Invasive argument

I’ve been reading a lot about invasive species lately, for reasons which will soon become apparent.

This morning, Ronald Bailey of Reason magazine has an essay about biological invasions: “Invasion of the invasive species!” Bailey notes that invasive species often increase local biodiversity. He then wonders why this is a bad thing?

The fear among opponents of "invasive species" is the aggressive outsiders will cause a holocaust among the native plants. That might initially seem reasonable because there are a few species, like kudzu, purple loosestrife, and water hyacinth, that grow with alarming speed wherever they show up. But that doesn't mean other species are in danger. There is no evidence that even a single long term resident species has been driven to extinction, or even extirpated within a single U.S. state, because of competition from an introduced plant species, Macalester College biologist Mark Davis notes [PDF]. Yet this spurious threat of extinction persists as one of the chief reasons given for trying to prevent the introduction of exotic species.

Here’s why it’s a bad thing: Exponential growth. It starts small, but once it gets going it’s very expensive or impossible to slow or stop. Gypsy moths. Emerald ash borers. Fire ants.

Bailey correctly notes that the effects on birds are much more noticeable than those on plants, but doesn’t observe that this is because the population sizes of plants are vastly larger than birds. It’s harder to make a plant extinct. With less than a couple hundred years separating us from the initial introduction of most invasive species, it’s too early to assess the extinction rate of indigenous species. And it’s disingenuous to say that we haven’t documented an extinction, when we’re spending millions of dollars to prevent them! Plus, he’s dead wrong when it comes to islands, where reductions in native flora have rapid impacts on native animal populations.

For Bailey, it comes down to aesthetics – people like their nature pure and unadulterated by species from the wrong part of the world:

Fair enough. But this is not a scientific argument. Sax and New Mexico University biologist James Brown correctly observe that whether the impacts of introduced species are considered to be positive or negative, good or bad is a subjective value judgment rather than an objective scientific finding. Scientists are no more uniquely qualified to make such ethical decisions than lay people.

But his essay isn’t about introduced species, it’s about invasive species. We can’t easily predict which introduced animals will become the next fire ants or zebra mussels. Who would have predicted that lionfish would become a huge problem in the Caribbean? It doesn’t take an ethicist to figure out that it’s hard to keep something manageable when you can’t predict its growth rate!

Sure, some people like the Everglades better with all those pythons. A few yokels was all it took to put them there.