There are many other examples of keystone species other than pisaster.
One of the most famous examples of a keystone species and their reintroduction to an ecosystem are wolves in yellowstone and the west. Another example are sea otters and the kelp forests off the coast of california. Sea otters keep the sea Urchin populations in check so they do not overgraze the sea kelp. When sea otters are removed the sea urchin population grows and dessimates the kelp.
Here are a few sites to check for more information on key stone species and their effects within ecosystems. Generally, keystone species are very important for conservation efforts because they have disproportionately large impacts relative to their numbers. Many keystone species modify ecosystem patches creating diverse habitat that supports many species.
Keystone species can be contrasted with ecosystem engineers, which are species that modify their habitat or create entirely new habitats. An example of an ecosystem engineer would be a beaver, who dams up a stream, creating a wetland that excludes previously existing species but also creates new habitat perfect for other species.
These species are also very important in conservation efforts, and often the question of whether to protect them or not is highly contested. Many feel beavers are a nuisance and ruin the land, while others feel they provide an important service and create unique habitats that need to be protected.
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