During the 19th century, sea otters were hunted almost to the point of extinction. when the number of sea otters became very low, the ecosystem of kelp bed "forests" also declined. scientists discovered that the sea otters were predators of sea urchins, which were, in turn, predators of the kelp. when sea otters declined, the sea urchin populations grew, destroying the kelp ecosystem and the many species that depended on it. it was then understood that sea otters were in this ecosystem.
a. predators
b. competitors
c. parasites
d. a keystone species
e. producers

Keystone species

Explanation:

A keystone species is the one that determines the structure, function and other aspects of the entire community. Other species of the system are greatly affected by keystone species. The keystone species are mostly not the most abundant ones but regulate the species richness and ecosystem functioning by affecting the availability of food, water or other resources.

In the given example, sea otters determine the species richness of kelp bed forests. By regulating the sea urchin population, they affect the population size of kelp as well. Removal of sea otters from the system destroyed the whole ecosystem. This makes sea otter a keystone species for the given ecosystem.

d. a keystone species

Explanation:

A keystone species plays a very important role in an ecosystem. It is crucial in maintaining the health of the ecosystem and without it the ecosystem would collapse.

Here, the otter species had an important role in keeping check on other species. It indirectly maintained enough number of kelp bed forests by consuming sea urchins since sea urchins consumed kelp. Without sufficient number of otters, sea urchins grew indiscriminately which lead to increase in consumption of kelp. Kelp population decreased when sea otter population decreased. Hence, sea otters were keystone species in this ecosystem.

if you know your abc's you should be able to get your answer or just use google