concept

Keystone species

A species whose effect on its ecosystem is wildly out of proportion to its numbers - like the wedge at the top of a stone arch that holds every other stone in place, though it is only one of many. Apex predators are often keystones: the ecologist Robert Paine pried a mussel-eating starfish off a rocky shore and its diversity caved from fifteen species to eight as the mussels took over. On Pandora the rare thanator and toruk shape the whole forest, holding down the giant grazers that would otherwise trample everyone's world.