Sea Otter

The Sea Otter (Enhydra lutris) is a classic example of trophic downgrading.  This animal was once widely abundant on the Pacific Coast of North America. Populations began to declined starting with the Pacific fur trade of the 18th and 19th centuries (Estes 1989).  The conservation and recovory projects of the sea otter have been a great success however its story serves as a well studied example of the the direct and indirect impacts of decreased predation and top-down forcing.

Image Source: National Park Service, nps.gov

Throughout he 1970's and 1980's, the decrease in sea otter population was discovered to have lead to large increase in  the abundance and distribution of sea urchins (Strongylocentrotus purpuratus), herbivores whose populations were historically controlled by sea otters (Estes 1989).

With the increased populations of sea urchins came the consequent decrease of kelp forests (brown algae or Phaeophyceae)  and other fleshy macroalgae.  Kelp was serves as sea urchins primary food source and therefore increased consumption resulted in kelp decline along coasts.  Kelp forests have been long understood as a vital ecosystem for near-shore aquatic life. They also provide essential ecosystem services in their role as primary producers (converting carbon dioxide to organic compounds) for many other herbivore species who graze the nutricious sea plant (Estes 1989).