ago. There are, of course, no records of accidental introductions, so we tend to assume something was introduced by humans if it’s the most likely explanation. The problem with this is that in the modern world of extensive and frequent travel, human involvement is almost always a possible explanation for species movement – have we reached a point where no newly discovered population can be considered native?

Finally, it’s worth noting that the whole notion of native vs. exotic, when defined in terms of human assistance, has the odd implication of separating humans from nature. Why does the involvement of just one species, Homo sapiens, determine the ecological legitimacy of all the others? We would certainly consider it natural for a plant’s seeds to be spread with the assistance of a bird, or for a fish parasite to follow its host on a trans-Atlantic voyage. So why are we different? The only thing I can think of is that our ability to spread other species around the world is extremely new (in terms of evolutionary time) and rapidly increasing. But that seems like a difference in degree, not in kind.

What do you think? What’s your definition of native? You can share your thoughts at our facebook page.