Text 1 Keystone species are organisms that have a disproportionately large effect on their environment relative to their abundance. For example, sea otters keep sea urchin populations in check, preventing them from destroying kelp forests. Text 2 The removal of a keystone species can lead to a 'trophic cascade.' Without otters, urchins multiply, consume all the kelp, and destroy the habitat for countless other species. The ecosystem collapses from the top down.