During the nineteenth century, sea otters were hunted almost to the point of extinction. When the number of sea otters became very low, it was noticed that the ecosystem of kelp bed "forests" was also disappearing. It was 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. What role did sea otters perform in this ecosystem?a. predators
b. competitors
c. parasites
d. a keystone species
e. producers

d. a keystone species

Explanation:

A keystone species is an organism upon which the balance of an ecosystem is dependent on its relative abundance in the ecosystem. In order words, their presence in an ecosystem is important in the structure and the functionality of an ecosystem. An abrupt change in their population will have exact great influence on their ecosystem.

Just as stated in the question above, a decline in the population of sea otters as a result of them being hunted, affected the ecosystem of kelp bed “forests”.  

Sea otters are a predatory keystone species. They are the glue that held the kelp bed “forests” ecosystem together.  

because they are eaten by the frog and the frog getseaten by the snake and the snake by the eagle