Wild Side : Invasive pests crash the biodiversity of Martha's Vineyard
Photo courtesy of eol.org
Outside, the season for insect activity is just getting under way; our earliest butterflies are finally active,
delayed by a good two weeks by the chilly spring we've had so far. But inside, a variety of little critters have been active, from tiny flies to spiders. In particular, I've noted respectable numbers of an odd, unfamiliar, blackish beetle clinging to the walls of our house in Oak Bluffs.
The stubby, rounded body and elongated head suggested I was dealing with a weevil of some kind. I noted that instead of the usual twin wing-covers that most beetles possess, this insect had an unbroken shell on its back: the wing covers are fused, resulting in a solid, protective layer but rendering the beetle utterly flightless.
A little time on my favorite insect identification website, bugguide.net, quickly produced a name for the critter: Otiorhynchus sulcata, or the black vine weevil. Adults of this species eat the leaves of a wide range of garden plants (I'll look for it on my hostas, phlox, and lilies later in the season), while the larvae feed on the root systems of a similarly wide array of (mostly horticultural) species. In short, this beetle is a pest.
While it is now widespread in the U.S. and Canada, it originated in Europe. I don't know if its first appearance in our house reflects a recent arrival on the Vineyard, or just to my immediate area. But I was left wondering: how did a flightless beetle make it from Europe to our living room wall?
We live, after all, on an island, and the surrounding water represents a barrier that arriving wildlife (if we exclude marine species) crosses only with some measure of difficulty. For most birds, of course, getting here is no big deal (though getting here in large enough numbers to establish a viable population is more complicated). And many insects are capable of flying surprising distances; while their path may be influenced strongly by the wind, it's not that hard for such species to make it here from the adjacent mainland.
But flightless insects, plants with seeds that are too heavy to drift on the wind, and even vertebrates too small or frail to swim across a few miles of ocean have a much rougher time making it to the Vineyard. Unusual things can, and do, happen to bring them here. But in general, this filtering effect of the surrounding ocean strongly influences the mix of species that occurs here.
Of the species now living on the Vineyard, a certain percentage will likely disappear over the coming years, as conditions change. And a certain number of new species will arrive, with some finding suitable conditions and replacing those that disappear. Over the centuries, a rough balance has prevailed: the rate of local extinction has stayed fairly low, more or less balanced by the rate of colonization by new species.
But times are changing. But the mechanisms — essentially random, rare accidents — that let new species get established here haven't changed much. So a reasonable prediction would be that the Vineyard faces decades or centuries of gradually declining species diversity.
But that prediction leaves out another important way in which conditions have changed: a steady increase in human travel to the Vineyard, and in the transport of goods from the mainland to our shores. For species that associate with human-altered settings, such travel and commerce represent vastly expanded opportunities to colonize islands like our own.
Unfortunately, a likely result is that the Island's wildlife will be enriched over time not so much by native species from the nearby mainland, but by species, many of them non-native and some of them potentially destructive, associated with human-modified landscapes. Cautionary examples abound. The winter moth, for example, one cause for the thousands of canopy trees killed in up-Island woodlands during recent caterpillar outbreaks, probably got here in the form of eggs attached to firewood or similar materials: females of this moth can't fly. And the decidedly residential distribution of some recently arrived invasive plants, like garlic mustard, suggests that they probably made it to the Vineyard with human assistance.
The result over time may well be that the Island's wildlife will gradually lose its distinctiveness, with specialized natives gradually replaced, not by other specialized native species, but by widespread, often exotic, and sometimes harmful species characteristic of human-altered habitats. Our species mix, in other words, will gradually grow more generic.
My little weevil, then, is a bellwether for the Island's future. Flightless and strongly associated with horticultural plants, this species surely didn't get here on its own. It probably reached my neighborhood (and probably the Vineyard itself) in a truckload of plants — one more widespread, potentially harmful generalist that is probably here to stay.