Keystone species
a keystone species is one whose presence contributes to the diversity of life and whose
extinction would lead to the extinction of other forms of life. a keystone species to
support the ecosystem of which it is a part.
an example of what can happen when a keystone species is removed occurred when fur
hunters eliminated sea otters from some pacific ocean kelp beds. otters eat sea urchins,
which eat kelp. with its major predator gone, sea urchin populations exploded and
consumed most of the kelp. fish, snails, and other animals associated with the kelp beds
disappeared.
the grizzly bear is another example of a keystone species. grizzlies transfer nutrients
from the ocean ecosystem to the forest ecosystem. the first stage of this transfer is
performed by salmon that swim up rivers, sometimes for hundreds of miles. salmon are rich
in nitrogen, sulfur, carbon, and phosphorus. the bears capture the salmon and carry them
onto dry land, scattering nutrient-rich feces (wastes) and partially eaten salmon carcasses.
it has been estimated that the bears leave up to half of the salmon they harvest on the
forest floor.

which sequence best represents the feeding relationships in a kelp ecosystem that has not been disturbed
by humans?
(1) sea urchins → kelp → fish (3) kelp → sea otters → sea urchins
(2) kelp → sea urchins → sea otters (4) sea urchins → snails → kelp