The dwarf island fox, denizen of the Channel Islands off California’s coast, have bounced back from the brink of extinction in record time. The adorable pups were nearly eliminated by golden eagle imports to the islands. Their numbers were down to barely 100 foxes by 1999. Now, thanks to relocation of the golden eagles and concentrated breeding efforts by conservationists, the foxes number about 2500.
The foxes came to the Channel Islands between 6000 and 10,000 years ago as normal gray foxes. Over time, they evolved into smaller dwarf foxes due to restricted resources and space on the islands. This is one of the best examples of the phenomenon of island dwarfism in the animal kingdom. The tiny foxes evolved into six subspecies and thrived on their separate islands. At the height of their population, before 1940, it is estimated there were tens of thousands of the dwarf foxes on the islands.
Beginning in the 1940s, DDT insecticide killed off all the native bald eagles on the islands. This left an unfilled niche, quickly taken over by predatory golden eagles. Normally the golden eagles would have been chased from the islands by the bald eagles. The millions of pounds of DDT discharged into the ocean by chemical companies between the 1940s and the 1970s had contaminated the fish that were the primary food of the bald eagles. The golden eagles fed mainly on sheep and feral pigs introduced to the islands in the 1850s. They also snacked on dwarf foxes, quickly cutting the fox population to around 100 by 1999.
The Nature Conservancy and the Park Service own about 76 percent of Santa Cruz Island, once home to the Chumash Indians. By the time fox recovery efforts began in 1999, the dwarf foxes lived on only three of the six islands. About 85 island foxes lived on Santa Cruz Island, and about 15 lived on San Miguel and Santa Rosa Islands. They were listed as an endangered species in 2004.
In 2006, captive-breeding programs for the foxes began on Santa Cruz, Santa Rosa and San Miguel Islands. The feral pig and sheep populations, which had stripped the islands of vegetation cover for the foxes, were removed. Bald eagles were reintroduced to the islands, where they fed mainly on their preferred marine diets and left the foxes alone. Forty-four golden eagles were relocated to the mainland.
Free once again to live and breed without fear of being eaten by eagles, the island dwarf fox population rebounded with record speed. In less than a decade, they are ready to come off the endangered list according to biologists. Currently approximately 1300 foxes live on Santa Cruz Island, 500 on San Miguel and 600 on Santa Rosa. Equally promising is their survival rate of nearly 90 percent yearly.
The San Miguel and Santa Cruz subspecies populations are large enough and healthy enough that they are no longer endangered. Santa Rosa should be at that point in a few more years. The main concern for scientists at this point is whether the breeding population is too small to fight disease. Genetic diversity is required to give enough variation in the gene pool for genetic health. This “population bottleneck” can negatively affect a species’ ability to cope with diseases and environmental change.
The NPS and Nature Conservancy will continue to monitor the fox populations for signs of disease, parasites and pathogens. If it becomes apparent this has occurred, scientists may breed together different subspecies of the island dwarf foxes to try to improve genetic health.