Wild Side : Wash-ashores
Wildlife on islands (including the Vineyard) pays a price for isolation. Since it is hard to get to an island, it can be difficult for new species to become established, or for new individuals to replenish existing populations. Therefore, with limited land area, dwindling natural habitat, and a surrounding water barrier, islands tend to see a gradual decline in biological diversity over time, as local extinctions occur with no replacement.
Still, all forms of wildlife necessarily have at least some ability to travel, and instances abound of dispersing wildlife turning up in astonishing places. A striking example would be the establishment of full, complex ecosystems on mid-ocean volcanic islands, like Hawaii, which begin their existence as sterile rock; all native Hawaiian wildlife descended from a random wash-ashore. So it's no surprise that the wildlife on the Vineyard changes over time, reflecting both alterations in our local habitats and the ability of new plants and animals to become established here.
One recent Vineyard wash-ashore is the indigo bunting. Breeding males of this smallish finch have exquisite blue plumage (though females and immature birds are a featureless brown). Fairly common in the eastern U.S., the indigo bunting used to be oddly absent as a nesting bird on the Vineyard, though it is frequent enough as a migrant, especially in fall. But this is one of a number of migratory species that is prone to "overshoot" events, in which birds migrating over the southernmost U.S. are caught by an early spring storm and flung far north of where they should be at that point in the season.
An exceptional example of this occurred in late March a few years ago, depositing many dozens of hungry indigo buntings on the Island a good six weeks before normal migrants would arrive. Some of these birds remained to breed (a pair at Waskosim's Rock may have been the first pair ever to nest here successfully). Now the bunting, though still far from abundant, appears well established as an addition to our breeding birds.
Another mode of establishment is illustrated by the broad-winged skipper, an inch-long, orange-and-brown butterfly that was formerly restricted to wetlands of the Southeast. But one of the plants its caterpillars use for food happens to be Phragmites, the tall, invasive grass whose feathery seed heads dominate portions of some Island pond shorelines. As the invasive strain of Phragmites marched north along the coast in recent decades, the broad-winged skipper expanded along with its essential resource, making it to the Vineyard by 1995 or so.
Then there's the Eurasian winter moth, the caterpillars of which played a role in the massive caterpillar outbreak of recent years. The expansion of this non-native moth in the Northeast has been rapid, which is surprising given that the female winter moth is entirely flightless: how can it colonize new areas, much less make it to an island, if its egg-layers are essentially immobile? Probably with an assist from humanity: winter moths stick clusters of eggs to nearly any kind of surface, and humans unwittingly transport them on lumber, firewood, or outdoor equipment.
So, in ecological terms, not all wash-ashores are equal. Some make it here as a result of natural processes, which others are carried here by human commerce. And while some new species play benign or even helpful roles in our local ecology, others are potentially harmful (winter moths are bad enough; some off-Island plant pathogens, like the fungus that causes the aptly named Sudden Oak Death syndrome, would be catastrophic if they conquered the Vineyard).
Moreover, even when harmless species arrive, it's not always a fair exchange. Wash-ashores tend to be species that are already widespread and common, good at dispersing and adaptable to a wide range of conditions. But the species we are most likely to lose are our rare or highly specialized ones, vulnerable to extinction because of their small numbers and specific requirements. So over time, the exchange of new species for old will tend to dilute the unique character of the wild Vineyard, making our natural communities increasingly similar to less distinctive communities of the mainland.
And while some of our new arrivals will get here under their own steam - or at least driven by wind or currents - more and more of the additions to our wildlife are likely to get here attached to, or mixed in with, goods imported by humans (landscaping plants are an especially likely means of transport). One unintended consequence of the "global marketplace" has proven to be worldwide dispersal of highly adaptable organisms, the kinds of species that are most likely to be invasive when established in a new setting. Increasingly, then, human activity will determine not just what native species can persist on the Vineyard, but what wildlife will join, replace, or overwhelm them.
Oak Bluffs resident Matt Pelikan, program director of The Nature Conservancy Islands Program, is an avid field naturalist. A former editor of a publication of the American Birding Association, he has published work on everything from butterflies to bats. His column appears monthly in The Times.
