Mapping an Invasion

They’re seemingly harmless trees, the blackberry, guava and Cinchona. But they are among 750 foreign plant species that have been invading the Galapagos Islands off the coast of Ecuador for at least the past half-century. Sprawling out comfortably in their new homes, they threaten the habitats of native plants and animals that exist nowhere else in the world. Rangers at Galapagos National Park are working to curtail these invasions. And new maps, published last month in the open-access journal, Neobiota, may guide the way.

A team of researchers from universities in Australia and Ecuador, the Charles Darwin Foundation and the Directorate of Galapagos National Park, used satellite imagery and field observations to identify individual invaders in the park. They honed in on Santa Cruz Island, host to the largest human population in the Galapagos. The resulting maps, which chart the invasive species’ extent, density and rates of spread, provide detailed information for conservationists.

Human introduced species thrive on isolated islands and threaten fragile ecosystems. With no natural competitors, native plants lack evolutionary defenses against fast-growing invaders that drink their water, eat their nutrients and steal their sunshine. Islands comprise less than five percent of the world’s land, yet are the sites of 80 percent of extinctions, said Jon Hoekstra, chief scientist at the World Wildlife Fund. “They are still home to about 20 percent of [the world’s] biodiversity and nearly half of all endangered species.” The biggest tortoise on earth and a daisy the size of a dinosaur (Scalesia pedunculata, a member of the daisy family, is actually a 65-foot tree) are among these endangered species on Santa Cruz Island. Controlling invasions, Hoekstra added, “is an urgent conservation challenge.”

Lead author and biologist Mandy Trueman, of Western Australia University in Crawley, agrees. “It’s such a big problem that it’s hard to know where to start and which bits to tackle,” she said. Although researchers have documented this problem, it became very real for Trueman when she moved to the Galapagos and experienced it firsthand. So she set out to map the invaders on Santa Cruz Island.

Trueman and her team focused on the humid highlands, an area that surrounds the island’s agricultural zone like a donut. Their novel methods produced maps that were relatively cheaper to produce and more accurate than traditional methods. First, they looked at satellite images of the island and divided them into manageable shapes. Usually, researchers categorize vegetation in these shapes by types, meaning they group together plants that often live around one another. But with vegetation as mixed up as it is on the island, the Galapagos team found this method nearly impossible. Instead, they identified individual species and confirmed their work in the field.

They found that invasive species occupied 92 percent of the humid highlands. And in the canopy, where the study was focused, nearly half the plants were foreign. The top three invaders were Spanish cedar, guava and orange cestrum. The researchers also analyzed eight species with known introduction times to see how far they had spread. In one year, wind, birds, tortoises and mammals can scatter Spanish cedar and guava over 100 meters, about the length of a city block. Depending on their ability to disperse seeds and find light and nutrients, plants invade at different rates, said Dr. Gregory Asner of the Department of Global Ecology at the Carnegie Institution for Science in Stanford, Calif.

Invasions like this aren’t new. “The results of this study echo findings we and others have reported in places like the Hawaiian Islands,” said Asner, who was involved in similar research. The mapping methods in Trueman’s paper suggest a practical management tool.  “I hope this study can help managers decide where to take action to protect our precious native flora and fauna,” she said.

At Galapagos National Park, rangers tackle 400 hectares a year of the most threatening invaders. Between 2011 and 2012, conservationists grew almost 18,000 Scalesia pedunculata in nurseries with plans to reintroduce them into the wild, according to the 2011-2012 Galapagos report.

Beyond reintroduction, park managers are also looking into biological control agents for blackberry, another major invader.

“If managers did nothing,” said Trueman, “the invasions would continue and perhaps take over to an extent that there are no native plant communities left.” Those unique species that define the Galapagos could vanish forever.