Over seven field seasons, I’ve documented the occurrence (to my own satisfaction at least) of 44 species of orthoptera — grasshoppers, crickets, and katydids, among them — on the Vineyard. Like any checklist, this one is provisional. Errors on my part, which I trust I’ll recognize at some point, may have mistakenly included one or more species. And the fact that I continue to add new species to the list, albeit just one or two a year now, suggests that I still haven’t found all there is to find.
But with a list that feels pretty solid in hand, I’ve begun wondering about the flip side of presence. What species are missing that could or should be here? Why are they absent? Will they ever get established on the Island, and if so, how?
These absences are not trivial. For example, one entire cricket subfamily, trigonidiinae, is represented in southern New England by several very common species. Yet I’ve never gotten even a whiff of a “trig” of any kind on Martha’s Vineyard. The common true katydid, with a loud, rasping “Cha-cha-cha!” call that makes it one of the most conspicuous of insects, has never been found here, despite being common in mainland Massachusetts.
Part of the explanation for the absence of wildlife here generally lies with the obvious fact that the Vineyard is an Island. The oceanic moat that surrounds us is a barrier even for insects that fly well, to say nothing of ones that only hop or crawl. Barring favorable winds, setting out on the right heading, and having energy reserves to support a long flight with no hope for a rest stop, an insect has a hard time getting here.
And orthoptera, though they tend to be relatively powerful insects, are with just a few exceptions mediocre at best in the air. Many are highly mobile on a small scale, hopping or flying, say, 50 feet in just seconds. But many orthoptera are ill-equipped for long-distance dispersal. Katydids are either flightless or labor to stay airborne; only a few types of crickets can fly at all; and while some grasshoppers take off like rockets, most are not efficient fliers, and land as soon as they’ve escaped whatever disturbed them in the first place.
Still, changes in the distribution of orthoptera are easy to point to in our region. The aforementioned true katydid, and probably the members of trigonidiinae as well, are essentially southern species that have marched steadily northward with a warming climate (minimum winter temperatures may be the limiting factor for where they can survive). Over the past half-century or so, they’ve colonized much of southern New England, and it seems likely that they have bypassed the Vineyard simply for lack of any way to cross the sound.
Will they ever make it here? I’d say yes, and probably sooner rather than later. There are a host of ways insects can get past physical barriers, and while most of these routes are low-odds propositions, there are a lot of insects out there, and sooner or later, the improbable happens. Arthropods have been documented colonizing mid-ocean islands by floating there on debris. Strong winds can carry insects surprising distances. And, most potently these days, human travel and commerce can provide free transport for wildlife.
Consider, for example, the differential grasshopper, Melanoplus differentialis, which can be locally abundant in eastern Massachusetts. It’s an impressive beast, among our region’s largest grasshoppers, and while old individuals fade to battleship gray, young ones are a surprisingly bright greenish-yellow. If they were regular here, I’d know it.
Yet the Vineyard has just two records for this insect. The ever-alert Nancy Weaver photographed one at Mermaid Farm, Chilmark, a few years back, on row crops. And I found a single individual on the ornamental plantings on Ocean Park, Oak Bluffs, last fall. The association with horticulture is suggestive. While evidence argues against the presence of a permanent population of this grasshopper on the Vineyard, individuals evidently make it here from time to time as hitchhikers in soil, on plants, or clinging to vehicles. Sooner or later, an egg-laden female or a mixed-sex group will arrive, and we’ll have what biologists call a “colonization event.”
So here’s the picture as I see it. Much of our orthopteran diversity likely dates back some
5,000 years or so, when a warming climate and rising seas cut the Island off from the mainland. Species surviving at our latitude then simply needed to be here, and then of course to survive the ensuing ecological changes, to make it onto my checklist.
Over the millennia, there was no doubt some churn: species being extirpated, new species arriving on rare occasions by largely natural mechanisms. Settlement by Europeans, with their intensive commerce and agriculture, probably stepped up the rate of change. More recently, many southern species have been extending their ranges northward, again as a result of a changing climate. While few if any of these have reached the Vineyard yet, some of them eventually will.
So keeping that checklist up to date is likely to be an ongoing task!