March 27, 2012
Saving the Santa Cruz Island Fox
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.
Santa Cruz Island FoxMainland Grey 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.
Golden Eagle preys on a Santa Cruz Island fox
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.
A wild island fox spotted on the ENST 495 class trip to Santa Cruz Island. (source: Melissa Krigbaum)
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.
A pup born in captive breeding program on Santa Cruz
Feral pigs allowed for the Golden Eagle to establish itself on the island
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.
Foxes being socialized before their release into the wild
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
The newest generation of Bald Eagles born in Pelican Harbor on Santa Cruz. They have been banded for tracking by the IWS
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.
Internet Sources:
http://www.biologicaldiversity.org/campaigns/esa_works/profile_pages/SantaCruzIslandFox.html
http://www.universityofcalifornia.edu/news/article/3817
http://www.nps.gov/chis/naturescience/restoring-santa-cruz-island.htm
The concern of learning through teaching via fox parents is a serious one as researchers can most likely only learn so much of what normal fox parents pass on to their young.
However, I think the foxes have a unique position as the apex predator on the island, so they will, most likely have a higher chance of survival than say an organism in the middle of the food chain.
It is truly unfortunate how much the island fox has had to go through but I am glad we have helped them recover. It is a very difficult dilemma because we can try and recover the island fox population by breeding them and having them captive, but that also means that they will not survive as well in the wild. The other option is eradicating the threatening species. This reminds me of how complicated natural systems are, including trophic chains and biological communities. I am glad that it all worked out, but it is amazing how complicated nature is and how important research is in order to make right decisions.
A notable issue came about due to the decline in the Santa Cruz Island fox population. Both the Island Fox and the golden eagle are endangered species, but while the golden eagle seemed to thrive on Santa Cruz Island, it was because it had an abundant food source: the Island fox. Not only was this invasive eagle protected as an endangered specie, but it was also protected under the Migratory Bird Treaty and the Bald and Golden Eagle Protection Act. Other scenarios certainly mimic the dilemma on Santa Cruz Island, and I believe that people should prioritize the protection of the native endangered animal even if its invasive predator is also endangered. This issue becomes controversial because sometimes humans feel strong connections to specific species, such as eagles, and they don’t want them hurt or disturbed. People should focus on protecting both species in these scenarios, but if only one can survive in a particular area, it should be the native animal.
This blog post brought up the interesting issue of whether it’s ethical to get rid of one animal that is endangered to save another. Such is th case with the golden eagle and the Santa Cruz Island fox. The golden eagle is protected by Golden Eagle Protection Act and the Santa Cruz Island fox is native to Santa Cruz ISland. Both animals are endangered, so why is Conley choosing the life of one organism over the other a decision to make in this matter.