Endangered species can be a keystone that prevents a struggling ecosytem from collapsing, but often it affects an ecosystem primarily through its absence. When numbers of keystone species fall too low (and a great many species are keystones), then there's a cascade of failures and imbalances that relate to it. (For example, if a plant is pollinated only by one species of moth, and the moth becomes endangered, the plant will decline and eventually become endangered as well. If an endangered species specializes in eating certain plants, those plants may multiply out of control in its absence).