Keystone Species O Total impact of species P Cr C Keystones B K G Dominants need and desire for clarity in standardized terminology, the adoption of the following terminology is proposed Species in communities can be defined as: • T VD VR Proportional biomass of species Figure Total impact of a species (absolute value of community impact  proportional abundance of a species) vs its proportional abundance, pi Species whose total impact is proportional to their abundance would fall along the diagonal line X¼ Y Keystone species have effects that greatly exceed their proportional abundance, both on a per capita (or per biomass) basis and a per population basis, and would cluster toward the upper left region of the diagram To illustrate, some organisms (e.g., rhinovirus, VR, which causes colds in wildlife) may have effects that are greater than expected from their biomass, but because the impact on the community is relatively minor, they are not keystone species Others (e.g., distemper virus, VD, a killer of lions or wild dogs) may have collective effects on the community that are disproportionately large, and would be keystone species Examples of keystone species are Pisaster (P), sea otters (O), predatory whelks (Concholepas, C), and freshwater bass (B) Dominants are high proportional biomass species whose large effects are not disproportional to their biomass, such as trees (T), giant kelp (K), prairie grasses (G), and reef-building corals (Cr) Positions of each species or group represent educated guesses Reproduced from Power ME, Tilman D, Estes JA, et al (1996) Challenges in the quest for keystones Bioscience 46: 609–620 herring, anchovies, and sea pens) ‘‘Foundation species’’ were defined as the ‘‘group of critical species which define much of the structure of a community’’ (Dayton, 1972) Related ideas include ‘‘interaction webs’’ and ‘‘functional webs,’’ which are defined as the subset of strongly interacting species that regulates community structure (Menge and Sutherland, 1987) A similar idea, the ‘‘extended keystone hypothesis,’’ states that ecosystems are controlled by a small set of key plants, animals, and abiotic processes Note, however, that the latter phrase adds environmental stresses (‘‘keystone processes’’) such as fire, wave-induced damage, substratum movement, and other kinds of disturbances to the biological forces stressed by the other concepts A contrasting view that biotic and abiotic forces are qualitatively distinct suggests the term ‘‘critical processes’’ to describe abiotic effects that can structure communities Recommended Terminology Although the various generalizations of the keystone concept and the lumping of several related concepts into a broad keystone species definition were well intentioned, this study agrees with Mills et al (1993) and Power et al (1996) that such usage sharply reduces the utility of the keystone species concept On the basis of current usage and the documented 449 • • • • Keystone species: Consumers having a disproportionately large effect on communities and ecosystems By this definition, keystone species can include predators, parasites, pathogens, herbivores, pollinators, and mutualists of higher trophic status, but not plants, sessile animals, or ‘‘resources’’ (e.g., salmon carcasses, salt licks, and deep pools) To date, no convincing examples of communities with more than a single keystone species are known Therefore, the suggested hypothesis is that most communities will have at most a single keystone species At the ecosystem level, which might include several distinct communities, there may be several keystone species Evaluation of these predictions awaits future research Strong interactors (critical species): Species having a large effect on the species (one or more) with which they interact Communities and ecosystems may have many strong interactors, and such species may occur at all trophic levels Strong interactors would lie toward the upper side of the abundance–impact diagram (Figure 7), and therefore include both keystone species and dominants Weak interactors: Species having little effect on other species, at least under average conditions Under some circumstances, weak interactors may occupy important roles in ecological communities as a result of changes that lead to temporary increases in abundance, size, or biomass (Berlow, 1999) Weakly interacting species would all lie toward the lower portion of Figure Dominant species: As noted above, dominant species are those strongly interacting species that owe their influence to their high abundance (Figure 7) Such organisms are the species that comprise a large proportion of the biomass in a community, and thus are the dominant components of community structure Trees in forests, mussels in rocky intertidal habitats, grasses in grasslands, and kelps or corals in nearshore subtidal habitats are all dominants Key-industry species: As defined earlier, key-industry species are prey that support a large group of consumers Following Elton’s (1927) usage, key-industry species are therefore animals of intermediate trophic status Groups of species in communities or ecosystems can include: • Interaction webs: Interaction webs ( ¼ functional webs) are the subset of species that through their interactions and responses to abiotic factors make up the dynamic core of food webs or communities Interaction webs include keystone species, dominants, and other strong interactors Identification of Keystone Species Lack of experimentation and other rigorous approaches to identify keystones continues to be a pervasive problem In many cases, species are named keystones based on superficial evidence such as natural history observations A lack of rigor in identification could result in mislabeling a species as a