invasives

There's an op-ed in today's New York Times by Hugh Raffles titled "Mother Nature's Melting Pot" which in essence says that invasive species aren't such a bad thing.

Many, perhaps most with ecology backgrounds will bash the opinions of Mr. Raffles. He's an anthropologist, and some of his ideas don't translate very well across disciplines. I'm going to critique him too, but I'll say up front that what he said needed to be said, and if he's guilty of gross oversimplification, then perhaps so are we ecologists.

First, the critique.

Mr. Raffles provides the words himself, in a metaphor which references human immigrants: "It is... the ever shifting diversity that immigrants like us bring to this country that keeps it dynamic and strong."

What he says may be true when speaking about humans, but it points out his lack of understanding of how plant and animal immigrants affect the ecosystem on at least a short-term timescale. Invasives usually decrease natural diversity, sometimes dramatically so. This is true at a variety of scales from local to continental. In extreme examples they displace most of the native community. Stands of Phragmites (common reed) are a good example. Not a lot grows under the monoculture. Not a lot lives there either. Even the lowly crayfish disappears.

What happens, typically, is that when an invasive plant takes over there is a loss of habitat structure. The complex interaction of hundreds of native species provides food and cover for a multitude of animals. Take that away, replace it with one kind of plant, all the same size and shape, and there's no place to hide, and not much to eat.

It's only a little better when the invasive is an animal. Carp do considerable damage to aquatic ecosystems. Bullfrogs (native to at least the southeastern U.S. but now found in most of the U.S.) are voracious predators on smaller native frogs.

So what we end up with is a simplified ecosystem. To revert to the anthropological example of Mr. Raffles,  it's as if when human immigrants arrived, they killed everything in their path instead of, at worst, displacing earlier residents from one neighborhood to another. Human immigration increases diversity at anything larger than a local scale. Plant and animal immigration, on a human time scale, more typically eliminates what was there before. There is no new neighborhood to move to, especially when humans have created barriers to migration in the form of cities and farmland.

The other problem is related to the pace of invasion. Mr. Raffles is correct that immigration is a natural process in the plant and animal world, and it has probably functioned as long as there have been plants and animals on earth. What's changed is the speed of immigration. Now, humans help the process along, either intentionally (carp, starlings) or unintentionally (gobies in ships ballast water, zebra mussels on the bottom of a pleasure boat).

Once, ecosystems had time to adapt to new arrivals, to assimilate a few at a time. Now the invasion is overwhelming. New things with no local predators arrive, and for a while they have a competitive edge. If a native community survives one, it may fall to the next one, or the one after that. There's no time to recover.

But now, here's my qualification. Not all invasives are created equal. While Phragmites or purple loosestrife can be devastating once they're well established, native biodiversity can persist fairly well in stands of some other species. It's a case by case thing.

More importantly, we ecologists are making the all too human mistake of looking at things only at the time scale of our own short lifetimes... or even less. We assume that what we see now will always be, or at least will be for a long time.

But even within the 25 years or so that I've been working in this field, I've seen changes. There's a place near where I grew up that was, when I was in high school, a thicket of buckthorn. When I went back there last summer, cottonwoods had overtopped the buckthorn and shaded it out. Most of the shrub cover had died back. What had been a successional field had become a forest. In another 100 years, those cottonwoods, a fast growing pioneer species with a relatively brief lifespan as trees go, will probably be giving way to something else.

Invasives undergo successional processes, just like anything else. Other species eventually outcompete them, or the presence of a near-monoculture eventually draws the attention of a browser or a parasite or a disease.

In most of the U.S., what was here at the time of white settlement was not natural. it was a fire-maintained ecosystem, kept artificially open by frequent native fire. Because native tribes had already been decimated by diseases which for the most part preceded scientists and other observers, the dynamic equilibrium which had existed for a few thousand years had probably already been altered to some extent. Fire suppression, logging, agriculture, and development thoroughly shuffled the deck, and invasive species stepped in to fill newly available gaps.

If humans disappeared today, it would take a while for the system to find a new dynamic equilibrium. It might take 150 or 200 years or more, but eventually new assemblages of species would find their niches.

Even if humans continue as we've been doing, which I expect to be the case at least until well beyond my lifetime and until whenever limits and cold hard reality finally catch up with clearly unsustainable economic systems, things will change. In another 20 years, some of the invasives we're very concerned about today will fade back, overtaken by other things. New species will arrive. It's all in flux, and we need to recognize that.

Fighting invasive species creates jobs, and it provides a mission for teams of enthusiastic volunteers, thus creating new networks, places for new connections to be made and creative new ideas forged and tested. Thus I'm going to advocate that we continue to fight invasives. However, it's best if we understand that we occupy a moment in time, and that very little is static. Change is the only constant.

It does not serve us well to take a simplistic viewpoint, no matter which side of this or any other debate we're on. Command and control mentalities are gradually being replaced with adaptive management for a good reason, although too many people seem to have not yet gotten the memo. We also need to think beyond this year, beyond this decade, beyond this century, even beyond several centuries.