Last week, I wrote about the biology and habits of the black oystercatcher, and I mentioned until recently, this species has not received much attention from biologists. In the past few years, though, researchers have realized the overall health and dynamics of oystercatcher populations can tell us a great deal about the health of the organisms that live within the intertidal zone. The black oystercatcher has been called a keystone species, but I think it would be more accurate to call it an indicator species. A keystone species is a species whose presence and role within an ecosystem has a disproportionate effect on other organisms within the system, while an indicator species is a species whose presence, absence, or relative well-being in a given environment is a sign of the overall health of its ecosystem. The well-being of black oystercatchers is thought to be an indicator of the overall health of the intertidal community along the North Pacific shoreline. Tlingit shamans identified with black oystercatchers and believed that just as shamans inhabit both the human and spirit world, oystercatchers live in the border world between land and water. As a tribute to these special birds, shamans often depicted oystercatchers on their rattles.
As the Tlingits noted, black oystercatchers live in a narrow band of habitat and are dependent on the intertidal area to breed, nest and feed, making them vulnerable to both natural and human disturbances. Monitoring black oystercatcher population trends and movements can better help us understand the health of this intertidal zone. In 2004, the International Black Oystercatcher Working Group was formed. This group includes federal and state agencies from Alaska, Washington, Oregon, California and British Columbia. The group’s focus is to learn more about black oystercatchers. In addition to understanding world and local population sizes, researchers hope to learn more about the life cycle of the black oystercatcher. They want to pinpoint natural and manmade threats to oystercatchers and to employ methods to minimize these threats. Understanding black oystercatchers will not only help safeguard these birds but could be key in protecting the intertidal community of organisms and the wide variety of animals that depend on this community for food.
Predation is the major cause of mortality for black oystercatcher eggs and chicks. In Alaska, predators include eagles, ravens, crows, Glaucous-winged gulls, foxes, bears, river otters, wolverines, marten, and mink. Because they live so low in the tidal zone, black oystercatchers are susceptible to flooding, especially in Alaska, where most nests are on low, sloping beaches. Flooding may be caused by natural causes such as extreme high tides, storm surges, or tsunamis or by manmade causes such as boat wakes. Since black oystercatcher nests are on the ground, they are also susceptible to disturbance by humans walking along the beach and stepping on the nests or disturbing the nesting birds.
Black oystercatchers are vulnerable to shoreline contamination, especially from oil spills. The 1989 Exxon Valdez oil spill in Prince William Sound, Alaska had a major impact on oystercatchers. Twenty percent of the oystercatchers in the area of the spill were killed immediately, and those that were not immediately killed had to either eat oil-contaminated prey or starve. Now, biologists know the short-term impacts of the oil spill on oystercatchers were only part of the story. Current research shows oystercatchers are still suffering from the physiological effects of feeding on oil-contaminated prey.
We need to learn a great deal more about the biology and habits of the black oystercatcher. Once we understand them, they can help us gauge the health of the intertidal zone from California to British Columbia and Alaska.
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