A Maverick View on Introduced Plants

Sunday, July 14, 2013

This post originally appeared on The Sieve.

We’ve all heard about the dangers of non-native plants: they outcompete natives; they carpet forest floors and smother roadsides; they cost us billions of dollars a year in control efforts. They’ve colonized huge swaths of the mid-Atlantic, where I live; I’ve written about them on this very blog. But is it possible that some introduced plants could prove beneficial in their new environments?

Palm fruit in El Yunque National Forest

That’s certainly what Ariel Lugo thinks. Lugo, the director of the US Forest Service’s International Institute of Tropical Forestry, has long promoted a more catholic attitude toward plants of diverse origin. I recently visited him at the University of Puerto Rico’s Agriculture Experimentation Station in San Juan, where his office sits among groves of eucalyptus and bamboo—both of which humans introduced to the island. According to Lugo, the immigrant vegetation reflects the welcoming Puerto Rican spirit. “Here, we don’t persecute trees,” he says. “The federal government is the only one that persecutes trees.”

As far as Lugo is concerned, any species that can help his island recover from past environmental devastation—near complete deforestation, large-scale cultivation of sugarcane and other crops—is welcome. In 1992 he published a paper comparing the understories of pine and mahogany plantations with those in regrowing native forests. Lugo found that similar numbers of species were growing in both places, and that many of the understory plants in the plantations were native. Moreover, he found the older plantations were starting to give way to native overstory trees. “The study challenges the conventional dogma…and underscores the dangers of generalizing about all tropical tree plantations or all natural tropical forests,” he wrote. According to science writer Emma Marris, it took Lugo almost a decade to get his paper accepted.

And when he did publish it, the backlash was fierce. “He almost got hung,” says Dov Sax, an ecologist at Brown University. A 1991 letter in the journal Conservation Biology accused Lugo of “rais[ing] the dangerous specter of legitimacy for exotic organisms in natural environments.” But since then, more ecologists have come to accept that exotic trees can be useful for restoring highly disturbed ecosystems. “That view was unpopular at the time, but it’s been born out,” Sax says. “It’s not true everywhere, but it’s true lots of places.”

To explain where he got his unconventional ideas, Lugo told me a story from his graduate student days. He was studying the tabonuco, a beautiful, blotchy-barked tree native to Puerto Rico’s rain forest. But Lugo noticed he didn’t find tabonucos sprouting from the abandoned farm fields that make up most of the island. Instead, he saw guava trees, native to the American mainland. He didn’t mind, though: “I love guavas,” he says. “I ate tons of guavas in the 1970s.”

Two decades later, those same fields were dominated by the African tulip tree, another exotic (as the name suggests), and now the most common tree on the island. Lugo realized at this point that he was seeing forest communities that were new to science: “novel systems,” he calls them. He also realized that all-native forests weren’t coming back—now or ever. “The oldest of these systems that we can surmise are 80 years old, and there’s no indication they will revert back to native systems,” he says. That’s because he’s seen them survive “the ultimate test for the Caribbean”—hurricanes. “If you don’t survive hurricanes you don’t belong here,” says Lugo. “These species have seen hurricanes and they lose branches like everybody else. I don’t think they’ll ever go.”

But novel systems don’t discriminate against native species either. They’re more like a neighborhood in Queens where long-time residents and fresh-off-the-boat newcomers go about their lives in dynamic harmony. And Lugo, for one, celebrates his island’s cosmopolitanism. “We have introduced rats in the native El Yunque forest; and we have African bees in the native palm forests; and we have endemic birds [birds unique to the island] in plantations constructed by people,” he says. “The species don’t see the distinction. They are now all engaged in a new order.”

Puerto Rico, often called the gateway to the Caribbean, has long been a place where people and species mix. It was settled at various times by groups from both North and South America, and eventually became dominated by the Taino people from the Orinoco River basin in present-day Venezuela. In 1493, Christopher Columbus encountered the Tainos, setting the course for colonization, exploitation, and an often brutal mixing of people and culture.

But, as environmental historian Alfred Crosby has pointed out, colonization has always been about ecology as well as culture. As early as 1509, the Spanish introduced cattle and other livestock to the island. They also introduced and grew crops from other parts of the tropics: coffee (originally from Africa), ginger (from Asia), tobacco (from mainland North America), rice, cacao, and tropical fruits from all over. Above all, though, Puerto Rico produced sugar (from south Asia); by the early 20th century island was practically one large sugar plantation. Only in the past 80 years have forests been allowed to regrow; a little over half the island is now reforested.

Non-native bamboo: invasive pest or carbon storage virtuoso?

Puerto Rico today is a stew of Spanish, African, and (in smaller proportions) Native American ancestry, and its flora is no less varied. So does it matter if part of the species diversity is imported? That depends who you ask. Many ecologists fear that introduced species will inevitably cause extinctions of native species, on the theory that there are only so many ecological niches to go around. This concern is especially pertinent on islands, which tend to be small and have large numbers of endemic species that occur nowhere else.

“That’s what first got me into trouble with my colleagues,” Lugo says when I bring this argument up, and he offers another story. In 1986, the famous evolutionary biologist E. O. Wilson invited Lugo to speak about tropical extinctions at the National Forum on BioDiversity (the term “biodiversity” was coined in connection with this forum). “So for the first time I examined extinctions in Puerto Rico,” Lugo says, “and to my horror I found there were hardly any. So that’s what I said there.” For sharing this unexpectedly positive finding, Lugo reportedly got yelled at in the Smithsonian cafeteria.

Lugo also rejects–for Puerto Rico at least–the argument that exotic species are less good than natives at providing vital “ecosystem services”: filtering water, providing food for insects, or moving or holding onto nutrients. “The services that El Yunque provides with its native species composition are the same services as we get from other mountains that have introduced forests,” he says. “The water is not different coming out of an introduced species forest or a native forest. The productivity, the carbon sequestration, the support of native species—we’ve written a lot about this.”

Lugo and 18 other scientists recently threw some new fuel on this long-simmering debate with an article in Nature entitled “Don’t Judge Species on Their Origins.” The authors were challenging what they view as an outdated orthodoxy, according to which native species are preferred to non-native ones for no reason other than that they are native. “Nearly two centuries on from the introduction of the concept of native­ness, it is time for conservationists to focus much more on the functions of species, and much less on where they originated,” the authors wrote.

Dan Simberloff, a University of Tennessee biologist and prominent invasive species researcher, fired back. In a response signed by 140 other scientists, Simberloff accused Lugo and his colleagues of “assailing two straw men”—the notion that invasion biologists oppose all exotic species, and the notion that they ignore the benefits of these species. In Simberloff’s view, Lugo and his coauthors are representing a fringe position that can “lead to bad decisions” in managing natural areas.

But Lugo defends the article; in fact he says the opinion he and his colleagues put forth was mild. “We’re more radical than what we showed there,” he told me. “We wrote something people could take.”

After talking to numerous people on both sides of the debate, I have to come to feel these rhetorical volleys may shed more heat than light on the ecological value of non-native plants. In fact, I suspect that Lugo and Simberloff—both first-rate scientists—would actually agree on most points of fact. But I also suspect that beneath the scientific debate are divergent—and passionately held—views about what kind of world these scientists want to live in. The Lugos of the world find novel species mixtures scientifically interesting and ecologically useful; the Simberloffs worry about unique species and communities being irrevocably lost. Both, it would seem, have a point.

In the end, though, what kind of world we want may not matter. We’re not going to turn back the ecological clock. Indeed, the rate of change is increasing: human population growth is putting tremendous pressure on every ecosystem in the world, while global trade is mixing species at never-before-seen rates. On top of that, we’re in the midst of the most profound ecological disturbance in human history—climate change—which may make the whole concept of native species obsolete. What difference does it make if something is native to a place where it can no longer survive?

In the future, we’re probably going to need all the help we can get. So if non-native plants can help stabilize disturbed ecosystems, let them, says Lugo. “To me, it’s good news that nature reacts to us by remixing and reforming and reshaping and restructuring,” he says. “Novel systems that are a reflection of our activities—I love those. They’re saving our ass.”

* * *

What do you think? Should we be holding the line against non-native species, or is it time to ditch the “native vs. non-native” paradigm? Is there a way to find a middle ground?

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