Butterfly numbers are down this year in Tidewater Virginia. Since 1999 when local naturalists began annual surveys there have been large fluctuations from year-to-year, but the numbers recorded this year on a hot July day were almost half the reckoning of last year, although most of the same species were seen (29 in all).
This scarcity was obvious from the moment surveyors set off for butterfly-favored habitats across the counties of Surry and James City. We had a hunch that a cold winter had killed the adults and pupae during their hibernation, but why then had so many other insects and arachnids fared okay? The only other reported paucity—and a welcome one—was the impression that there were fewer mosquitoes around. But the dragonfly population had soared. The abundance of golden-winged skimmers was astonishing. You could see hundreds (countless) at a time and in many locations throughout the study area buzzing hither and thither in frenzy. They were probably searching with their large, bulbous eyes to catch a fly or a date. But there was an Aha! moment when we saw another species on a branch munching an orange sulphur butterfly.
There is not much in common between a dragonfly and the serpentine, fire-breathing dragon of fables, but they are both voracious carnivores. Dragonflies have complete mastery of the air—they can fly forwards, backwards, sideways, even upside down, at over 30 mph after prey, and can eat as much as their own body weight in a day. With such aerobatic skills and all-round vision they should be the inspiration of drone modelers at the Pentagon. We wondered if dragonflies were responsible for the dearth of butterflies. Maybe.
As I write this post in the Allegheny Mountains of West Virginia butterflies are as common as they were a year ago. In a fifteen minute walk across a meadow I spotted two species of swallowtails, two of fritillaries, American coppers, wood satyrs, sulphurs, whites, a bevy of skipper species, and others too fast-flying to identify. All were plentiful, but there were no dragonflies. I saw none throughout the district because there is no standing water for them to breed.
These observations seemed to clinch a simple explanation—one of cause and effect. When prey species were abundant in Tidewater last year the dragonflies produced a bumper brood that emerged as adults in 2014 to consume all the insects they could catch. They are highly efficient predators, even more so than lions and tigers, capturing prey 95 times out of 100 attempts. The butterfly population may take a few years to recover and, unless they are a minor fraction of the diet, dragonflies will go hungry next year and not peak again until after a good butterfly year.
It’s a neat story and a familiar one of population cycles that Charles Elton (1900-1991) pioneered. He was an Oxford biologist who, as much as anyone, transformed the craft of the naturalist to the science of ecology. He was fascinated by the 3-5 year cycles of Arctic lemmings; he found their numbers rose year after year until they ate out their food supply and died in a Malthusian catastrophe of starvation (not by jumping off cliffs). Bringing the lemming story up to date, there was an irruption of snowy owls as far south as the Carolinas in the 2013-14 winter, which may be explained by a crash in the lemming population after the 2012 summer of plenty.
Neat explanations are sometimes true, but history records that after the first simple hypothesis is planted a thicket of other explanations grow up as scientists probe deeper, and not only in ecology.
When I was a newly-minted faculty member in Edinburgh, Professor L. Mary Pickford (1902-2002) casually made a remark in the common room one day that I have never forgotten. She had a retired as an eminent neurophysiologist who would have known Elton as a contemporary Fellow of the Royal Society (Britain’s national academy of sciences). You can imagine her like a twin of the old movie star, the very English Margaret Rutherford. Mary said that of all the changes she had witnessed in a long career in medical science one of the most dramatic was the replacement of single causes and effects by multiples. She probably had in mind her favorite research subject, the pituitary gland, for which textbooks in her youth listed a function, mostly only one, for each of its hormones. But nothing is so simple today. Prolactin, to take one example, is now claimed to have over a hundred different actions in various animals. And for another example, oxytocin has been found since that conversation with Mary to be involved in maternal behavior, anxiety, and sexual orgasm, and can no longer be considered as just a ‘birth and milking hormone.’
Likewise, Elton’s theory that lemmings periodically eat out their food supply has been extended to include complex prey-predator relationships. And, moreover, ecologists predict that their population cycles will lengthen in future as Arctic warming affects snow pack and vegetation. Nature appears more complicated and expansive the closer we examine it and on every scale—from the menagerie of sub-atomic particles through the structure and biochemistry of cells to galaxies at the known edge of the universe.
As a student cramming for exams I remember craving for simple stories in physiology, and found the deepening and broadening of knowledge to be frustrating, if not annoying, jolts to work harder! But I now think that complexity is something to celebrate, and if it seems bottomless perhaps we ought to approach it with reverent awe. And so I am musing whether our hunch about dragonflies is too simplistic, although I suspect we will never be sure. I hope we are becoming a little more humble in claiming knowledge and cast off the certainties that old cartographers had when they printed on the far side of oceans, Here Be Dragons.
Next Post: Doc Bamboo