Mapping Butterflies, by Saskya van Nouhuys

There are about 3,500 habitat patches in the Åland Islands, Finland, that are suitable for the Glanville fritillary butterfly. We used to think there were only 1,800, but then we found 1,700 more by consulting areal maps in 2001. The butterflies live in 500 of the habitat patches, on average. Some patches have been occupied for at least two decades, but most of them are only used for a few years at a time, or even just one year. Some patches are pristine looking meadows held open by grazing sheep or cattle, or by mowing. There are also dusty roadsides, inexplicable forest clearing, well-tended yards, garbage dumps, and desolate open coastal strips. Many of the patches are scattered with agricultural debris — foundations of barns and houses made from huge rectangular red stones, unmarked abandoned wells, crushed cement drainage pipe, rusted Soviet tractors, apple crates, and decomposing rubber boots.

In a good year we find about 12,000 caterpillars in the spring, just after they wake up and begin to feed on the tiny green leaves that start growing even while still under the snow. We estimate, based on resampling experiments, that we miss on average a third of them. So maybe there are actually 18,000 caterpillars. Two thirds of them die because they are parasitized, and some are eaten by stink bugs or green lacewing larvae, and some starve too. So it isn’t a surprise that come summer the islands are never overrun by clouds of fresh new adult butterflies.

Each habitat patch has a number ID that, to me, is more like a name. There is patch 22, which is the first one I ever visited, where later I stepped on a dead cat, and later still Päivi’s mother, Elvi, lost her cell phone. There is 1668 where an old man walked compassionately from his house, leaning heavily on his cane, along an uneven path across the stones, to bring me a sun hat. At patch 576 a boy used to follow me around while I looked for caterpillars. First when he was very young, he gave me his hand and I took him to see them. Later we found them together, and then later he would mark where they were with sticks, before I even got there. He disappeared. Then two years ago he reappeared as a grown man, sitting in his car drinking with his friends. Resting his pale hand on the crest of his beer belly he watched me, not saying anything.

Then there is patch 3484, up in northern Eckerö. That is where I always take visitors because of the walk through the forest, the surprising beautiful view of the sea upon arrival at the patch, the constant breeze and smell of drying sea weed, and the absurdity of hundreds of big black caterpillars seeming to rush as they move slowly across bare rock. That is where I went last year the day that Ilkka died. I had been to 3484 with him once, and to patch 1, with the giant snails, and to many others. It was warm even though it was still May.  I lay down on the flat hot rock, alone, to begin to mourn.