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"Tourist" Bison Devastate California Island |
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Bijal P. Trivedi |
| December 17, 2001 |
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Rumor has it that the bison that now wreak havoc on Santa Catalina, about a 15-minute helicopter ride from Los Angeles, California, were brought to the arid island as movie extras for the silent film The Vanishing American. This is a completely believable scenario given the island's proximity to L.A.—but for the fact that there are no bison in the film. Another theory is that the animals were originally brought to the island as a tourist attraction. Whatever their origins, these majestic "invaders" are destroying Santa Catalina's fragile plant life and turning grassy areas into dust bowls. But exactly what should be done about the bison is controversial as they attract a steady stream of tourist dollars to the local economy. "There is an ambivalent feeling about the bison on Catalina," said Deb Jenssen of the Catalina Conservancy, which oversees 88 percent of the island. "They are beautiful. They are incredible. We all love to see them, but when you see some of the impacts we say, oh boy, what are we going to do about this one," said Jenssen. Originally there were 14 bison. Now there are almost 400 roaming the island, which is only 76 square miles (197 square kilometers)—an area just slightly larger than Washington, D.C. The bison, which are natives of North America's Great Plains, have overwhelmed the island's resources as they devour the slow-growing grasses that cover the steep slopes. Grazing, trampling, and wallowing by the hefty animals have destroyed the vegetation in many areas, leaving the soil soft, crumbly, and exposed. Without plants and grass, the loose soil is vulnerable to erosion. "The bison is a big animal that needs to eat a lot," said Jenssen. "The plants are not fast-growing plants that can recover from that kind of grazing and browsing." Although the bison seem to be thriving on Catalina, there are signs that the herd size may be getting too large for the island to support. Bison Eat Prickly Pear Cactus "We have also witnessed bison eating prickly pear cactus fruit," said Jenssen. Cacti are not typical fare for bison. Their diet consists only of grass. But whether the animal is merely adapting to local food sources or whether it is a sign of a desperate, hungry animal not finding enough grass to graze is unknown. "There is a lot of material to contemplate when we introduce a foreign animal to a habitat, such as the potential implications of that introduction," said Jenssen. The Catalina Island Conservancy is removing other nonnative animals such as pigs and goats in an effort to return the island to its natural state. But so far bison have been reprieved from deportation. Catalina receives over a million tourists every year—many who come specifically to see the bison. 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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