Ranking high among the Vineyard’s ecological treasures are our great ponds — those shallow basins, fed by groundwater and penned in by barrier beaches, that line the Island’s southern shoreline. Of great scenic, recreational, and economic value (oysters!), these idiosyncratic water bodies also support a rich and unusual mix of wildlife.
Part of the secret is the dynamic nature of these ponds. Water levels, temperature, and salinity vary widely with pond openings (either natural or manmade) and storm overwash. Anything that persists in or around a great pond must have some strategy to accommodate sudden, extreme, and unpredictable variation. But as always in nature, a challenge like that also represents an opportunity for species that can solve the basic problem.
One fascinating great pond microhabitat that recently caught my eye is the border of wet sand and sediment exposed when a pond’s water level drops as a result of an opening. This strip of muck may be narrow, often only a few feet wide. But it is vast in linear extent, amounting to a significant area. And while the strip is transient, submerged when the pond is full, it’s also reliable: Sooner or later, the pond opens, recreating this unique habitat. Inevitably, plants and animals find ways to use it.
While it looks austere, that band of wet pond bottom evidently offers the makings of a robust food web. At the foundation of that web is, no doubt, humble stuff: algae, I surmise, along with decaying detritus of various kinds, and perhaps even bacterial films. Whatever eats those things is, in turn, fodder for larger species. On a recent visit to the Tisbury Great Pond, I took a close look at what exploits this unique setting.
Flies, predictably, were plentiful and varied, ranging in size from too small to photograph, or even get a good look at, to quite respectable. One of the larger species I found was probably in the genus Tachytrechus, maybe the widespread T. vorax. A “long-legged fly” in the family Dolichopodidae, this insect is probably at least partly predatory, taking smaller flies as prey. But perhaps it also scavenges detritus off the wet sand. In any case, they were doing well: An attractive species with a glossy thorax, Tachytrechus was numerous all across the 100 yards or so of shoreline I explored.
Another interesting find was a small, blue-spotted bug that turned out to be in the genus Ochterus. This is a “velvety shore bug,” in the small and obscure family Ochteridae. (I had never even heard of this family, much less encountered it, and in North America, it is represented by just six species in this single genus.) These, too, are predators upon smaller insects; I watched and photographed one individual as it rambled across the sand, investigating every irregularity of the surface for a possible snack.
More familiar to me, though rather out of their usual context, were water striders in the genus Gerris. These slender-bodied, long-legged bugs are best known for skating over the surface of water, riding the surface tension as they search for prey. But they are versatile insects, perfectly capable of sustained flight (I often find them on rain puddles distantly isolated from permanent water), and also, it turns out, capable of hopping across terra firma. I photographed several as they basked and foraged on the wet sand.
While they seemed confined to the slightly drier sand on the higher, inland portion of the exposed sand, spiders also used this band of habitat. I found a number of medium-sized wolf spiders that I identified, somewhat tentatively, as the shoreline specialist Pardosa milvina. Pardosa is an odd genus with many specialized, water-loving members. One, P. lapidicina, can be mind-bogglingly abundant among the rocks of cobble shoreline, and another, P. litoralis, turns up on the edges of saltmarsh. In all these shoreline habitats, Pardosa spiders are probably generalist predators, running down or pouncing on any available arthropods as prey.
At a much larger scale, the wet sand of a drawn-down great pond is clearly a favorite foraging site for raccoons. The lanky-fingered footprints of this versatile mammal were all over the sand I examined. A broad selection of blue crabs of various ages and in various stages of disassembly suggested what resource drew the raccoons here.
For all of these creatures, this narrow strip of exposed pond bottom is clearly just one habitat option. Most of the time, water covers this area, and therefore all these species must have been using it either opportunistically or as one habitat among several that they exploit.
But that doesn’t mean the wet sand of a great pond shoreline has little value for wildlife. My exploration turned up surprising diversity, including several uncommon, specialized species using this habitat. This narrow band of exposed pond bottom, easily overlooked by a naturalist, turns out to be yet another way that our great ponds contribute to supporting the Island’s famous biodiversity.