On islands, imported plants and animals can spell ecological disaster. The Aleutians, the Galápagos, the Falklands, Hawaii, and countless other archipelagoes have seen species such as rats, goats, brown tree snakes, and exotic grasses delivered by human visitors. Many of the newcomers have flourished to the point of driving unique island species extinct.
People are now trying to reverse the damage of these ill-advised translocations. Step one is obvious: eradicate the invaders. Step two, however, isn’t so clear. Experts argue over whether expensive, time-consuming, and labor-intensive restoration is necessary, or if simply removing non-natives is enough.
Now a study set in California’s Channel Islands indicates eradication alone can jumpstart recovery. Led by two UC Santa Cruz undergraduates, the research shows native shrubs reclaiming the island on their own nearly three decades after removal of sheep and other grazers. The study is published in the journal Restoration Ecology.
“Taking out those sheep triggered a large change in the vegetation; it was going from grassland back to coyote brush or California sage, to a more diverse community,” said Nissa Kreidler, one of the study’s student authors.
“People spend millions of dollars on restoring island ecosystems. So it’s a huge finding that passive recovery can be sufficient and under certain circumstances, active restoration is sometimes not necessary,” said Roxanne Beltran, the paper’s lead author.
Intensive supercourse
The paper grew out of an assignment in the Conservation and Ecology in Practice “supercourse” both students took in spring 2012. Taught primarily by UCSC professors Don Croll (ecology and evolutionary biology) and Erika Zavaleta (environmental studies), and UCSC Natural Reserve System director Gage Dayton, the class spends the entire quarter learning techniques for ecological and conservation research while visiting the protected wildlands of the UC Natural Reserve System. Students read previous research pertaining to local ecology and management, then design and conduct their own short-term field studies.
On Santa Cruz Island Reserve, humans have had a major impact on land conditions. The first sheep arrived 150 years ago, to be joined by cattle, horses, and feral pigs. These large herbivores completely denuded some areas, and transformed others into non-native grassland.
The effort to eradicate sheep, cattle, and horses from the island began in 1981 and was largely complete by 1999; feral pigs were eliminated by 2007. Then the land was left alone. At nearly 100 square miles in size, Santa Cruz Island is far too large to actively replant native vegetation.
Initially, removing grazers caused unforeseen consequences. Ten years on, non-native grasses had taken over a larger share of the island. Dirk Van Vuren, now an ecology professor at UC Davis, documented that initial shift in the late 1980s and again in the 1990s. He compared the plants on either side of a sheep exclusion fence using photos and vegetation transects.
While deciding on a class project for the island, Beltran said, “we looked at his old black-and-white photos and saw herbivores had caused more damage than we could see currently. We decided to see exactly how much the island had recovered passively.”
Promising results
The results were so promising that Beltran and Kreidler decided to develop them into a publishable paper.
“We said, we can do this,” Kreidler recalls. “The supercourse had given us the tools. We knew how to design a project, run the statistics, talk with other people, and collaborate to write a paper.”
That research confidence did not arrive by accident; it’s what the supercourse was designed to do. “We have them do study after study; when they’re done with the quarter they’ve finished five, one on each reserve they’ve visited. They are constantly writing up their work and getting more efficient at study design and data collection. It takes the mystery out of doing research, and they’re not afraid to try things and make mistakes,” instructor Croll said.
