When deciding whether a particular species is really a keystone or not, it's important to consider what effect the removal of that animal will have on the habitat. There are two broad categories under the keystone designation.
The first is as habitat engineers. These are animals that dramatically change the habitat from one form to another. A beaver for example is a classic example of an engineer by turning a moving river section into a pool by the creation of a dam.
The second category is by controlling populations of other animals that are in the habitat. This is the category that the majority of marine keystone species fall into. The following are three examples of marine keystone species.
Ochre Stars: Keeping the Mussels at Bay
Ochre stars, Pisaster ochraceus, can be found in the intertidal area of rocky coasts. These colorful stars are active predators who feed especially on the California mussel, Mytilus californianus. These mussels form huge beds in the high zone where they spend part of their time exposed to the air at low tide.
The stars are considered to be keystone species because if they are removed from the habitat then the mussels will quickly populate any free space they can find. Eventually the mussels will out-compete all the other species.
It is through their very active feeding habits that the ochre stars maintain a high level of diversity by not allowing the mussels to push all other species out of the area.
Sea Otters: Balancing Sea Urchin Populations
Sea otters, Enhydra lutris, are important keystone species in the kelp forests especially off the coast of California. Sea otters are mammals who feed on a variety of marine invertebrates but who feed especially on sea urchins.
Sea urchins are voracious herbivores who tend to feed on the base of the kelp, eating through the stipe until the whole plant detaches from the bottom and floats away. An overabundance of urchins can lead to overgrazing of the kelp and the kelp forest can be converted to an "urchin barren".
Once abundant in California, sea otters were hunted down for their pelts from a population of around 16,000 to near extinction. The resulting effect on the kelp forest was an increase in the barren areas because of the urchin populations.
After they were placed under federal protection to increase their numbers and save them from extinction, sea otters were able to begin naturally controlling the urchins so that the kelp forests had a chance to recover.
Tiger Sharks: The Marine Trash-Man Holds a Bay Together
Tiger sharks, Galeocerdo cuvier, commonly known as the "wastebasket of the sea" for their habit of eating anything and everything, may not seem like the most obvious keystone species, however their presence in Shark Bay in Western Australia serves to control populations of dugongs and sea turtles who graze on the sea grass beds there.
Sea grass is a common nursery habitat for many fish, so overgrazing on the beds could lead to a decline in fish populations. Removal of the sharks would cause an increase in the grazer population and a decline in the sea grass beds, which in turn would cycle back to a decrease in overall species diversity.
By keeping the grazers under control, the sharks help to maintain the overall diversity of the bay.
Esther Eder - Born and raised in Salem, Oregon, Esther graduated from Oregon State University in the spring of 2007 with a Bachelor of Science degree in ...
