March 27, 2012
The island fox is one of the many endemic species that inhabit the Chanel Islands. The fox is the largest native mammal on the islands and was the top predator for thousands of years. It is believed that the island fox evolved from the gray fox, which came over from the mainland more than 18,000 years ago by rafting on pieces of debris from storms. Since the arrival of the grey fox, six subspecies of the island fox have evolved on the islands. The island fox is one-third the size of its ancestor. Urocyon littoralis santacruzae is the scientific name of the Santa Cruz Island fox.
While historically, the Santa Cruz island fox stood at the top of the island’s food web, a process known as hyperpredation caused ecosystem interactions to be restructured. Hyperpredation refers to a scenario in which an indigenous species is subject to increased predation from an exotic predator that is able to live because of availability of an exotic prey. On Santa Cruz Island, the indigenous island fox experienced increased predation from nonnative golden eagles that were able to colonize the island because of the abundance of feral pigs.
Pigs came to Santa Cruz Island in the 1800s when European settlers brought them over with sheep to serve as domestic livestock. Pigs that escaped or were let loose quickly established large feral populations on the island and reeked havoc on its ecosystem. Since pigs reproduce at alarming rates, large litters of piglets attracted golden eagles to the island in the 1990s. The piglets served as an abundant year-round food source that allowed the golden eagles to establish themselves on the island. Golden eagles would have become a problem sooner on the island, but a population of territorial bald eagles prevented them from establishing themselves. Unfortunately, by 1960, the bald eagle populations had disappeared because of hunting and DDT contamination.
While golden eagles prey on skunks, pigs, and foxes, the fox population has taken a disproportional hit in comparison to the other two prey populations. Pigs have the advantage of being able to reproduce throughout the year. They are also able to outgrow predation. Skunks avoid a lot of predation due to the fact that they are nocturnal creatures. Foxes, on the other hand, only reproduce once a year, are mostly active during the day, and can’t outgrow predation. In 1994, the island fox population was at around 1,5000. By 2001, the population had fallen to around 60. This amounts to a 95% reduction in population size in less than a decade. The Santa Cruz Island fox was declared an endangered species in 2004 by the U.S. Fish and Wildlife Service.
With the survival of the island fox at risk, a multi-layer approach was executed to return balance to the Santa Cruz ecosystem. This comprehensive eco-system recovery plan included captive breeding of the island fox, relocating the golden eagles that preyed upon the foxes, eradicating the feral pigs, and re-establishing the bald eagle on the island. Each component of this plan worked together to restore balance and successfully led to one of the fastest recoveries of an endangered species in US history. Since the foxes were first listed as federally endangered the population has grown by 20 to 30% annually. The total number of wild foxes has increased to over 410, with the island fox survival rate around 96% on the tracked and monitored foxes, all in less than a decade.
In order to boost the total fox population, a captive breeding program was established in 2001 on Santa Cruz Island. First wild foxes were caught, and breeding pairs were established. In 2005, 20 pups were born in captivity, with a total of 85 pups born over the programs six years. While captive breeding always brings about the issue of human interaction altering the behavior of wild animals, such a program was essential for boosting the total population in the wild. Additionally, the establishment of a successful breeding program represents a future for island foxes, because when numbers get exceptionally low, scientists can conceivably save the population from extinction by breeding pups in captivity. On advantage that the captive breeding program on Santa Cruz Island held over some of the other programs on other Channel Islands is that the large remaining wild population allowed for the program to increase the number of founders in captivity on a regular basis. If mating pairs were unsuccessful, they were able to bring in other foxes to breed with. This helped to increase the genetic diversity of those bred in captivity. These foxes were also vaccinated against canine distemper virus and rabies in order to limit the risk of disease causing a population threat down the line.
At first the pups released to the wild were quickly being eaten by the golden eagles, but as the Golden Eagle population on Santa Cruz was relocated to the mainland, the survival rate of pups increased dramatically. The relocation of the eagles was a crucial part of the fox recovery, as predation by eagles was identified as the cause of death for over 72% of the monitored foxes between 2000 and 2006. However, it was no simple task to be able to remove these eagles as they are legally protected by the Migratory Bird Treaty as well as the Bald and Golden Eagle Protection Acts. Once given permission to relocate these animals, 32 golden eagles have been captured and released on the east side of the Sierra Nevada Mountains. These magnificent birds are often captured using dug-in nets placed in areas that eagles frequent. The nets are baited with dead feral pigs or live rabbits, and radio-controlled to capture the eagle. The eagles are transported in large commercial sky kennels and are always released on the mainland within 24 hours. Today, Santa Cruz is thought to be home to less than 10 Golden Eagles that have continued to evade capture. Thanks to a very successful relocation program by the Santa Cruz Predatory Bird Research Group, the fox’s main predatory has been eliminated allowing for the foxes to recover.
However, relocating the current eagles to the mainland is an insufficient solution, without also removing the prey that first established the colony of golden eagles on the island. As a result, an intense feral pig eradication program has also taken place, resulting in the removal of over 5,000 pigs from the island by 2006. Today, no feral pigs remain. Santa Cruz sectioned off the island, and proceeded to hunt the pigs in each section, often shooting from a low-flying helicopter. Relocation of the pigs was not an option as Federal and State laws prevent moving these pigs to the mainland, due to potential diseases. This removal of pigs has also benefitted many native plants that were being destroyed by the pig population, such as the blue dick flower. The eradication of the pigs is thought to be the most important step in restoring the natural Santa Cruz ecosystem.
They also are working to re-establish the bald eagle, in order to prevent the golden eagle from recolonizing and re-threatening the fox in the future. The bald eagle used to occupy the island but was destroyed by high DDT levels off the coast. While the bald eagles eat fish, seabirds, and animal carcasses, they do not eat live foxes and therefore do not present a danger to the recovering species. Additionally, the bald eagles are extremely territorial, and since golden eagles and bald eagles hardly co-habitat anywhere, the increase in bald eagles would keep golden eagles from moving back to Santa Cruz. This effort is being led by the Institute for Wildlife Studies and the Montrose Settlement, who have released over 50 bald eagles since efforts began in 2002. For further reading on the bald eagle efforts, follow along at http://www.nature.org/ourinitiatives/regions/northamerica/unitedstates/california/explore/santa-cruz-island-bald-eagles-hatching-hope-2010.xml
Although the eco-system is certainly well on its way to recovering its natural balance, a few issues remain. The greatest concern is that the original depletion to merely 70 foxes created somewhat of a genetic bottleneck, reducing the overall genetic variability within the fox population. This may lead to the fox population being more susceptible to diseases and other threats that will impact all foxes with similar genetic traits. Additionally, there remains the question of how well the pups bred in captivity will be able to teach their young how to behave in the wild, since they lacked that form of training and it is believed that foxes parent their young through the first year. Whenever humans involve themselves in the raising of wild animals, there is a question of how that human interaction will impact the animal’s behavior in the long run. Outside of the fox problem, the Santa Cruz ecosystem is still threatened by non-native plants such as fennel overwhelming the natural vegetation which the animal populations rely on.
All in all, the eradication of feral pigs and relocation of golden eagles have allowed for the island foxes to recover at an unprecedented speed, making Santa Cruz Island an endangered species recovery success story.
This post was authored by Melissa Krigbaum ’12 a double major in Environmental Studies and Economics and by Alex Anthony ’12 majoring in Environmental Studies.