A keystone species is a species that has a disproportionately large impact on its environment relative to its abundance. These species help maintain the structure of an ecosystem, influencing the types and numbers of other species present. When a keystone species is removed, it can lead to significant changes in the ecosystem, often resulting in a decline in biodiversity. Examples of keystone species include the sea otter, which helps control sea urchin populations, and the gray wolf, which regulates herbivore populations in its habitat. The presence of these species is crucial for the health and stability of their ecosystems.