Wild Side : Manmade changes affect natural world on Martha's Vineyard
Photo by Matt Pelikan
A naturalist's life is full of questions you hope to have a chance to answer: How does a species live? What forces or other species help or hinder it? Where, exactly, can you find it, and what governs its distribution? But before these questions become meaningful, you need to answer much more basic questions about presence, absence, and numbers: What species occur here? Which ones don't that you might expect? What species are common, uncommon, increasing, fading away?
To be sure, one grows used to fluctuation in the abundance of wildlife, which can be especially pronounced in species with short life spans. Some insects, for example, pass through three or four generations in the course of a season on the Vineyard, and one generation may easily be ten times the size of the one that precedes or follows it.
Say that a female American copper butterfly lays 100 eggs. If just two of these survive to reproductive age, she will have replaced herself and her mate. And if, on average, all of her colleagues do likewise, the population will remain stable. The natural world is based on the assumption that most offspring won't survive, and therefore a surplus of eggs or babies is necessary. For most generations of American coppers, a survival rate of two percent may be all they can muster.
But imagine that some factor responsible for the death of about half of the young coppers (perhaps a wasp that fatally parasitizes larvae) declines for some reason. Suddenly, 50 percent of copper eggs may live to maturity, and an observer will find this butterfly 25 times more common than usual. And if several causes of mortality take time off all at once – well, you end up with a lot of American coppers. These things happen: one generation of a butterfly may be barely detectable, while the next one may be abundant.
Such short-term volatility is fun to observe but tells you little about the actual status of the species in question; only over the long term can you detect a population trend up or down (if one exists). And it is those long-term trends, resulting in the loss or gradual rise to prominence of a species, that really matter.
One pattern of decline that Vineyard observers are very familiar with is the gradual loss of grassland species as formerly open habitat grows up into woodland. Good examples would be birds associated with pasture and grassland. This guild of birds flourished during the Island's sheep-farming days, but species after species of open-country breeder — vesper sparrow, upland sandpiper, short-eared owl, bobolink, eastern meadowlark, grasshopper sparrow — is gone already or is on its way out.
This works in reverse, of course, at least in theory. Woodland birds like the wood thrush, the rose-breasted grosbeak, and the scarlet tanager must have been scarce here at the height of the sheep industry, rediscovering the Island only after it had sufficient habitat to make a visit worthwhile. But for contemporary Island observers, most of these woodland birds have been established here for longer than we can remember. What we notice tends to be the disappearances, as the last vestiges of open land grow less viable; the occasional gain of a woodland species (like the resumed breeding by Cooper's hawks after decades of absence) barely offsets a depressing sense of loss.
It seems likely that the ebb and flow of species on the Vineyard will grow more intense in years to come. A wholly new factor, climate change launched by the buildup of carbon dioxide in the atmosphere, has entered the game. And because temperature is an important determinant of what wildlife can survive, climate change will alter the distribution of species. For some wildlife, the Vineyard will simply become too hot; for others, our region, currently off-limits because of winter temperatures, will gradually become more suitable.
Clear examples of this process are already evident, I'd maintain. Several species of southern butterflies, for example, have been turning up with increasing regularity in southern New England. To some extent, this trend results from the fact that more people are looking for butterflies, and the general level of skill of butterfly observers has increased. But in the case of a species like the Sachem, a small, orange grass-skipper, it's clear that real changes are occurring in distribution.
The Sachem was first recorded in Massachusetts in 1997 (on the Vineyard, as it happens, where two individuals were found). In recent years, though, this butterfly has turned up annually in the Bay State; on the Vineyard, a second record occurred in 2008, and this season, the species has already appeared twice (and may have bred here). Records from elsewhere in the northeast suggest that this species is surviving the winter farther and farther north; in any given year, the odds are increasing that Sachems will start the season with field position making it possible for them to wander to Massachusetts. It seems safe to predict that within a few decades, this butterfly will be a regular part of the Vineyard's insect life.
Climate change will undoubtedly bring both gains and losses; which will predominate is impossible to predict, but you can be sure that Island naturalists of future generations will be paying attention. Numbers matter.