Keystone species are those species within an ecosystem that have a disproportionately large impact on the community structure and function, relative to their abundance. They play a crucial role in maintaining the health and balance of the ecosystem. Keystone species help regulate the population of other species, control the spread of invasive species, and promote biodiversity. Their presence is essential for the overall stability and resilience of the ecosystem. Examples of keystone species include sea otters, wolves, and bees.