Thursday, September 30, 2010

The invasive species problem

Although climate change has gotten the lion's share of the attention when dealing with environmental problems (and with good reason), there are other issues that shouldn't be ignored either. I'm reminded of this by a report that garter snakes (Red-sided, judging from the picture that accompanies the article) are now established and breeding in Newfoundland. Odds are, this won't be the end of the world, of course. Garter snakes are no threat to humans, and the smaller ones prey mostly on things like slugs, most of which breed fairly fast. However, larger ones do eat rodents, and this is a very double-edged thing. On the one hand, they might bring down the numbers of the House Mouse, an invasive species whose drawbacks are obvious. On the other hand, there's apparently a distinct subspecies of the Meadow Vole that is endemic to the island and has no evolutionary reason to fear long slithery critters. If I were the type to lay bets on this sort of thing, I'd bet that the voles will survive and the introduction of garter snakes will not lead to disaster... but nonetheless, this sort of thing is best avoided, because sometimes it becomes a major problem. If the "Asian carp" (actually two different species, the Bighead Carp and the Silver Carp) become established in the Great Lakes, they could devastate the entire ecosystem... unless perhaps another invasive species messes things up for them. Unfortunately, if that happens the mussels will probably also mess things up for the native fish of the ecosystem.

This leads to another point. Quite often, an invasive species has no serious enemies in its adopted home, and when it breeds out of control, like the Gypsy Moth has in North America, the temptation is to introduce another species to control it (swallow the spider to catch the fly and all that). In many cases this is the best option available; the preferred approach for invasive insects is to introduce some parasitic wasp or fly that is fairly specific to the pest you want to control. And indeed, a good many parasitic wasps and flies attack only one or a handful of species. It was most unfortunate that the folks who tackled the Gypsy Moth problem selected a parasite that wasn't host-specific - the tachinid fly Compsilura concinnata, which happily attacks and kills the larvae of various giant silkworm moths such as the Cecropia and Luna. A bit of research should have told them that this could happen; it attacks a lot of species in its native Eurasia too.

What is the moral of this story? It's a lot better to prevent the problem in the first place than to try to fix it after the fact. And if you think dealing with invasive species is a hard problem, imagine how complex geo-engineering is likely to be...

No comments: