Keystone Species: what are we talking about?

Science is often challenged by misuse of terminology. In most cases, this is due to an incomplete review of the literature. In spite of recent recommendations (Paine 1996), terms like "species' functional roles" are still used in a confused way, and original definitions are often misinterpreted. The "keystone species" concept (Paine 1966, 1969) is a case in point. In recent years, the overly expansive usage of the keystone concept has led to a redefinition of the term. According to the current interpretation (Power and Mills 1995, Power et al. 1996), keystones are only those species having a large, disproportionate effect, with respect to their biomass or abundance, on their community. Some operational tools are now available to distinguish not only between strong and weak interactors, but also between keystone and dominant species (Paine 1992, Navarrete and Menge 1996, Power et al. 1996, Hurlbert 1997). Khanina (1998) is the latest (but certainly not the last) author attributing the keystone status to dominant species or even to dominants with structural roles (see, for instance, MacIsaac et al. 1995, Procaccini et al. 1996, Hacker and Gaines 1997). Moreover, those species driving ecosystem processes or energy flows are generally referred as "key" species, but only a few of them are keystones. Putting keystones and key species in the same melting pot, as Khanina (1998) does, should be avoided. Therefore, trees and bisons are not keystones, just as the original keystone species identified by Paine was not the dominant mussel, but its starfish predator.