Dragons of Change?
19/05/09 21:55
Most of the people I work with today don’t know that I began my career as a biologist studying dragonflies. For at least a year or two, I was probably the world’s expert on a single species of North American dragonfly. Seem obscure? My dissertation didn’t start off as a study of climate change impacts on this species (the common green darner or Anax junius), but four years of fieldwork in rural Ontario revealed that 40 years of shifting precipitation patterns correlated closely with an altered the rate of development of the species’ larvae.
Insects have in many ways proven to be the climate “canaries” — the signs of larger changes looming. For many reasons, insects are far more sensitive to small shifts in precipitation, air temperature, or the frequency and severity of extremes of weather. It is no coincidence that many of the best-documented biological impacts of climate change were made by butterfly biologists such as Camille Parmesan (University of Texas) and Terry Root (Stanford University).
Dragonflies are quite different. First, they belong to a different biological “order” than butterflies, which means that they are not closely related to butterflies at all — roughly as close as rodents (rats and mice) are to primates like humans and chimpanzees. Dragonflies are far more ancient as well. The dinosaurs walked among fields and swamps with swarming dragonflies — some of them a meter in wingspan — for some 100 million years.
The lasting presence of dragonflies has an interesting side effect: there aren’t very species of them left. North America has perhaps 1,000 species — roughly the same number of bird species here too. While the newer, more successful butterflies and moths probably include between 20,000 and 30,000 for North America. Global species totals are probably five to seven times higher than the North American figures.
Moreover, most people tend to think of dragonflies as a terrestrial, land-borne species. But all but a handful of dragonflies develop in water — and almost always in freshwater. Thus, they span aquatic and terrestrial habitats across different parts of their life cycle. Dragonfly larvae are voracious predators that will even eat fish and amphibians and can be — by insect standards — quite large. Most species are habitat specialists as larvae too, found in particular types of freshwater: shaded sandy-bottom streams in southern forests, small isolated ponds on Great Plains, or deep fast-flowing rivers in the Sierra Nevada. Of course, I always appreciated that dragonflies feasted voraciously on butterflies too. But perhaps that simply came fro personal jealousy.
This week, a multi-national team published a global survey of the status of dragonflies from a conservation perspective. Led by Viola Clausnitzer and sponsored by IUCN and published in Biological Conservation, this team found that only about 10 percent of the surveyed species were “species of concern,” meaning that they showed evidence of a decline. One of the many reasons the study was significant was that it was the first significant survey of an insect order. Most other insect orders (like Lepidoptera, or butterflies and moths) have too many species to characterize accurately. Or like “ice bugs” (Grylloblattodea) have so few species that they are not globally distributed and spatially representative.
The authors’ overall conclusion was that dragonflies were less threatened than many other groups that are well known, such as bird, amphibians, and mammals. These groups show up to 30 percent of their species as being threatened — “of concern.” Does the Clausnitzer team’s findings have implications for climate change? Do we finally have a climate canary for freshwater?
My hunch is that dragonflies represent a lot of challenges to studies of this kind. One of the most important may reflect that dragonflies span the aquatic and terrestrial worlds. Like birds and butterflies, they can fly as adults. Unlike most butterflies, dragonflies tend to like to fly and are very robust and aggressive fliers. (They’re clearly much better fliers than amphibians, which are doing very badly globally.) Perhaps the most important research issue is that most dragonfly species have not been studied or observed very long, while naturalists have been collecting observations about birds and butterflies for centuries. Dragonflies seem to have drawn interest only recently, in the past century or so. Our long-term records are far more spotty.
I have another hunch, however. Most of the early studies of butterflies and climate change were really documentation of shifts in phenology, or time- or season-specific behavior such as breeding date, migration timing, the rate of larval development, and so on. Or these studies looked at how the ranges of butterflies species had shifted — can you find the species in the same place now as 100 years ago? Such records only exist for a tiny handful of dragonfly species. But given the robust nature of an insect order capable of surviving more than 350 million years (and many thousands of significant shifts in global climate over that period), surely these changes are the most important ones to focus on from a climate perspective.
Does an absence of knowledge mean that dragonflies are useless to us in documenting ecological impacts for the future? I think not. Many parts of the world lack reasonable records of water quality, for instance, much less data on biological trends. But dragonflies are large, charismatic, and easily studied. To better understand the impacts of climate change (and to design appropriate climate adaptation interventions), we need to apprehend the relationship between climate and phenology for many species. And the moves to increase the global resolution of freshwater ecosystem monitoring and evaluation methods mean that dragonflies may actually be a powerful means of engaging public consciousness and strategic planning in climate-aware water resource management.
blog comments powered by Disqus
