Introduction:
The southern sea otter (Enhydra lutris nereis), also known as the California sea otter, is a charismatic marine mammal that lives off the west coast of the United States. Prized for its luxurious fur, the species was heavily hunted in the 18th and 19th centuries, until it was thought to be extinct. A small population was discovered in the 1930s, and the number of individuals has been growing gradually since then. In 1977, the species was listed as Threatened under the Endangered Species Act.
DATA SETS:
1. Scientists have discovered that the sea otter plays a pivotal role in limiting the populations of sea urchins, one of their primary food sources. In the absence of this predation, sea urchin populations can expand and lead to the overgrazing of kelp forests, which provide habitat for a wide range of species. This suggests that the sea otter is a(n) _____ in its ecosystem.
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2. Scientists have been actively monitoring the recovery of the sea otter population for years. Select “Number of Individuals” from the Data Set Menu to view a line graph showing the annual population of sea otters from 1990 to 2014. In what year was the greatest number of sea otters counted? How many were counted in that year?
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3. What was the number of individuals when the population was at its smallest between 1990 and 2014?
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4. What has been the total percent increase from 1990 to the year with the largest population? (To determine the percent increase, first find the difference between the two populations and then divide by the starting population size. Multiply the resulting value times 100 for percent.)
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5. Compare the number of individuals in 2007 with 2014.
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6. Because of uncertainty in the sampling method and annual fluctuations in population size, the 3- year running average is the number used to determine population trends and to evaluate the status of the population. Find the 3-year average for 2007 and for 2014. (Hint: To determine the 3-year running average, add the total population number from the year in question with the 2 previous years and then divide by 3.)
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7. What general statement about the difference in the population size between 2007 and 2014 is true?
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8. Which of the following could be responsible for the years during which the population has declined?
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9. Another metric that is of interest to scientists is the number of juveniles or pups that are counted each year. The ratio of pups to adults can give an indication of how quickly the population is growing. This number can be interpreted as the number of pups per 100 adult individuals. From the Data Sets, select “Pups per 100 Independents”. Does the ratio of pups to independents appear to be rising, falling, or remaining stable?
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10. Sea otters feed very heavily on filter-feeding invertebrates that concentrate pollutants. As a result, they are particularly susceptible to anthropogenic influences on the environment. Given that they can be relatively easily observed and counted, the relative health of their population can be interpreted to signify overall ecosystem health. What term describes species such as this?
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