May 29, 2002 The California sea otter is in trouble and people may inadvertently be part of the problem. Nearly half of sea otter deaths are associated with infectious diseases and several of these may be indirectly caused by people.
"The diseases that cause the most mortality in otters…seem to be newly emerged. In some cases, humans may have influenced the spread and emergence of these new diseases," say Kevin Lafferty of the U.S. Geological Survey at the University Of California, Santa Barbara, and Leah Gerber, who did this work while at the National Center for Ecological Analysis in Santa Barbara and is now at Arizona State University, in the June issue of Conservation Biology.
Managers rarely explicitly consider the threat of infectious diseases to wildlife populations. However, many of the factors that threaten species -- from habitat degradation to introduced species to pollution -- may also affect the transmission of infectious diseases. These factors typically limit the spread of infectious diseases, making most pathogens die out in populations of rare species. That said, domestic animals can harbor diseases that can spread to wildlife, which can be particularly devastating to rare species. For instance, domestic sheep diseases have wiped out populations of bighorn sheep. In addition, an introduced infectious disease can make common species become rare because they are not adapted to the new pathogen. For instance, introduced bird malaria has decimated many Hawaiian song birds that used to be common.
"Many of these wildlife diseases are viewed as emerging, particularly as we introduce species around the globe and as our domestic plants and animals encroach on natural habitat," says Lafferty.
In their analysis of the link between disease and conservation, Lafferty and Gerber highlighted the California sea otter. While the Alaska population has been growing at about 18% per year, the California population has been declining by 2-3% per year since 1995. The primary reason is that young adults have been dying, according to marine ecologist Jim Estes.
Lafferty and Gerber analyzed data from previous studies by other researchers about the causes of California sea otter deaths. "Because existing data indicate that non-otter diseases are responsible for a large proportion of otter deaths, we wanted to see if there was a link between disease and sea otter population trends," says Lafferty.
A parasitic worm caused 14% of the otter deaths and the researchers found that more otters died in years when the worm was common, suggesting that it could be keeping the population from recovering. The worm (Polymorphus kenti) can penetrate the intestine and cause peritonitis, which is most likely to be lethal in pups and juveniles. The worm is a shorebird parasite that uses sand crabs as an intermediate host. Sea otters usually don't catch it because they prefer molluscs and sea urchins. But people also like molluscs and sea urchins and when these prey are scarce, an otter can eat several sand crabs in a row. "Humans may have increased the prevalence of this disease by competing with otters for food, causing them to seek alternative prey," say Lafferty and Gerber.
Toxoplasmosis caused up to 8% of the otter deaths. Toxoplasma gondii is a cat parasite, and the otters may catch Toxoplasma cysts from sea water contaminated by cat feces. This disease could be decreased by improving waste disposal.
Valley fever caused 4% of the otter deaths. This disease is caused by inhaling spores of a soil fungus (Coccidiodes immitis) in dust raised by construction and agriculture. "The emergence of valley fever as an otter disease corresponds with an increase in human cases in California," say Lafferty and Gerber. Dust control efforts that reduce human exposure to valley fever may also help protect sea otters from the disease.
The good new is that if we are contributing to these sea otter diseases, we can do something about them. "Perhaps the biggest benefit to be gained by understanding the role of disease in [California] sea otters is that it will help focus attention on the sources of mortality that will influence recovery efforts," conclude the researchers.
Lafferty and Gerber call for more research on how diseases affect wild populations. "The effects of most infectious disease probably go unnoticed," they say.
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