Orcas, or killer whales, are natural predators of sea otters. It has been observed that in regions where the sea otters are not present, sea urchins grow rapidly and destroy much of the natural kelp vegetation. Which of these statements is correct in this context? A) Kelp is the keystone species because their survival is key to the survival of the entire ecosystem. B) Sea otters cannot be considered the keystone species because they are not the top predators in the ecosystem. C) Sea urchins are the keystone species because they multiply rapidly in the absence of their natural predators. D) Sea otters are a keystone species as they prevent a herbivorous species from completely destroying a plant species.

The species are keystone species as they directly affect the ecosystem thus they maintain the structure of the community. The absence of these species modifies or change the structure of the community.

In the given, since the sea presence of sea otters maintains the structure of the community whereas the absence of the sea otters affect the vegetation of the kelp and thus food chain or food web of the ecosystem, therefore, sea otters are the keystone species.

Thus, option-D is the correct answer.