The wildlife question I’ve heard most frequently during the summer of 2013 is, undoubtedly “Where are all the monarch butterflies?” It’s a good question, and it’s prompted by a valid observation. The monarch, a large, orange-and-black butterfly famous for its migrations, is usually a common sight over Vineyard meadows and fields, but this year it has been nearly absent. What Islanders may not know is that shortage of monarchs appears to be continent-wide this year, and that several other species of migratory butterflies have also been abnormally scarce this year across much of the United States.
The basic outline of the monarch’s peripatetic lifestyle is probably known to nearly everyone. Concentrating in a few very small areas in Mexico (especially) and the southernmost United States, nearly all the monarchs in the world spend the winter in communal roosts. With the lengthening days of spring, the members of these roosts disperse and move northward, females laying eggs as they go. Over the course of several generations, the species spread northward to colonize the U.S. and southern Canada. In late summer, shorter days and lower sun angles prompt a reverse migration, and the species returns to its ancestral wintering sites, mainly at fairly high elevations in Mexico’s Sierra Madre mountains.
In a typical year, the first monarchs reach the Vineyard around the middle of May, and in favorable habitat, the species is common by the Fourth of July. For monarchs, “favorable habitat” means open land rich in plants in the milkweed family, on which the caterpillars of this butterfly feed exclusively. By this point in late summer, the southward migration is usually obvious, and it’s often easy to tally 50 or 100 monarchs at sites, like Katama Farm, that are favored by this species.
Not so in 2013. I didn’t see my first monarch until late July, and so far my entire season’s total is four individuals — a small fraction of a good single-day count in a typical year. This pattern has prevailed widely across the range of this species, and discussion of possible reasons has been vigorous among butterfly enthusiasts. But a definitive explanation has proven elusive, and it is not even clear that this year’s dramatic decline in numbers is any particular reason to be concerned for this butterfly’s welfare.
Part of the problem is undoubtedly a long-term process of habitat impacts in and around the sites where monarchs over-winter. Logging has reportedly reduced the amount of suitable roosting trees at the main sites in Mexico, and this has probably resulted in steadily smaller numbers over-wintering and beginning the northward migration.
But studies have also shown that the productivity of much of the monarch habitat in the U.S. has declined in quality, mainly through shrinking milkweed populations. One recent study implicates modern agricultural methods involving widespread herbicide use on crops genetically engineered to be herbicide resistant: the “Roundup-ready” crop plants flourish but native vegetation, including milkweeds, that formerly grew among the crops is killed. The result appears to be a downward trend in monarch numbers. But the size of this butterfly’s population varies widely year-to-year, even under the best conditions, making the decline in monarch numbers difficult to quantify.
If you combine this background with some other factor — a harsh winter at the wintering sites, for example, or bad spring weather as the monarchs moved north in spring — it’s easy to imagine how this year’s monarch deficit might have come about. What’s not clear is what will happen next year: while some pessimists predict that this year’s decline marks the beginning of the end for the entire species, other biologists see this year as a blip, with the fecundity that this butterfly is capable of leading to a resurgence over the next few years. Time will tell.
But an odd twist this year is that several other migratory butterfly species are also in dramatically short supply this year. Painted ladies, American ladies, and red admirals have all been virtually absent, like the monarch, on the Island and around our region. The latter two may over-winter irregularly at our latitude, but migrants from the south undoubtedly augment numbers here (which can be vast: there have been days on which I’ve seen hundreds of red admirals). The painted lady is even more strongly migratory, recolonizing most of the U.S. each year from a tropical and sub-tropical core population.
These four missing migrants use different caterpillar food plants, travel on somewhat different schedules, and in general show significant differences in their ecology. What could have caused a simultaneous, dramatic decline in all four? A full explanation may never emerge; probably some unusual convergence of multiple factors is to blame.
If the explanation is simply coincidence, then the odds are good for a rebound in some or all of these butterflies next season. But this year’s pattern of missing migrants is striking and, in my admittedly limited experience, unprecedented. I worry that this bizarre butterfly pattern is a symptom of some large-scale, fundamental ecological shift. Are the combined effects of factors such as habitat loss, the replacement of native plants by exotic ones, and rising levels of chemicals in the environment starting to add up to serious impacts on butterflies and other insects? I hope I’m wrong.