Interbreeding Threatens Rare Species, Experts Say

Sharon Guynup
for National Geographic Today
December 26, 2002
Throughout the forests of its Pacific Northwest home, the spotted owl, listed as a threatened species, is facing a new challenge.

An interloper from the Midwest, the barred owl, has moved in and the birds are interbreeding—creating fertile, hybrid "sparred owls."

"It's a nasty situation," said Susan Haig, a wildlife ecologist at the U.S. Geological Survey in Corvallis, Ore. The spotted and sparred owls are hard to tell apart, and hybrids are not protected under the Endangered Species Act. "This could cause the extinction of the Northern spotted owl," she said.

Like an increasing number of creatures around the world, the barred owl may interbreed itself out of existence—and scientists and government agencies are taking notice.

When unrelated species meet and mate in the wild, it can create an ecological nightmare—especially if it involves endangered animals. When populations dip so low that every baby that's born—or hatched—counts, interbreeding can further limit reproductive success.

Some hybrids are born sterile, but that doesn't mean they don't cause other problems, explains Judith Rhymer, a conservation geneticist at the University of Maine. "It's still a conservation issue because parents are contributing less and less to the next generation," she said.

Although hybridization is a natural evolutionary process, "problems arise when it's human-caused," said Nina Fascione, vice president of species conservation for the Defenders of Wildlife.

Human Introductions

And that's the crux of the problem. Humans are bringing together animals that have never seen each other before, either through habitat destruction, the international pet trade—or travel.

The owls are just such an example. "Barred owls may be invading because there's been so much deforestation from logging," Haig explains. Logging creates the more open environment that the newcomers prefer, and the owls continue to move as forests are leveled.

The endangered red wolf is another case in point. Hunting and habitat loss slowly decimated wolf populations until they finally went extinct in the wild in the 1980s.

Coyotes traveled east and took over their territory—and when U.S. Fish and Wildlife Service officials reintroduced the wolf into North Carolina, they soon discovered "coywolf" pups, coyote/wolf mixed-breeds.

Biologists have controlled the coyote population, and the wolves are thriving.

A Global Problem

The interbreeding issue is global. Because of human intervention worldwide, hybridization of both plants and animals is now occurring at much greater rates than ever before.

"Human activity is changing the dynamics between species and evolution," said Benjamin Evans, a biologist at Columbia University's Center for Environmental Research and Conservation in New York City. "It is happening at a speed that hasn't been seen in the past."

There are many examples. On the Indonesian island of Sulawesi, all seven species of macaque monkey interbreed where their territories overlap, but natural selection seems to be keeping the situation in check: the hybrids are not as hardy as their purebred parents.

In sub-Saharan Africa, about 20 species of African clawed frogs have so successfully crossbred that they've produced at least seven genetically-distinct new species.

There are more: Mallards ducks are interbreeding other ducks out of existence in New Zealand and Hawaii. In the U.S., threatened golden wing warblers are breeding with blue winged warblers and endangered cutthroat trout are crossbreeding with rainbow trout.

Breeding Barriers

So why don't different animals, like salamanders and snakes or bears and buffalo, readily pair off? There are many breeding barriers that safeguard the purity of most species. Animals evolve in ways that ensure their survival, with certain traits that help them adapt to climate, eat and digest what's on the local menu, and avoid or fight off neighborhood predators. The wrong genetic concoction could breed out these traits.

Sometimes, the main barrier is a simple difference in habitat or breeding areas, like preferring thick jungle over wide open spaces, explains Brian Charlesworth, an evolutionary biologist at the University of Edinburgh. Breeding seasons and mating cues also differ widely between species.

Even if an interspecies coupling does occur, the female's eggs may not be fertilized. But even if sperm and egg do merge, both parents' genes need to work together in order for the embryo to develop, said Eric Hallerman, a geneticist at Virginia Polytechnic Institute and State University in Blacksburg.

"Genes need to turn on and off at the right time, in the right places—millions of times—in order to make limbs and other body parts. If they don't, the embryo dies or becomes grossly malformed and then dies," he said.

Some hybrids that do survive suffer blindness or other birth defects, while others die young. Others are infertile. But some survive—and thrive.

Scientists are slowly recognizing this new threat. For the past decade, the U.S. Fish and Wildlife Service has been wrangling over a policy on "intercrossing" of endangered species. They've decided it isn't a legal question, but is instead a science and management question, says biologist John Fay. For now, the agency is working on a case-by-case basis.

In one recent case, they actually chose to interbreed the Florida panther with a closely-related "cousin" species, the Texas cougar, in order to save it. By the 1980s, Florida's panther populations had dropped to such low numbers that inbreeding was causing serious birth defects—and driving the animal toward extinction. USFWS brought in Texas cougars to widen the gene pool. Panthers are now birthing healthy fertile cubs and numbers are growing.

But while the government works out how to deal with interbreeding species, nature keeps turning up new hybrids.

Haig believes it's a growing problem. "As habitats become more fragmented, we're going to find more and more examples of hybrids, and it's going to be a prime problem for conservationists. The tragic part is that I don't know if there are clear answers to the problem."

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