Case study: How prey choice and habitat use affects the sea otter population
A study conducted by Johnson et al. (2009), examined how two diseases affect the population growth of sea otters. More specifically, it investigated how the choice of prey and habitat use affects the population of the southern sea otter (Enhydra lutris nereis). Regardless of federal protection, sea otters are struggling to recover and it is believed that infectious diseases are an important cause of mortality.
This study looked closely at infectious brain diseases caused by two parasites, Toxoplasma gondii and Sarcosystis neurona, which are typically transmitted to a carnivore through ingestion of infected prey. The eggs of these parasites are shed through the feces of terrestrial animals: opossums for S. neurona and members of the cat family for T. gondii. The infected feces are picked up in run-off water and the eggs are transported into the ocean, where they enter into the marine food web. Sea otters become infected when they eat shellfish that contains these eggs (although the shellfish themselves do not become infected).
In order to investigate risk factors for these diseases in sea otters, the researchers used two methods, observational field studies through radio-tagged otters and epidemiologic tools in the lab. The study included 118 sea otters whose health and behavior was monitored closely over a 3-year period. The otters were tested for disease exposure when they were captured by collecting and analyzing a small blood sample: 56 (48%) tested positive for T. gondii and 39 (33%) tested positive for S. neurona. The researchers also noted the prey choices of the sea otters: they found that most of the individuals in this study specialized on just one or two of the most common diet items, including Cancer crabs, abalone, sea urchins, marine snails, clams, sea stars and kelp crabs. This individual diet specialization turned out to be a key to understanding parasite infections.
The findings of the study showed that becoming infected with these diseases was related to the individuals’ choice of prey and habitat use. Otters that consumed many marine snails had twelve times greater probability of being infected with T. gondii then otters that rarely consumed marine snails. Also, otters living and foraging in San Simeon and Cambria, California, were four times more likely to become infected with T. gondii then otters living in other places. In contrast, those otters feeding on prey from sandy-bottom habitats (such as clams and worms) were more likely to become infected with S. neurona.
In conclusion, the study finds that diet preference and habitat location are high risk factors for contracting this type of brain disease. The authors also point out that there is an association between pathogen exposure and consumption of lower quality prey types. Depleted resources and high levels of disease may be acting together to limit the rate of population recovery.
Johnson, C. K., M. T. Tinker, J. A. Estes, P. A. Conrad, M. Staedler, M. A. Miller, D. A. Jessup, and J. A. K. Mazet. 2009. Prey choice and habitat use drive sea otter pathogen exposure in a resource-limited coastal system. Proceedings of the National Academy of Sciences of the United States of America 106:2242-2247.