Ecological Extinction - Examples

Examples

Estes et al. (1978) evaluated the potential role of the sea otter as the keystone predator in near-shore kelp forests. They compared the Rat and Near islands in the Aleutian islands to test if “sea otter predation controls epibenthic invertebrate populations (specifically sea urchins), and in turn releases the vegetation association from intense grazing”. Estes and his colleagues found that different size structures and densities of sea urchins were correlated with the presence of sea otter populations, and because they are the principal prey of this keystone predator, the sea otters were most likely the main determinants of the differences in sea urchin populations. With high sea otter densities the herbivory of sea urchins in these kelp forest was severely limited, and this made competition between algal species the main determinant in survival. However, when sea otters were absent, herbivory of the sea urchins was greatly intensified to the point of decimation of the kelp forest community. This loss of heterogeneity serves as a loss of habitat for both fish and eagle populations that depend on the richly productive kelp forest environment. Historical over harvesting of sea otter furs has severely restricted their once wide-ranging habitat, and only today are scientists starting to see the implications of these local extinctions. Conservation work needs to focus on finding the density threshold that render the sea otters an effective population. It must then continue and artificially repopulate the historical range of the sea otter in order to allow kelp forest communities to re-establish.

The California spiny lobster, or Panulirus interruptus, is another example of a keystone predator that has a distinct role in maintaining species diversity in its habitat. Robles (1987) demonstrated experimentally that the exclusion of spiny lobsters from the intertidal zone habitats led to the competitive dominance of mussels (Mytilus edulis and M. californianus). This results shows another example of how the ecological extinction of a keystone predator can reduce species diversity in an ecosystem. Unfortunately, the threshold of ecological extinction has long passed due to over fishing now that many local extinctions of the California spiny lobster are common.

Jackson et al. (2001) took a much needed historical perspective on the role of ecological extinction caused by overfishing of oysters in the Chesapeake Bay. Commercial oyster fishing had not affected the bay ecosystem until mechanical dredges for harvesting were utilized in the 1870s. The bay today is plagued by eutrophication due to algal blooms, and the resulting water is highly hypoxic. These algal blooms have competitively excluded any other species from surviving, including the rich diversity in faunal life that once flourished such as dolphins, manatees, river otters, sea turtles, alligators, sharks, and rays. This example highlights the top-down loss of diversity commercial fishing has on marine ecosystems by removing the keystone species of the environment.