August 9, 2012
By: Lauren Stoneburner
Santa Catalina Island—a coastal Mediterranean island off the coast of California—is the least likely place to find America’s quintessential prairie animal, the American Bison (Bison bison). However, the filming of a Western movie in 1924 summoned these beasts from the grasslands of central North America to this isolated island, which they have come to call “home.”
As residents and tourists grew accustomed to their presence, they became an iconic feature on the islands. More than one million tourists visit Catalina Island every year, and many of those tourists come specifically to see the bison (Trivedi). The bison has not only brought revenue to Catalina’s economy, but residents have adopted a sense of pride over these animals, contributing to the island’s culture (Sweitzer). The economic and cultural value that these beloved animals provide has protected the species from being completely eradicated. However, their numbers must be limited, for their impact on the native island ecosystem is undeniable.
Because of its long-term isolation, Santa Catalina has developed an intricate ecosystem, composed of specialized species that play very specific roles within the ecosystem. Prior to the bison’s arrival, Santa Catalina had never before hosted large, grazing ungulates (Sweitzer), so their presence has greatly disturbed the dynamics of this fragile ecosystem. The bison’s primary impacts stem from their grazing, trampling, and wallowing behavior.
Catalina’s bison currently graze on a variety of native and non-native vegetation. Though they primarily eat grasses and forbs, they have added to their diets cactus and some woody schrubs, including the endemic coastal scrub oak (Sweitzer). Thus, bison do control the exotic species populations through grazing, but they directly limit the success of native plants as well. In total, the pressure that bison have put on native plants is far greater than the benefit of their controlling invasive competitors (Sweitzer).
Additionally, bison wallowing has also promoted the spread of invasive plants. As bison wallow, they embed plant seeds into their fur and later release the seeds into the environment as they wallow elsewhere. This behavior directly spreads non-native seeds across the island, allowing the plants to establish populations in new regions and further outcompete native species (Sweitzer). These impacts can be reduced by limiting the bison population size, for fewer bison directly lessens non-native plant dispersal (Sweitzer).
Trampling, along with grazing and wallowing, causes the worst damage—soil degradation. These behaviors kill the vegetation, which hold the soil in place. As a result, the soil becomes loose and exposed, leaving it vulnerable to erosion (Trivedi). Erosion washes away fertile soil, transporting critical nutrients, such as nitrogen and phosphorus needed for plant growth, and microorganisms living in the soil that perform ecosystem functions, such as nitrification, decomposition of dead organics, and recycling of other nutrients. Thus, the impact that bison have on soil quality is catastrophic, for the soil ecosystem is lost and no longer provides nutrients that native plants need to support the rest of the ecosystem. This provides further opportunities for invasive plant species with less specified soil requirements.
The Catalina Island Conservancy manages the bison herd size, keeping the population between 150 and 200 individuals (“History”). This status quo compromises the environmental integrity of the island and the cultural and economic pressures. Considering how strongly attached the residents and tourists are to the American bison, it is not currently feasible that the bison will be entirely eradicated. However, the current status quo still sacrifices more environmental integrity than should be allowed.
Catalina Island holds an especially unique and fragile ecosystem that is currently threatened by the proliferation of invasive species. As bison continue to persist in high numbers, restoration attempts will be undone. Soil quality will continue to decline, and invasive species will further dominate native landscapes. Many decades or centuries from now, the bison may become naturalized, becoming integrated into the island’s intricate ecosystem. However, if Catalina’s native ecosystem continues to deteriorate by the bison’s doing, residents may have to accept that this island is not a permanent home, but a makeshift residence that is breaking down, long expired, and in need of significant repair.
About the author: Lauren Stoneburner is a sophomore undergraduate majoring in Environmental Studies and Biological Anthropology at the USC Dana and David Dornsife College of Letters, Arts and Sciences.
“History.” Catalina Island Conservancy. Catalina Island Conservancy, 2009. Web. 09 Aug. 2012. http://www.catalinaconservancy.org/index.php?s=wildlife .
Sweitzer, Rick; Van Vuren, Dirk. “Bison Study Executive Summary.” Santa Catalina Island Conservancy. Catalina Island Conservancy, 2009. Web. 07 Aug. 2012. http://www.catalinaconservancy.org/userfiles/files/Bison%2520Study%2520Executive%2520Summary.pdf .
Trivedi, Bijal P. “”Tourist” Bison Devastate California Island.” National Geographic. National Geographic Society, 17 Dec. 2001. Web. 07 Aug. 2012. http://news.nationalgeographic.com/news/2001/12/1217_TVbison.html .