A keystone species in an aquatic environment is a species that has a disproportionately large effect on its ecosystem compared to its abundance. Such species play critical roles in maintaining structure and balance within the community. For example, predators like sea otters regulate sea urchin populations, which in turn helps maintain kelp forest ecosystems. The removal of a keystone species can lead to dramatic changes in the ecosystem, often resulting in declines in biodiversity and alterations in habitat structure. These species are essential for the overall health and stability of their environments.