"Tourist" Bison Devastate California Island

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The economic importance of these animals to the local economy cannot be understated. In 2002 the Catalina Chamber of Commerce has planned a "Buffalo in Paradise" promotion to lure visitors to the island. Concerns have been raised that reducing the herd size to about one hundred may not be enough bison for the tourism industry.

A two-year study is underway to determine what should be done about the bison and their impact on the environment. Two options being considered are reducing the herd size and restricting the animals from the more fragile areas of the island.

Pigs Are Also a Problem

But bison are not the only invasive species causing problems on California's eight Channel Islands—of which Catalina Island is the most southern.

A new report, published in the December 18 issue of the journal Proceedings of the National Academy of Sciences, has shown how the introduction of the domestic pig to the islands has indirectly caused a massive reduction in the endemic fox populations, placing the species "near extinction."

The research, led by Gary Roemer of New Mexico State University in Las Cruces, suggests that an abundant supply of pigs enabled golden eagles to colonize and breed in the Channel Islands, whereas before the birds were infrequent visitors. While the pigs provide a steady source of food for the eagles, their preferred prey is the fox.

Before the golden eagles took up residence, the fox (Urocyon littoralis) was the largest carnivore on Santa Cruz Island, followed by the skunk. But these two animals alone did not provide enough food to sustain a resident eagle population.

Pigs Lured Raptors That Ate Foxes

An abundant supply of pigs led to an increase in golden eagle sightings beginning in the 1960s. Roemer and his colleagues estimate that the eagles first began colonizing in about 1994.

A chemical analysis of golden eagle breast feathers and blood samples from the fox, skunk, and pig revealed that in 1994—the time when the fox population began its precipitous decline—the eagles' diet consisted of 51 percent fox, 34 percent pig, and 15 percent skunk.

Although the pigs were introduced to the Channel Islands about 150 years ago, Roemer speculates that bald eagles, which prey primarily on fish and resided on the islands till about the 1960s, deterred golden eagles from taking up residence on the Islands.

But a decline in the bald eagle population, especially in the Channel Islands, left an open niche for golden eagles.

Between 1994 and 2001 the fox population on Santa Cruz island decreased from about 1,300 to less than 100. During the same period the fox population on Santa Rosa suffered a similar decline. On the island of San Miguel the population fell from 450 to 15.

In 2001 Roemer's team found 21 golden eagles on Santa Cruz, 14 of which were relocated to save the fox population. Efforts have also been launched to eradicate the islands' pigs.

"Our study brings to light how an exotic species can directly and indirectly change the structure of an entire community," said Roemer.

The direct effect of pigs on the vegetation could be predicted. "But who could anticipate that 150 years after their introduction, the pigs' presence would lead to golden eagles colonizing the island and the concomitant extirpation of the fox," said Roemer.

"The bottom line is that the indirect effects of introducing nonnative animals are not easily seen and can have devastating implications," Roemer said.

The foxes are endemic to six of the eight Channel Islands, with each island harboring its own unique subspecies. A petition has been filed for foxes from San Miguel, Santa Rosa, Santa Cruz and Catalina to receive protection under the Endangered Species Act.

Research Supported by the National Geographic Society

Roemer's work was supported by the National Geographic Society, University of California at Los Angeles, the Institute for Wildlife Studies, the Switzer Foundation, the University of California-Santa Cruz Island Reserve, and Channel Islands National Park.

Roemer is one of a distinguished group of scientists from around the globe, in fields ranging from astronomy to zoology, who have been awarded grants from the National Geographic Society's Committee for Research and Exploration (CRE).

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