January 13, 2012
Santa Catalina Island, as well as the other Channel Islands, off the coast of California exhibits unique, or endemic, biodiversity that makes it a place of interest for study by scientists. Due to its consistent isolation from the mainland, species that have colonized Santa Catalina Island have evolved into distinct counterparts to their mainland relatives making them endemic. These species originally have come to the island by various means such as the deposition of seeds of mainland plant species by flying birds or the “rafting” of animals on vegetative debris washed out to sea from mainland floods. However, these endemic species have been threatened more recently by invasive species that have been brought by human activity. Mitigation efforts have had moderate success although many native species have already been lost.
One particular example of a species endemic to Santa Catalina Island is the Santa Catalina Ground Squirrel. The Santa Catalina Ground Squirrel is differentiated from its mainland counterpart by being noticeably larger. Biologists have characterized the larger size of the endemic species as ‘gigantism.’ Gigantism is a common evolutionary trait for smaller vertebrate and many plant species that colonize islands. Due to the lack of competition and predators on islands in comparison to the mainland, it is hypothesized that these organisms are able to evolve into larger forms of their mainland counterparts.
The Santa Catalina Ground Squirrel is endemic only to Santa Catalina Island causing it to be an interesting species to study for biologists and biogeographers. Schoenherr, Feldmeth, and Emerson in their book, Natural History of Islands of California, state that it is not currently known why the Santa Catalina Ground Squirrel has not colonized the other Channel Islands as many other species have done. Rafting on vegetative debris from the mainland is one possible explanation for how the endemic squirrels arrived on Santa Catalina although it would seem just as likely that the Santa Catalina Ground Squirrel would over time have rafted to and colonized other Channel Islands. Schoenherr, Feldmeth, and Emerson suggest that due to evidence of the squirrel in Native American stool deposits on the island that they may have been brought to Santa Catalina for food. Again, it is not fully understood why Native Americans would not have brought the squirrels to other islands for this same purpose. One hypothesis for their endemism to only Santa Catalina may be that the island has remained isolated by the sea from its neighbors. This is different than the Northern Channel Islands that conglomerated into one larger island in glacial periods when sea levels were much lower than they are today. The land bridges formed between what are individual islands today would have allowed species from those islands to intermix and colonize the larger landmass that subsequently was broken up when sea levels rose again.
One species that underwent further evolutionary differentiation due to the isolation of individual populations on separated Northern Channel Islands is the Island Fox. The Island Fox is present on most of the Channel Islands, with the subspecies of the Santa Catalina Island Fox being specifically endemic to Santa Catalina Island. Notably, the Island Fox has differentiated more than any other island mammal. It is closely related, however, to the nearby mainland’s Gray Fox. The Santa Catalina Island Fox is much smaller than its mainland relative representing a common evolutionary trait for larger vertebrates that colonize islands called ‘Dwarfism.’ Large animals’ tendency to be smaller when isolated on an island is attributed to the limited supply of food on islands as opposed to the mainland. However, it is not completely certain that the Gray Fox is the Santa Catalina Island Fox’s most direct ancestor as there is some dispute about how the Island Fox arrived at Santa Catalina Island.
Based on mitochondrial evidence and other scientific indicators, it is estimated the Island Fox’s common ancestor arrived to the Northern Channel Islands 16,000 years ago. How the fox arrived at the Channel Islands, and more specifically Santa Catalina Island, is not known for certain, but evidence has indicated several possible methods. The vicariant distribution hypothesis is supported by the close relation between the Island Fox to three small fox species of the Yucatan region of Mexico and in Guatemala. Vicariant distribution relies on plate tectonics and the concept of a “land raft.” About 29 million years ago, the Pacific plate and the North American plate, which meet on the west coast of North America, changed in their interaction from a head-on convergence to sideways motion also known as a transform system. The Pacific plate carrying the Channel Islands traveled north, breaking from near Sonora, Mexico, and brought the islands’ inhabitants with them acting as a “land raft.” Some suggest that when the plates shifted and caused the formation of the Channel Islands, the foxes came with the land. As mitochondrial evidence suggests a much more recent colonization by the fox, this hypothesis is not as well supported. Others suggest that the foxes may have come to the Channel Islands when sea level was low enough to make rafting on vegetative debris from the mainland. However, the more widely held belief is that Native Americans who may have used the foxes as pets brought the fox to the Northern Channel Islands and Santa Catalina around 16,000 years ago.
The Santa Catalina Island Fox and Ground Squirrel are among the endemic species that are threatened by exotic species brought more recently to the island in the last few centuries. As Spanish missionaries settled California in the late eighteenth century, many domesticated animals like pigs and goats were introduced to the Channel Islands. Invasive species quickly disrupted the ecological balance of the islands by destroying vegetation and competing with the native island species. Invasive plants took up valuable nutrients, water, and space from native shrubs. Pigs and hogs, rooting in the ground for food, exposed large amounts topsoil that was soon eroded into the sea causing a loss of fertility in the soil. Goats, being indiscriminant eaters, ate vegetation so close to the ground that it was unable to recover.
Another invasive species, bison, was introduced to Santa Catalina Island not by settlers but by moviemakers. In 1924, bison were introduced to the island for the filming of the movie The Vanishing American. While bison populations have caused harm by eating and trampling native grasses, bison populations are now limited and graze on mostly introduced grasses preserving the natural plants of Catalina. Also, the bison do not eat the vegetation as close to the ground as goats allowing for the vegetation to recover to a greater degree.
However, the negative impacts of these invasive species and human activities has not been able to rid the Channel Islands, and particularly Santa Catalina Island, of its fascinating and rich natural history and biodiversity. While some hypotheses suggest that the Santa Catalina Fox and Ground Squirrel were brought to the island by humans and therefore may be considered invasive, they have undergone thousands of years of evolution to become distinct species. Mitigation, conservation, and preservation efforts have been put in place and have had success. The Endangered Species Act has protected many of the endemic species of the island for years. In 1975, 42,135 acres, or 86 percent, of Santa Catalina was given over to the Catalina Conservancy for protection and management. In 1989, efforts to rid Santa Catalina Island of the Feral Hogs and goats began including shooting or trapping the animals from helicopters and creating a sadly unsuccessful goat adoption program on the mainland. Today, it is estimated that there are less than 1000 goats, 3000 pigs, and 500 bison on the island with the western side of the island almost totally free of the destructive animals. Regardless, many species unique to Santa Catalina Island regrettably have become endangered, or gone extinct, as their habitats are destroyed or taken over by invasive plants and animals. While Santa Catalina and the other Channel Islands provide great places to study endemic species and evolution, they also provide opportunities to study the management of invasive species, and conservation and preservation efforts that informs the work of others around the world combating these global problems.
This post was written by Brian Rodysill ’12, who is pursuing a BA in Environmental Studies with a minor in Natural Sciences; and by Alice Hall-Partyka ’14 who is pursuing a double major in Environmental Studies (BA) and Global Health.