Giant Aldabra tortoises are a non-native species on the islands of Mauritius. The country introduced them in 2004 after losing all its large tortoises.
How can we best come to terms with the exotic species that surround us?
PUBLISHED JULY 24, 2014
What should be done with the wattle-necked softshell turtles on the Hawaiian island of Kauai?
It's a head scratcher: Should we remove the turtles from Kauai to preserve the native ecosystem there—the turtles could potentially eat native fish—and risk the extinction of their species, or should we keep them alive in Hawaii?The turtles came from China, starting in the 1850s, brought by sugarcane farmers who liked them as soup. Today, they're endangered in China and considered invasive—the term for non-native species that cause undesirable effects—in Kauai. But conservationists don't believe the animals are safe from hunting in their home range, so there's little point in boxing them up and sending them back.
Those kinds of knotty questions are becoming more commonplace in ecology, as global change accelerates. And so a new attitude is emerging that's less reflexively hostile toward invaders. It was much in evidence at a symposium held last week at the North American Congress for Conservation Biology in Missoula, Montana. I participated as a journalist but not a disinterested observer: I've argued in the past that it's time for a more nuanced approach to the non-native plants and animals among us.
Invasive species are scary. It was ecologist Charles Elton, back in the 1950s, who introduced the militaristic "invasion" metaphor to describe exotic plants and animals—but there's no question some can be extremely destructive.
The brown tree snake has eaten a dozen kinds of forest birds in Guam to extinction; zebra mussels clog pipes around the Great Lakes; the common house cat turns out to be, in Australia, a mercilessly effective killer of cute, fluffy marsupials like the bilby and the numbat.
As scientists have sounded the alarm about these pests, the public has gotten the message. Citizen groups rip out non-native plants. Native gardens have become increasingly popular, both as ways to celebrate the unique flora of each region and as tiny hot spots of diversity. Native trees provide food for native bugs, which feed native birds. Food chains developed over thousands of years of co-evolution unfold in our backyards. We're even going native in the kitchen, with fine restaurants increasingly focused around locally hunted, foraged, and grown ingredients.
So we've learned, scientists and lay people alike, that native species are good and non-natives are bad.
Julian Olden, a biologist at the University of Washington, Seattle, who co-organized the symposium, recently polled nearly 2,000 ecologists. Among his findings: A substantial number of them said they would immediately eradicate a hypothetical non-native forest plant, even if it were shown to have no effect on the forest. Olden calls this the "guilty even when proven innocent" approach.
That kind of approach is not very useful on a rapidly changing planet.
Exotics Are Everywhere
Climate change is making it harder even to decide who the invaders are.
How, scientists at the symposium wondered, do you define "native" on a warming planet, when plants and animals are already moving toward the poles or up mountainsides in search of climate conditions they can tolerate? Should we consider them "invasive" in their new homes? Regardless of what we label them, conservationists will be reluctant to remove them from their new environs—to do so would stymie their chances of adapting to the warmer future we're creating.
And then there are the non-natives that we actually like. Most domestic crops are exotic in most of the places they're grown, but there are even wild exotics that "do good," forming useful relationships with native species.
Edwin Grosholz of the University of California, Davis, told the recent symposium about one such relationship. On beaches in his state, non-native spartina grass has become important habitat for the endangeredCalifornia clapper rail, a plump shorebird with a downward curving bill more at home on land than in the air. A project to rip out and poison the spartina—which grows in dense swaths that exclude many other shorebirds—saw clapper rail numbers go tumbling downward.
There are other examples like that. The endangered southwestern willow flycatcher nests in "invasive" tamarisk shrubs. Many native (and beautiful) Hawaiian flowers are now pollinated by the Japanese white-eye bird—because the native pollinators have been driven extinct by other non-native species.
Should we impose further risk on already endangered natives by severing these relationships? Or should we admire the resilience of nature and let such "well-behaved" exotics stay?
Weirdest of all is the puzzle of the invasive species that are themselves endangered, like the wattle-necked turtle. Dov Sax, an ecologist at Brown University in Providence, Rhode Island, and symposium co-organizer, reported that 15 percent of mammals and 10 percent of birds that have been introduced in non-native habitats are under threat in their home ranges.
Hippos, vulnerable to extinction in sub-Saharan Africa, are multiplying in the lakes of Colombia, after being imported by drug lord Pablo Escobar. The red-crowned Amazon parrot is endangered in its home in northeast Mexico, but flourishes in noisy flocks in cities in California and Hawaii. What do we do with such cases?
Leave them alone, more and more conservationists are arguing, and stop focusing obsessively on categorizing species as native or non-native. Mark Davis, an ecologist at Macalester College in St. Paul, Minnesota, once considered himself an "invasion biologist"—but not anymore. "I am actively trying to get the field to retire the invader narrative," he said in Missoula.
A Good Thing, Not the Only Thing
After all, nativeness is just one environmental value, and arguably not as important as preventing extinctions and preserving biodiversity. In some cases we can best serve biodiversity by leaving the non-natives alone or even—brace yourself, now—introducing them on purpose.
This is the thinking behind, for example, installing the Aldabra tortoise on the islands of Mauritius. The islands lost their own large tortoises, and the fruiting plants that formerly had their seeds moved around by these fruit-loving reptiles have been on the decline. A tortoise that's related to the island's large tortoises—a non-native from the Seychelles in the Indian Ocean that was intentionally introduced in 2004—is now handling some of that work.
Most of the time, for the time being, conserving species still means focusing on supporting them in their historical habitats, planting natives and removing non-natives. We can and should do that in places where it is feasible and important to us.
As native gardening guru Douglas Tallamy has shown us, nothing beats a native tree for supporting biodiversity. Keeping whole landscapes completely native will require more and more active management as time goes on—but for some special places, it will probably be worth it.
But the weirder, tougher cases will keep coming up. As climate changes, as the species we've already moved around establish themselves in their new homes, we'll be called on more and more to choose between the needs of a threatened species and the historical continuity of an ecosystem.
Which matters most to you is a personal decision. For me, though I highly value the particular distribution of life that makes each place unique, species survival trumps historical fidelity. Ultimately, I care more that species exist than that they stay where they're "supposed" to be. Let the turtles stay on Kauai.
I also believe that hating non-native species is counterproductive and unfair. Even the deadly tree snakes in Guam, responsible as a species for so many extinctions, are not evil as individuals. They have no idea they aren't in the right place. They're just snakes being snakes.
It makes more sense to be angry at the humans who moved a harmful species to a new place—but in general, harmful introductions were accidents or were undertaken by people who meant well.
And most of the extinctions and population declines that mar our beautiful Earth aren't caused by exotic species. They're due to development that is destroying habitat, often needlessly. That's the real bad guy. If you must hate something, hate mindless development.
When my kids and I see Queen Anne's lace by the roadside or the lemony yellow flowers of the common mullein—both very common non-native weeds in the United States—I don't scowl. I don't see those flowers as evil villains or even as a blemish on the landscape. They're unlikely to drive any other species extinct. They don't know they're on the wrong continent. And no matter where they are, they're still beautiful.
Emma Marris is the author of Rambunctious Garden: Saving Nature in a Post-Wild World.
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