Common question: Why should we worry about species moving around, anyway, haven’t they always done that? A forest is a just a forest and a grassland is just a grassland after all. What are you getting so worked up about? Aren’t humans the worst invasives?
Answer: Yeah, humans are the worst invasives, but I can’t really justify mass extirpations for humans. Not yet, anyway. Though limiting birth rates probably isn’t a terrible idea.
No, a forest is not just a forest. Each ecosystem is unique, with differences both dramatic and small between it and its neighbors. Introducing new species to an ecosystem can have big consequences. Invasive species can alter ecosystems they invade by direct competition with other species, by changing the chemical and structural landscape, by facilitating diseases and other invasive species, and by genetic swamping by hybridizing with related native species. 42% of North American species on the Threatened and Endangered list are there primarily because of invasive species. Relatively few species can be shown conclusively to have been driven extinct by an invasive species. However, something that is true of both invasive and endangered species, is that it takes the most effort and the most time to get rid of the last 10% of a species; extirpations are happening all the time, which can have dramatic consequences on the ecosystems left behind, but a full on extinction is a bit harder to manage. But don’t worry though, given enough time and the helping hand of humanity, those invasive species will eventually manage to push that 42% into extinction.
And yes, species have always moved around – dispersal can be a natural process. Seeds that travel by wind can be blown half way around the world. Hurricanes and rafting events can move chunks of whole ecosystems. Without the ability of species to disperse, new volcanic islands would remain barren, and retreating glaciers would leave only rocks in their wake. Like climate change, it’s not the fact that species move around that’s the problem; it’s that they do so at historically and geologically unprecedented rates. They move farther, faster, and there are way more of them because of us, and our capability of global travel.
But it’s not black and white. There can be shades of gray. Some species are non-native, but incapable of invading ‘wild’ landscapes because they require the care and maintenance of humans. Some species have established self-sustaining populations in the wild without the help of humanity, and have even become wide spread, but not had a large impact on their new ecosystems, or have even been beneficial to some native species. But some of them are a big problem, a real big problem. And if we’ve learned anything from evolution, it’s that things rarely remain static. Some of the species that don’t seem like much of a problem now will become a problem in the future; it’s just a matter of time, dispersal and/or mutation, and Natural Selection.
At some level, it comes down to an ethical debate. Is biotic homogenization something we want to avoid? Do we value global biodiversity? How much do we value it? In the US alone, invasive species cost nearly $120 billion in damages to natural and economic interests per year (Pimentel 2005). That’s money we are already spending. I’d say we value it quite a bit. So that’s what I’m getting all worked up about.
Pimentel, D., Zuniga, R., & Morrison, D. (2005). Update on the environmental and economic costs associated with alien-invasive species in the United States Ecological Economics, 52 (3), 273-288 DOI: 10.1016/j.ecolecon.2004.10.002