The northern broken-dash is a tiny brownish butterfly, not much over a half-inch long. Its peculiar, hyphenated name comes from the stigma on male individuals – a bar-shaped, scent-producing organ on the forewing that is, in this species, distinctively divided into two pieces.
The northern broken-dash is a member of a subfamily known as the “grass skippers,” since their larvae feed exclusively on grasses, and the adults prefer dry, open habitats dominated by grass. While common on Martha’s Vineyard, the northern broken-dash is fussier than some of its relatives about habitat: It doesn’t require high-quality native grassland, like some other skippers do, but neither does it tend to occupy low-quality sites dominated by exotic grasses. The species used to be common in my yard, but it no longer is. What happened?
First, the evidence. Since my arrival in Oak Bluffs 22 years ago, I’ve recorded virtually every butterfly I’ve seen on Martha’s Vineyard, including (indeed, especially) ones that turned up in our yard. During my first decade or so on the Island, my records show northern broken-dash appeared. Counts of four or five were pretty routine during the latter half of July, which sees the peak of abundance for this species. Over the past five years, in contrast, I’ve missed this species entirely in three years, and never recorded more than a single individual.
Part of the difference, I suppose, may stem from observer coverage. I worked from a home office in those good old days, making multiple sweeps through the yard to look for butterflies nearly every day. But coverage has not exactly been bad in recent years, and this season, when I was home most of the time as a result of being unemployed, more frequent observation did not produce more northern broken-dashes. The decline is real.
You can’t blame the condition of our yard for the decline. Starting with a meager lawn hydroseeded onto compacted subsoil, I’ve gradually converted my lawn into a meadow. It features a rich mix of grass species, both native and introduced, a variety of native and exotic wildflowers, and tall herbaceous vegetation (I mow it just once each year, in late fall) of the kind that makes grass skippers happiest. So the decline must stem from conditions around my yard — its landscape context, as an ecologist would say.
And indeed, while it has happened so gradually as to have been hard to notice, development has steadily filled in my section of town since we moved here. Formerly vacant parcels now have houses; old houses with disorderly “Vineyard lawns” have changed hands and been “upgraded” to more conventional landscaping. And if one looks farther back, the changes have been even more pronounced. Longtime residents have told me that our neighborhood, not too many decades past, was much more rural in character — not just with fewer houses, but with small-scale commercial gardens, and much less landscaped area.
Viewed with a 40-year perspective, the dearth of northern broken-dashes in my yard makes sense; indeed, it seems inevitable. At the start of that sweep of time, local conditions favored insects like this one. Small populations occupied snippets of remnant grassland along roadsides, next to truck gardens, or in vacant lots. Like all small populations, these were probably temporary, eradicated after a few years by predators, poor larval survival, or sundry other forms of bad luck.
But these localized extinctions didn’t matter. There were other small populations nearby, and individuals dispersing from those discovered and recolonized the sites that had lost their skippers. What ecologists call a “metapopulation” probably existed: Multiple small populations loosely linked to one another in a larger, constantly shifting, but basically stable configuration.
Probably before I even arrived on the scene, this system was breaking down, undermined by conversion of habitat and fragmentation of the landscape that made it harder for dispersing individuals to find new homes. The metapopulation weakened. During my first years on the Vineyard, I suspect I was the beneficiary of the waning remnants of a once robust system. Northern broken-dashes from small local populations found my yard regularly. But the populations they came from were doomed, too small to persist, too isolated to be recolonized. Today, my meadowized yard could probably support northern broken-dash as a breeding species. But our property is now too remote from other populations for the yard to be effectively colonized — or even just visited regularly.
This process is a small version of what is happening across the Island, indeed across much of the continent. Alteration and fragmentation of habitat (mostly as a result of residential development, in the case of the Vineyard) degrade the numbers and connectedness of wildlife populations. The process is masked, for a while; the persistence of relict populations helps you pretend that nothing is wrong. But the writing is on the wall: A metapopulation of a particular species has broken down into small, isolated populations that aren’t viable on their own, and gradually blink out.
Then one day you wake up and realize that a formerly common species is gone.