Perhaps the type of wildlife inquiry I receive most often is about abundance: “Butterflies (or birds, or dragonflies) seem especially scarce (or especially common) this year. Are their numbers declining (or increasing)?”
These are fair questions, and important ones. But the answer is rarely a simple yes or no. A host of factors obscure discussions of wildlife abundance, and conclusions based on one observer’s experience can be wrong, or at least misleading, as easily as right.
One set of complications arises from the limited perspective of any particular observer. Unless you’re extraordinarily active, for instance, you only visit a very small percentage of the habitat on the Island. Indeed, for many of us, impressions of abundance may be based mainly on observations in our own yard or neighborhood. The numbers you observe may indeed be high or low. But it does not follow that the rest of the Island — much less the broader region — is experiencing the same thing. Other habitats may be more productive; other populations or other species may be having a much better or worse year than the ones you’re observing.
Assessments of abundance can also be distorted by the varied histories of different observers. Human memory can be short and selective. If you’ve only been paying attention to butterflies for a couple of years, you really don’t have much basis for comparison: Which year was the “normal” one, against which other years should be evaluated? Without a longer perspective, it’s impossible to say.
And even long perspectives on butterfly abundance can be warped by memorable experiences in the past. You might, for example, have witnessed a year with a strong flight of migratory butterflies such as red admirals or American ladies. When such a flight is at its peak, butterflies are everywhere! And if that experience leaves a strong enough impression, it may set a challenging benchmark for butterfly abundance; almost every year is going to seem disappointing.
It’s not just limitations on the part of observers that muddle the abundance picture. Wild populations tend to behave in ways that further obscure the true status of a species or group of species. Insect populations, for one thing, vary hugely from year to year. Sustained, abnormal cold in the winter, for example, can kill large numbers of overwintering individuals, so numbers are meager the following spring.
But most insects are hugely prolific, with females laying scores or hundreds of eggs. When conditions improve, the result can be high survival and a huge surplus, in one species or across a wide range of insects. These year-to-year changes may be real. But they don’t necessarily correlate with long-term trends in numbers across a wide area, a range of habitats, or a variety of species. So, often, butterflies truly are scarce during a certain year. But that’s better thought of as a temporary blip in numbers, rather than an indication of how populations are trending in general.
Seasonality is another confounding factor. Most kinds of insects are active as adults only for a limited period each year (or, in some cases, multiple periods, as a species passes through several generations). The rest of the season, that species may be undetectable. If the “off-season” for a large number of species occurs around the same time, the result is a perceived shortage. Among Vineyard butterflies, such a flat spot usually occurs in early June, and another at some point in July. We are just between flight periods for several common and widespread species all at once, creating the illusion of a shortage.
Even when a long-term trend can be identified, it may be a local phenomenon. I’m confident, for example, that butterfly numbers and diversity have steadily declined in our Oak Bluffs yard during the 23 years we’ve occupied it. I have detailed records that prove this. But the change probably reflects the loss of habitat locally to development and habitat succession, and these results may say nothing at all about what’s happening in Chilmark.
All that said, of course, there truly can be long-term, large-scale changes in population size, and these changes are vitally important from a conservation perspective. A steady decline in numbers of the monarch butterfly, for example, has been well documented. And a host of large-scale studies suggest that insect populations generally are on a downward trend, the result of habitat loss, fragmentation of the landscape, the spread of nonnative species, and an increasingly toxic environment.
But the key to these studies (at least the meaningful ones) is that they find ways to address all the confounding factors I’ve discussed. They look at data from a long time period and over a large area, and they find information sources that are more reliable than imperfect human observations and memory.
So are butterfly numbers low this year, and are numbers declining? I can give you a definitive answer: Yes, no, and maybe.