Where Did All the Foxes Go?

I had moved to Channel Islands in 1992 (from Death Valley) to work in the park’s ecological monitoring program, a relatively new National Park Service initiative to, as we would say, conduct a health check on a park’s ecosystem every year. Island foxes were an entirely logical element to monitor, since they were unique, somewhat rare, at the top of the island’s food chain, and were thought to be vulnerable. For one, to diseases, since island foxes had evolved in splendid isolation on the islands, and likely had not  been exposed to diseases to which mainland species had been exposed. Their naturally small population sizes also put them at risk of just “blinking out” due to chance events. That being said, we were not entirely worried about them at that time; in fact, island spotted skunks were thought to be even rarer, and maybe more at risk.

We monitored island fox populations by trapping them every summer on large grids on the islands. They were not that hard to trap; they readily went into box traps baited with dog food and a scent attractant. They were pretty easy to handle, as well. Being so small, one could grab them and control them without having to knock them out with drugs. Perhaps because they had never been hunted, they were a little more amenable to being handled. We marked them with “PIT” tags – passive integrated transponders – the same small (grain of rice -sized) tag now used on domestic pets.

Those first couple of years, we found LOTS of island foxes on San Miguel Island, where we had started the island fox monitoring program. In fact, we found that island foxes lived at higher densities, as much as 10-15 foxes per square kilometer, than any other canid species. Plenty of foxes out there. However, in the third summer of monitoring (1995), there were less foxes, a fact we attributed to the natural swings that wildlife populations can exhibit due to annual weather differences, for example. Perhaps we were in denial (my stock-in-trade move). Foxes declined further in 1996 and 1997, and by that time we were fully engaged in figuring out why.

There are so many factors potentially affecting wildlife populations. We looked, obviously, at food; island populations can be affected by food swings that come with the El Nino climate swings. However, there had been no drought in those years in southern California, and we had also been monitoring some of the island fox’ food items – vegetation, mice, lizards and birds. No decline in any of those. We feared a disease outbreak, since foxes were “naïve” to most diseases, having no history of exposure to them. Along with Dave Garcelon of the Institute for Wildlife Studies and Gary Roemer of UCLA, we conducted a specieswide disease survey to smoke out any effect disease might be having on island foxes. No smoking gun, however; no sign of lethal canine diseases such as canine distemper virus or rabies.

Finally, in 1998, we obtained funding for radiocollars, so we could see directly what was taking out our foxes. Radiocollars work by sending a radio signal every minute or so, and you can track where a radiocollared animal is by triangulating the signal, by homing in. Their real value was telling us when – and how - an animal died. If a fox was motionless for six hours, the signal changed; it sped up. When we received the mortality signal, we triangulated to find the carcass, and then performed a necropsy, or autopsy, to determine the cause of death.

We affixed radiocollars to a handful of island foxes on San Miguel in late 1998, when we estimated there were less than 50 foxes left on the island, down from a high of about 500 in 1994. Within two months, half of our collared foxes had died, and most of those had been killed – by golden eagles! We knew this because of telltale signs at the kill sites. The carcasses had been ripped apart in a manner typical of golden eagles. There were claw marks on the carcasses that matched the talon marks of golden eagles, and there was whitewash left at the carcass sites. The kicker was the feathers we found at kill sites. They were certainly eagle feathers, and we sent them to the USFWS forensics lab in Oregon for confirmation. Those guys were good. They reported to us that not only were they golden eagle feathers, but in some cases they were able to be more specific: this was a rump feather from an immature golden eagle, for example.

Golden eagles on the Channel Islands? We were dumbfounded. Golden eagles didn’t breed on the Channel Islands, and had never done so in the past. Bald eagles, sure…but they were gone from the islands due to the effects of DDT, which had been manufactured in Long Beach, where the runoff took it out to sea and to the Channel Islands. We worked with raptor biologists to investigate the golden eagles on the islands. They ended up finding half a dozen nests on Santa Cruz and Santa Rosa Islands, and those nests were like archeological digs: by excavating them they found prey remains, which told us what the golden eagles were eating. Turns out they were eating, during the nesting season, the non-native ungulates (hoofed animals) brought to the islands by early ranchers: European boars (pigs) on Santa Cruz, and introduced mule deer on Santa Rosa.. Specifically, piglets, and fawns, which were in the eagles’ prey size range. When those weren’t in season eagles were eating everything else on the islands, including island foxes (those were found in the nests, as well). It was a case of what Gary Roemer called hyper-predation: the presence of one prey base (pigs and mule deer) allowing a predator to severely impact a different prey species (island foxes).

Because island foxes had not evolved with an aerial predator, they were very vulnerable to the sudden appearance of golden eagles in the mid-1990s. Death came from the sky, and foxes never saw it coming. I often said it was a perfect storm of factors that made island foxes so vulnerable. Had bald eagles still been on the islands, had they not been decimated by DDT, they may have prevented colonization by another large eagle species. And if the islands’ habitats not been altered by decades of grazing, foxes may have found sufficient cover in native island shrubs, which in many areas had been replaced by European grasses. And the presence of non-native grazing animals provided a prey base for eagles which otherwise would not have been able to breed on the islands.

In 1999, then, we knew what we had to do: stop eagle predation on island foxes, bring island fox numbers up from perilously low levels, and implement ecosystem changes that would favor island fox persistence.