This is a simplified food web for a typical kelp forest from the west coast of North America (kelp forests are found from Baja California to Southern Alaska).
Kelp are large brown algae that grow attached to rocks on the sea floor. A kelp forest can only develop where rocks are present and sunlight penetrates to the floor of the ocean (typically depths of less than 50 meters).
The primary consumers on this food web (herbivores) feed on the kelp in different ways. The crustaceans and snails primarily feed on old kelp blades ("leaves") and partially decayed dead kelp. They thus have very little effect on the overall growth of the kelp.
In contrast, sea urchins feed on healthy kelp tissue, and will eat their way right through the stipes ("stalks") of the kelp, setting them adrift. Sea urchins can thus have a dramatic impact on the productivity of kelp forests.
Note that in the food web above sea otters are the only animals that feed on sea urchins in large numbers. The sea otters are considered the keystone predator in this ecosystem, because the whole system can collapse if they are removed. Have a look at the video called "Sea_Otters-keystone" to see what happens when sea otters are eliminated from the kelp forest.
What is the definition of an "urchin barren?"
Look at the food web above. Why are sea otters considered keystone predators, while the sea lions are not?
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