Essentially all species play a role in nature, and everything is inextricably linked. But like in the theatre or in a movie there are species that play major or key roles. In the theatre of life many species, with respect to total biomass or relative abundance within an ecological community, disproportionately contribute to the maintenance of co-evolved functions, structures and processes in nature. Without them ecosystems can degrade to simplified systems lacking the former diversity and dynamics; even completely collapse (read more). Animals like the sea otter along the pacific coast of North America and the wolf in Yellowstone National Park are notable or textbook examples of these so-called keystone species. The reintroduction of the wolf in Yellowstone resulted in the return of certain animal and plant species, the restraint of others, and stimulates the recovery of a more balanced and complete ecosystem (read more).
However, conservation management and ecosystem restoration should be based on complex interactions among species (read also trophic cascades) and not solely on keystone species. Defining a keystone species is ambiguous and it’s easy to forget the small and inconspicuous in the equation. For instance organisms like bees and other pollinators are more key to the biosphere and our survical than say a lion on the Serengeti plains; where termites are perhaps most key. Another good example is the Pacific salmon in catchments of Canada and Alaska, exerting tremendous knock-on and wide ranging effects through protein provision and nutrient cycling (read more). Keystone species exert their effects top-down (e.g. the wolf as apex predator) as well as bottom-up (e.g. the beaver as ecosystem engineer). It again boils down to understanding as much about the intricacies of nature as possible, through integral and holistic science. Hence the great desirability to restore ecosystems to as complete as possible.
Keystone species may have a great public appeal and at the same time be indicative for the health of certain ecosystems and human goodwill (e.g. the tiger and the forest). These species can also be denoted as flagship species. The ecological needs (often in terms of space and habitat quality) of keystone or other species may encompass the needs of a range of species, hence functioning as an umbrella species.