Overhunting also has an impact. Terrestrial mammals, such as the tiger and deer, are mainly hunted for their pelts and in some cases meat, and marine mammals can be hunted for their oil and leather. Specific targeting of one species can be problematic to the ecosystem because the sudden demise of one species can inadvertently lead to the demise of another (coextinction) especially if the targeted species is a keystone species. Sea otters, for example, were hunted in the maritime fur trade, and their drop in population led to the rise in sea urchins—their main food source—which decreased the population of kelp—the sea urchin's and Steller's sea cow's main food source—leading to the extinction of the Steller's sea cow. The hunting of an already limited species can easily lead to its extinction, as with the bluebuck whose range was confined to and which was hunted into extinction soon after discovery by European settlers.