Art Shapiro's Butterfly Site

This story was originally published as the Tuleyome Tales column on Sunday, May 6, 2007, in The Davis Enterprise

Butterflies in our part of California have had some tough times lately.

Most people say there aren’t as many butterflies now as there were when they were kids. Because I’ve been monitoring butterflies on a biweekly basis at up to 10 sites in this part of California since 1972, I was in a position to say whether or not that was true. And by and large, it wasn’t. I used to argue that butterflies are just more conspicuous to kids than to adults, and it was all just a matter of perception. Until 1999, that is. In 1999 more than a dozen species in our area showed a sharp downturn. I began to sit up and take notice. Was something actually going on? In a word, yes.

The North American Butterfly Association (NABA; check out its Web site) sponsors a Fourth of July butterfly count modeled on the Audubon Christmas Bird Count. This is a free-form, rather unconstrained activity, and it’s questionable what, if anything, the data it generates can tell us about trends in butterfly numbers or diversity. But since the 70s I had been doing my own count right here in Davis, and it was completely standardized in methodology—so it was potentially informative. The question was, what if anything can one day’s sampling a year tell us? Several years ago a group of us set out to explore that question, using a variety of statistical tools. One of them was a diversity measure called "Fisher’s alpha," brought to our attention by Professor Michael Rosenzweig of the University of Arizona. There were no grossly obvious trends in the data set, but when it was analyzed using Fisher’s alpha, a grim outcome resulted. The analysis told us we were already experiencing a decline ("subclinical," in epidemiological parlance), and it was likely to accelerate to the point where it would become obvious. Well, in 1999 it did. But in retrospect, we can see signs of it already in 1998.

Most of the species that went downhill in 1999 recovered to some degree in subsequent years, but not completely—and some began drifting downward again. The most alarming trend was the loss of common, weedy species – the antithesis of the stereotypical endangered super-specialist. The Large Marble butterfly disappeared from one after another of my long-term sites, despite the fact that it breeds on weedy mustards and wild radish – which are not exactly in short supply. The Common Sootywing breeds on amaranth pigweeds and garden cockscomb and was in every weedy yard and vacant lot in Yolo County (it doesn’t occur in wildlands!). It disappeared from all my sites but one. Some of the other species obviously in trouble, while not weedy, had been superabundant not so long ago and hardly seemed good candidates for extinction. The Willow Hairstreak used to swarm by literal millions along Putah Creek on the Valley floor. Now it was gone – completely gone.

Meanwhile, several billion migrating Painted Ladies originating in the waterlogged deserts of Southern California filled our skies in the spring of 2005, masking from the general public the dire straits faced by most of our butterfly fauna.

When we did statistical analyses comparing butterfly trends to climatological data, we got no consistent patterns. Some climatic variables were better correlated with the trends than others, but their relative importance was inconsistent from place to place as well as from species to species. At any rate, correlation does not prove causation. "Green" folks were quick to suggest agrochemicals or genetically-engineered crops were behind the declines, but they did not "fit" the picture well. (For example, despite all the hoopla about BT corn pollen being toxic to Monarch butterfly larvae, not only had there never been any convincing evidence of impacts in the field, but there wasn’t any BT corn here anyway and besides, the pollination season didn’t mesh with the life cycles of most of the declining species.) The most likely villain seemed to be the changing pattern of land use in our region, and especially the loss of connectivity among habitat patches. That’s what we’re working on now.

Meanwhile, the weather in late winter and spring 2006 was just about the worst possible for butterflies in the Central Valley – cool, cloudy and very wet. Areas of the Yolo Bypass and the American River floodplain remained under water into June! Last year was catastrophic for many butterflies, and for the species already in decline it appeared to be the coup de grace. It looked like a substantial number of species had been lost, perhaps even regionally. In 2006 there weren’t even any significant numbers of Painted Ladies; it was a very dry year in the desert.

Well, it’s been a dry year here in water year 2006-07. So dry, in fact, that the Yolo Bypass never filled up with water. Hell, even the vernal pools never filled up with water. Drought may be bad for Painted Ladies in the desert. But the winter of 2006-07 brought a miraculous rebirth to our moribund butterfly fauna! Species that seemingly had disappeared were back, sometimes in quite respectable numbers. Some things, including the Large Marble, were seen at some localities for the first time in five or six years. The Purplish Copper, which had been on the skids, had a very good summer in the Yolo Bypass in 2006—but normally would have been largely or completely drowned out overwinter. It wasn’t, and the first adults came out extremely early. And then on April 10 I counted 39 of them in my study site, which is more than I had seen at all my sites put together in most of the last several years! Similarly, the Pygmy Blue, which is normally eradicated by flooding in the Suisun Marsh every winter, flew all winter and resumed breeding in March—for the second time in 35 years!

So is the crisis over? Hardly. My "take" on this is as follows: butterfly populations normally suffer heavy mortality overwinter, probably due to a mixture of drowning, freezing, and bacterial and fungal diseases favored by mild and wet conditions near the ground. This droughty winter providentially reduced this mortality to the point where populations that had declined to the point of invisibility became visible again. But the underlying mechanisms of decline, whatever they are, are still in place. What is happening right now may just be a detour on the highway to Hell. But the message for butterfly lovers is "Enjoy it while it’s here!"

Arthur M. Shapiro is Professor of Evolution and Ecology at U.C. Davis. His book "Field Guide to the Butterflies of the San Francisco Bay and Sacramento Valley Regions" will be in bookstores sometime in May.