Human-enabled hybridization with coyotes is a major threat to endangered red wolves. [Image Credit: U.S. Fish and Wildlife Service Southeast Region]
One of the world’s most endangered canines, the red wolf, faces a surprising threat to its survival. The 50-75 red wolves living in the wild in North Carolina sometimes mate with the more numerous coyotes that moved into their territory, resulting in hybrid offspring. When coyotes interbreed with red wolves, their genes creep into the small red wolf population, threatening its genetic integrity.
Human actions enabled coyotes — which thrive in human-modified environments — to spread beyond their historical range. Coyotes thus now inhabit red wolf territory, where they are altering the genetic makeup of the endangered species.
In addition to watching hybridization between coyotes and red wolves, researchers studying how humans are renovating the tree of life have identified a half-dozen recently hybridized plant species in the United Kingdom, plus a new kind of maggot that feeds on North American apples.
“We are causing an explosion of new species,” says James Mallet, an evolutionary biologist at Harvard University.
Before humans, a species generally stayed in one place. But we began moving species around the planet, introducing them to potential mates of other species they’d never have met otherwise. This movement of species across continents has created novel opportunities for interbreeding and hybridization, says Chris Thomas, an ecologist at the University of York in the U.K.
Hybridization has brought us brand new species, including the Oxford ragwort, a yellow, daisy-like hybrid flower that originated on Mount Etna in Sicily, in an area where two different varieties of ragwort grew together and cross-pollinated inadvertently. Once a sample of the hybrid offspring was brought to an Oxford University garden in 1690, it came into its own as a species. By 1794, the flower had escaped Oxford, its seeds spreading across the country during the Industrial Revolution by hitching rides on trains. Now regarded as an invasive species, the Oxford ragwort grows in pastures and along roads and railway lines, where it poisons cattle and horses. It has spawned two more new hybrid species with native British ragwort varieties.
Citing the ragworts and others, Thomas recently wrote in the journal Trends in Ecology and Evolution that humans are speeding up the natural processes of evolutionary speciation, at least for plants in the U.K. He identified six or seven plant species that came into existence in the U.K. between 1700 and 2010 as a result of human action — about one species per 50 years. That rate of speciation, which is the process of a new species emerging, could be faster than any other time in history, Thomas proposes.
“It looks likely that the rate of new species coming into existence is accelerated as a result of us being here,” Thomas says.
Thomas also noted that, while those six or seven species evolved in the U.K., only one plant species went extinct in all of Europe, which would put Europe’s plant speciation rate higher than the extinction rate — but he says scientists would need more data about speciation and extinction to say for sure.
Not all ecologists are on board with Thomas’ theory. Comparisons between speciation rates in the past and present may not be accurate because we’re better at noticing new species emerging in front of us than in the past, says ecologist Philip Hulme of Lincoln University in New Zealand.
Plus, any new species begins with only a few members — already endangered, in a way — which makes it difficult for that species to survive very long. It’s too early to tell what new species will just be a blip in history. “The key judgment will be, in a hundred years’ time, how many of these species are still kicking around?” Hulme says.
It’s true that new species, which only contain a few individuals in perhaps only one location, are likely to die out quickly, says Mallet. But he also thinks it’s still “quite likely” more speciation events are happening these days, though probably not more than extinctions — we may be causing more extinctions than we actually know about. Many new species are pests, he says, including an insect that afflicts apple trees.
Before Europeans brought apple trees to North America, the ancestors of apple maggots, a type of fruit fly, fed on hawthorn trees. Once the maggots spread to human-introduced apple trees around the year 1860, they had to change slightly to adapt.
Apples put out their fruit earlier in the summer than hawthorns do, so maggots on apple trees shifted their life cycles to match that of their new host. Since they lived in different homes and their life cycles were no longer in sync, maggots that lived on apple and hawthorn trees split from one species into two.
“Essentially, this species evolved a hundred years ago,” Mallet says. “It’s pretty good evidence of speciation happening before our eyes.”
Some amount of hybridization and speciation between animals is natural, says Scott Mills, a biologist at North Carolina State University who studies how animals respond to human-caused changes. But human action has made meetings between different species — and thus hybridization — more common.
Not only do we move species around, we also modify environments in ways that allow animals to expand their ranges, as coyotes have. Climate change, which also shifts animals’ ranges, “puts animals in each other’s backyards in perhaps surprising ways,” Mills says. Species that weren’t neighbors before are now running into to each other and interacting, because of how we’ve changed ecosystems.
Hybridization can contribute to speciation, but it also has the potential to wipe out a species instead. The U.S. Fish and Wildlife Service’s management plan for red wolves recognizes hybridization with coyotes as a “serious threat.”
A few hybrids might not seem like much of a threat to the whole species of red wolves, but if those hybrids breed with full red wolves, they introduce coyote genes into the red wolf population. After a while, if interbreeding between coyotes, hybrids and red wolves continues, the red wolf’s particular set of genes may be lost.
Hybridization with coyotes wasn’t always a problem for red wolves. After decades of habitat loss and intensive predator control programs reduced wild populations, red wolves were declared extinct in the wild in 1980, when biologists captured the few remaining wolves to start a captive breeding program. Red wolves were reintroduced to a wildlife refuge in northeastern North Carolina in 1987. A few years later, coyotes moved into the area and the trouble began.
It’s still an unanswered question whether hybridization with coyotes is jeopardizing the existence of red wolves as we’ve known them to this point, Mills says. Red wolves also face other threats, including competition with coyotes for resources and territory, which doesn’t help their cause.
Overall, Mills says it’s too soon to tell whether the human-influenced increase in hybridization is leading to more new animal species, or rather, dissolving existing ones.
Even if we are adding new species to the Earth, looking only at the grand total of species ignores biodiversity and the uniqueness of species that are disappearing. For example, Hulme argues that there’s no other animal like the kiwi, a tubby brown flightless bird that’s native to New Zealand, while there are many varieties of finch in the Galapagos Islands. Losing one type of finch to extinction would be like plucking a leaf off the tree of life, Hulme says. It would be gone forever, but many more, similar leaves would still be around. If the kiwi went extinct, the tree would lose a whole branch.
But rather than thinking about human beings as just destroyers of other life forms, Thomas suggests that we think of ourselves as transformers, taking a hand in the remaking of ecosystems that has happened across our planet’s long history.
“Life that comes on Earth after us,” Thomas says, “will be different because of us being here.”