April 20, 2012
One characteristic shared by the two islands is how susceptible their ecosystems are to disturbance, as exhibited by the crashes of their island fox population. Although different in cause, each demonstrated that a small island ecosystem, evolving under sheltered protection from mainland disturbances can create unique and fragile ecosystems that do not handle major disturbances well. This is largely due to their relatively small gene pool of the population and small geographic range.
In other more traditional geographic regions, a disturbance in an ecosystem that leads to a population crash can often be followed by an easier recovery. Either there is a large enough, well-adapted surviving population that can repopulate, or organisms from another region can gradually be reintroduced into the area. However on an island, often neither is possible. If the species experiencing the crash is endemic, then it is possible that the crash will result in the species extinction as no other existing members of the species exist in the world. Even if some individuals survive the initial disturbance, with the population, small to begin with, may leave so few survivors that the gene pool does not carry enough diversity for a proper recovery and the species may die out. As such, a disturbance in an island ecosystem is much more likely to lead to species extinction.
On Catalina Island, the collapse of the fox population was primarily due to the introduction of the canine distemper virus. In 1999 an outbreak occurred causing the population to drop from 1300 to only 100 animals. The outbreak swept across the west side of the island but fortunately did not reach the eastern island, which was separated by a narrow isthmus. In 2000 the Catalina Island Conservancy and the Institute for Wildlife Studies instituted the Catalina Island Fox Recovery Plan, which consisted of monitoring, captive breeding, vaccination, and relocation of the foxes. The program was a success and by 2004, the population had climbed up to 300. Although it is not entirely known how the virus was introduced into the population, one theory is that it was brought to the island by an infected domesticated dog or a stow-away raccoon.
On Santa Cruz Island, a collapse also occurred, but for different reasons. Over-predation by the golden eagle, an exotic species, was discovered to be the primary cause. However indirect blame could be placed on the human introduction of pigs to the island. A study by Roemer et. al. indicated that the colonization of Golden Eagles onto the island could only be sustained by the existence of a feral pig population. However, even though the foxes alone could not sustain the eagle population, they were much more affected by eagle predation than the pigs. The foxes were ill adapted to evade eagle predation and as such faced possible extinction.
Like the island fox’s unfortunate fate at the hands or claws of introduced species and viruses, many native and endemic plant species on both Catalina and Santa Cruz islands have suffered from human introduced grazers. While both islands have gone under some form of plant restoration from the damages done by past-introduced grazers, Catalina currently still has resident populations of non-native grazers while Santa Cruz Island does not. This provides an interesting contrast between the islands because there are many similar native plant species that exists on both islands but in different quantities and manifestations. Through this comparison one can clearly see the tremendous impact that grazers have on the plant communities of the Channel Islands.
Catalina currently has a small population of 150-200 bison that roam the island. The bison population is controlled both by a birth control that limits the number of calves a female bison can have a year and by shipping the bison back to the mainland to supplement mainland herds on tribal lands. The birth control method was introduced in 2009 and was greeted by animal rights activists who opposed the Catalina Conservancy’s earlier eradiation of feral goats and pigs with high power rifles from helicopters. The Los Angeles Times reported that the birth control option for controlling the bison herds was suggested by an animal activist Avalon shop owner named, Debbie Avellana. Other non-native grazers that continue to roam the island are mule deer that are kept under control by recreational hunting as well as the Conservancy, and a very small population of black buck antelope. Historically Catalina was used for grazing goats, pigs, sheep and cattle but have since been irradiated.
Catalina’s current native plant population has suffered as a result of the current non-native grazers on the island. The effect of the grazers can be seen all too clearly in the example of the native Giant Coreopsis (Coreopsis gigantea). On Catalina this “Dr Seuss plant” is only found with in the confines of the Ackerman Nursery where grazers are kept out. There are also reports of some wild species on the sea bluffs or steep gullies of the island where grazers can’t get to them. On a whole plants on Catalina tend to be bush-like where they otherwise would be more like trees. The only “trees” you will find on Catalina are either non-native or are the native toyon, lemonade berry, sugar bush or Catalina Cherry trees because they are so resilient. Some native plants have changed their pollination season to try and outcompete not only the grazers but also invasive plants.
Restoration on Catalina is difficult because there is a permanent human population there and the island attracts around a million tourists a year. This constant stream of visitors means the potential for foreign species introduction is more likely. Fennel is still a problem on the island being an aggressive invasive species, but a management strategy including weeding around campsites and populated areas outward seems to be working in its early stages. Another invasive species is the eucalyptus, which was brought to the island on purpose to beautify areas like Avalon and was a favorite of the Wrigleys. Santa Cruz Island also struggles with both eucalyptus and fennel.
Santa Cruz Island does not have any non-native grazers currently living on the island. Historically Santa Cruz Island was a ranch raising some of the most well known beef and sheep products on the west coast. Since then it has been brought under the control of National Park Services and the Nature Conservancy. The only human presence is that of campers and eco-tourists, and researchers. There are a few people that live there to maintain the research and historic ranch facilities. These conditions have allowed a recovery of many native plants and allows for these plants to grow large and where on Catalina you may have a sparse bush, on Santa Cruz Island you will have a large bush as tall as a man. On Santa Cruz Island, Giant Coreopsis and Bedstraw are significantly more common than on Catalina as are buckwheats (including one species of buckwheat that is endemic to Santa Cruz Island), Manzanita (also including a endemic species), and Sunflower bush. Santa Cruz Island has around 600 native plant species.
These cases, exhibit how island ecosystems are incredibly susceptible to disturbances, which can often be brought upon by the interference of humans. In the case of Catalina Island Fox, the introduction of a virus, possibly by a colonizer’s pet dog, is to blame for the collapse of a species. Santa Cruz’s population collapse was brought upon by the human introduction of pigs to the island, which facilitated the entry of yet another harmful invasive species. It is believed that in both instances, had humans not brought in these disturbances that such a collapse would not have occurred. Just as these collapses wouldn’t have occurred without human interference one can use Santa Cruz Island to “see” how different a landscape Catalina would have if it didn’t have the human introduced grazers still shaping plant communities on the island. As such these cases serve as a reminder that humans should exercise extreme caution when interacting with such isolated ecosystems, as they can be as fragile as they are unique and beautiful.
This post was written by Mariah Gill ’12 and Jefferey Nakashioya ’12 both seniors in Environmental Studies.
Carlos de la Rosa, Personal Communication/ Lecture