The Warning Provided by Monitoring
From 1993 to 1999 the National Park Service monitored the population of island foxes on San Miguel Island. Each summer, foxes were trapped and tagged. When each fox was first captured, it was implanted with a device about the size of a grain of rice called a passive integrated transponder tag, or "PIT" tag. The tag gives each fox a permanent, unique number that can be read with a special scanner and allows scientists to monitor individual foxes over the years.
Park biologists estimated there to be over 400 foxes on San Miguel in 1994, but by 1995, an alarming decline had begun. With each passing year, fewer foxes were trapped in the monitoring grids. At the same time, rangers and visitors reported seeing fewer live foxes and more fox skeletons and carcasses. By 1998, the San Miguel population was down to a few dozen. Island fox populations on all three islands were naturally small and had historically fluctuated, but as far as was known had never been as low as they were during this period and had never come close to extinction.
In fall 1998, National Park Service biologists initiated a radio telemetry study of island foxes on San Miguel Island to determine causes of mortality. Of 15 radio collared foxes tracked from 1998 to 1999, 5 were believed to be killed by golden eagles and 2 died of other causes. This confirmed the results of an earlier study on Santa Cruz Island where golden eagle predation was identified as the cause of death for 21 of 29 fox carcasses. Golden eagle predation was unprecedented, and was considered unnatural because golden eagles had not previously bred on the islands and were, until this time, rarely observed.
Similar declines occurred simultaneously in the island fox populations on neighboring Santa Rosa and Santa Cruz Islands Fox mortality rates due to predation were so high that by 1999 the San Miguel and Santa Rosa fox subspecies were nearing extinction; on each of those islands total abundance had declined from approximately 450 and 1,500, respectively, to 15.
Eliminating the Major Cause for Decline
While the threat of critically low island fox population size could conceivably be improved through captive breeding and reintroduction, such efforts would be futile unless the threat from golden eagles was eliminated. Historically, the native bald eagle most likely deterred golden eagles from exploiting the Channel Islands as a food source. Golden eagle did not colonize the islands until their larger fish-eating relatives were wiped out by DDT, hunting, and egg collection. With the golden eagle's sharp talons, swiftness of flight, and 4 times the body mass of a fox, they easily preyed upon the vulnerable fox.
Interestingly, the first documented decline on San Miguel in 1995 coincided with the first golden eagle sightings on the island. Golden eagles had been live-captured and relocated from some areas in western North America to reduce depredation on livestock and it was surmised that similar translocation methods could be used to golden eagles from the Channel Islands. In late 1999 the National Park began working with its partners to relocate golden eagles to the mainland. Golden eagles nested on both Santa Cruz and Santa Rosa Islands from the mid-1990s to as recently as 2006.
In order to mitigate golden eagle predation on island foxes, The Santa Cruz Predatory Bird Research Group, with the support of the Park Service and The Nature Conservancy, trapped and relocated a total of 44 golden eagles, including 10 eaglets born on the islands to distant sites in northeastern California. Monitoring indicated that , none returned to the islands.5 Today the occasional golden eagle visits the islands, but the level of predation on island foxes is negligible; all three island fox subspecies in the park are recovering rapidly.
Predation by golden eagles was the primary mortality factor for foxes on the northern Channel Islands, but there were other threats as well. Introduced diseases or parasites could conceivably devastate island fox populations. In 1999-2000, on privately-owned Santa Catalina Island, about 90% of the fox population was recently lost due to canine distemper virus.
Due to their insular existence, island foxes have no built up immunity to parasites and diseases brought in from the mainland and are especially vulnerable to those a domestic dog might be carrying. In addition, it is extremely difficult to vaccinate against or treat foxes for these parasites and diseases in the wild. For this reason, pets are not permitted in Channel Islands National Park.
Eliminating Other Factors Contributing to the Decline
Domestic pigs had been brought to Santa Cruz Island in the mid-1800s as a food source, and subsequently became feral. By the mid-1990s the thousands of feral pigs on the island became a factor in the decline of island foxes. Golden eagles were able to successfully colonize the island, and begin breeding, because of the availability of piglets.
Simply relocating the existing golden eagles would not assure island fox recovery. With the substantial prey base provided by island pigs, dispersing golden eagles would continue to breed on the island. It would therefore be necessary to implement longer-term actions that would prevent sustained use of the islands by golden eagles. This resulted in a strategy that would require the removal of the feral pigs on Santa Cruz Island beginning in 2004, in a project implemented by the NPS and The Nature Conservancy, its partner in management of Santa Cruz Island. By 2006, no pigs remained on Santa Cruz Island.
In a similar manner, introduced mule deer supported golden eagle breeding on neighboring Santa Rosa Island. Deer and elk on that island were to be phased out as part of an agreement between NPS and the former owners of the island, and by 2012 non-native ungulates had been all but eliminated on Santa Rosa Island.