Which of the following would make a good example of a keystone species?
Wheat, without which humans would have to turn to eating other grains
A diatom species living in a lake, which is eaten by all animal plankton species but not most of the diet of any
A tree species in a forest, where the entire forest is made up only of that tree species
A beaver, which creates types of habitat that almost no other animals make
A keystone species does not necessarily have to be the most abundant species in a particular ecosystem. However, it's niche is important because it influences the entire ecosystem in terms of how it functions.
For example-gray wolves were eradicated in many western states and have only been recently re-introduced to Yellowstone National Park. As predators, they kept the deer population in check. When they were gone from the ecosystem, the deer population grew exponentially until their resources ran out and many starved to death leading to a population crash.
Without the keystone species, the entire ecosystem is affected from the predators down to the producers. In the question you have asked, the answer has to be the beaver. Because of their niche, they help to create new habitats for other species to live in. The beaver is a rodent that chews on trees and builds dams by streams in the forest. This creates ponds which other animals use for their home. By removing beavers, wetlands dry up and this in turn affects all members of this type of community. Beavers were trapped for their fur in the 1700's and onward. It was greatly prized by Europeans. They were practically wiped out in different parts of the country. We now recognize what an important role they play in maintaining their ecosystem and this is why they are a keystone species.
It should be noted that it is important to maintain species richness in an ecosystem. This refers to the number of species in an area and since they are interrelated, each is valuable to the ecosystem as a whole.