A common green darners (Anax junius). Judy Gallagher / Flickr
By Jason Bittel
It's that time of year again: Right now, monarch butterflies are taking wing in the mountains of northwestern Mexico and starting to flap their way across the United States.
The great monarch migration always makes headlines, in part because climate change, along with our pesticides and agricultural monocultures, threatens to erase the natural spectacle, but also because this natural phenomenon is simply amazing. The trip of nearly 5,000 miles is an unlikely one for an insect, but these butterflies don't have a monopoly on the massive, multigenerational migration game.
It's time to give some love to the epic feats of dragonflies.
According to a study published by the Royal Society last fall, common green darners, which are found from Cuba to Canada, make a long, complex journey that takes three generations and spans a distance of more than 1,500 miles. Scientists are still trying to figure out exactly how they do it, but temperature seems to play a key role in telling the animals when to move. Unfortunately, this means climate change could well wreck the whole event even before we fully understand it. Worse, it would leave much of eastern North America without an important member of its food web.
The news that dragonflies migrate probably won't shock people who study insects, says the study's senior author, Colin Studds, a wildlife ecologist at the University of Maryland. "We've had an inkling of how many insects migrate," he said. "There are moths, there are beetles, and there are probably about 20 species of dragonflies that we have expected of migration. But we don't know much about it other than that it's a phenomenon."
One of the reasons why insect migration remains so much of a mystery is that bugs are tough to track. They're tiny, they die all the time, and they're quick as heck — especially dragonflies. "Unlike monarchs, common green darners are fast fliers," said MaLisa Spring, state coordinator for the Ohio Dragonfly Survey at the Museum of Biological Diversity just north of Columbus. "This makes them hard to catch and potentially add identification tags or stickers, and even harder to photograph. Most photo observations are a blur of blue and green, so a tiny sticker doesn't work so well."
This is why Studds and his coauthors decided to study where the dragonflies had been, rather than where the critters were heading. To do this, they took tissue samples from the wings of more than 850 green darners, some collected from the wild and others from museum specimens going back 140 years. A molecular analysis of the wings allowed the scientists to identify hydrogen isotopes that revealed the latitudes at which the dragonflies were born. (These isotopes vary at reliable rates as one moves north or south, so determining which ones are present and in what quantities can drop a pin of origin within about 100 to 200 miles.)
The team also made use of 21 years' worth of citizen science data to understand when and under what conditions dragonflies appear across their vast range. With all of these pieces put together, common green darner migration patterns are no longer so much of a blue-green blur.
Tina Vance / Flickr
In February the dragonflies appear only in warm latitudes, which is reflected in data showing dragonfly emergences at this time in Florida, Louisiana and Texas, as well as Puerto Rico, Cuba and southern Mexico. But come March, this first generation has already winged it up to the American Northeast, the Great Lakes region and southern Canada. And then those well-traveled dragonflies mate, lay eggs and die.
In May and June, the second generation hatches onto the scene. Some live their whole lives locally, but others — driven by what, we do not know — point their beautiful compound eyes south. Between September and November, these snowbirds don floral shirts and sunglasses and can be found flitting around the southern U.S. and Caribbean. Like the first generation, these dragonflies mate, lay eggs and die.
This leaves us with the third generation in the cycle, which are the population's homebodies. Because winter hits the south soon after they hatch, and they definitely don't want to head north to even colder climes, these lucky darners live out their life span locally. And then they ... mate, lay eggs and die like the rest. Their offspring then hatch around February, transform from larvae to adults, and head up north, starting the cycle anew.
The great dragonfly migration has likely been taking place for many millennia. Each generation has a job to do and an itinerary to follow, and every step is tied to a specific temperature range.
"So one of the things that we figured out from the citizen science data of over 20 years across the whole eastern United States is that the dragonflies don't begin migrating until it gets to about 9.5 degrees Celsius, or just shy of 50 degrees Fahrenheit," said Studds. "They kind of wait for it to go up to that point and they follow that temperature north."
Enter climate change. Temperature patterns are shifting all over the U.S. (indeed, the whole planet), and research on monarchs is already showing that such fluctuations may affect how the butterflies inoculate themselves against parasites. Could there be similar consequences for other migratory insects?
"If we are moving into a period where winter temperatures are warming and spring temperatures are getting warmer through time, then that will probably affect the entire timing and pace of dragonfly migration," said Studds. "It will affect how far north they go and how quickly they develop."
In some areas, warmer temperatures year-round may mean dragonflies won't even need to migrate at all. This isn't necessarily bad for the darners, mind you. But because dragonflies are important predators of mosquitoes, aphids, and other insects, as well as nutritious food for ducks, toads, newts, fish and even carnivorous plants like sundew, losing a seasonal influx of these creatures could ripple out through North America's ecosystems. (Plus, dragonflies are beautiful, and people up north will miss them.)
The truth is, scientists still know very little about how these seasonal shake-ups will affect wildlife, and common green darners and monarch butterflies are just two species among many that are facing big changes. But given that insects are experiencing dramatic declines worldwide, we need to unpack as many of these little mysteries as we can now — instead of trying to piece the puzzle back together after it's too late.
Reposted with permission from our media associate onEarth.
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