Climate Change Is Causing Some Mixed-up Wildlife
Species separated for millennia are coming into contact, and mating. The debate over what, if anything, we should do about their hybrid offspring is heating up.
To gauge what kinds of effects these shifts were having on Arctic animals, Kelly teamed up with biologist David Tallmon at the University of Alaska and conservation geneticist Andrew Whiteley at the University of Massachusetts-Amherst. The trio coauthored a seminal report for the journal Nature in 2010 that chronicled the hybridization that wildlife managers and First Nations communities had been seeing in the Arctic, including the mixing of beluga whales and narwhals, bowhead and right whales, Dall's and harbor porpoises, hooded and harp seals, spotted and harbor seals, and North Atlantic minke and North Pacific minke whales, in addition to polar and grizzly bears. They also outlined the devastating effects the new genetic exchanges could have on biodiversity, such as parent species being driven to extinction or creating hybrids unable to survive in the environments they are born into.
The scientific community at large quickly recognized that the genetic mixing wasn't limited to animals in the rapidly changing Arctic. Today they're finding it all over the place, in owls, petrels, squirrels, big cats, and wild canines.
Between 2007 and 2009 researchers from several Australian universities caught 57 hybrid blacktip sharks while doing routine marine surveys off the northeast coast of Australia. Genetic tests confirmed that they were crossbreeds of Australian and common blacktips. The result of several generations of interbreeding, they were found south of the tropical areas where Australian blacktips typically live.
Elsewhere, scientists are discovering that hybridizing species are exchanging behavioral and physiological traits, not just physical ones. Mark Scriber is an entomologist and professor emeritus at Michigan State University who studies swallowtail butterflies. In 1999 he began noticing hybrids in northern ranges that could and were eating plants previously tolerated only by southern swallowtail species. He also discovered hybrids in the north whose emergence had been delayed by four or five weeks, so that they arrived too late to mate with the previous generation of butterflies and too early to mate with the next. They could mate only with each other, essentially creating a new species.
These sorts of interactions are, in their purest form, a kind of evolution, points out Kelly. For millennia, wildlife was forced together and pushed apart as climate, ecosystems, and landscapes changed. During these periods of upheaval, genes flowed between animals, creating new species and driving others to extinction. But genetic mixing that frequently takes centuries now takes only decades or even years, because modern climate change is altering the earth so quickly and drastically.
Regardless of the cause, Jim Mallet, an evolutionary biologist at Harvard University who has studied hybridization in European and South American butterflies, argues that we should let nature take its course. And while he isn't completely alone in his thinking, most other scientists interviewed for this story were divided over whether to take action or let the interactions play out unimpeded. "My feeling is that hybridization is natural," Mallet says. "It is the result of a mating decision by an individual, and different individuals have different desires and interests. You don't want to label a mating decision as unnatural when it's found in the wild."
Still, recent human-driven hybridization could have catastrophic results for species. "The climate warming that we have induced is closer to a meteor strike [for species] than to the gradual evolution of green plants," says Kelly, who is now the assistant director of polar science for the White House's Office of Science and Technology Policy. "We're forcing change to happen so quickly that it is more likely to promote extinctions than provide adaptive responses."
Unnaturally speeding up the hybridization process can significantly affect biodiversity and the animals themselves. Pairings in which one parent species is threatened usually hastens its decline, though scientists aren't certain why one set of genes wins out over the other, as Vallender has seen with blue-winged warblers surviving while golden-winged warblers die off.
Across the continent, Eric Forsman, a biologist with the U.S. Forest Service, has watched closely while spotted owls, a threatened species in the United States, lose their tenuous foothold in the Pacific Northwest, in part because of interbreeding with newly arrived barred owls. Barred owls have expanded their range from their native Midwestern homeland to the Pacific Coast, likely due to wildfires and climate change. "One hypothesis is that because of warming temperatures, the forests of northern Canada expanded," scattering woodlands across the Great Plains and creating a migration corridor, Forsman says. "That may have allowed barred owls to expand west- ward across what was once a physical barrier and into spotted owl territory." What spurs the two species to interbreed isn't well understood, says Forsman. It may be that since there are fewer of their own species to mate with, spotted owls pair up with the first owl they see--and that's likely to be one of the many barred owls that have moved into the area. Most barred owls, meanwhile, continue to mate with their own species. The end result is fewer spotted owls.