Stabilizing and Equalizing Mechanisms

An important distinction that can be applied to species coexistence mechanisms is between stabilizing and equalizing mechanisms (Chesson 2000). Equalizing mechanisms relate to the concept of average fitness differences between species. These average fitness differences define dominance relationships within a community. In the absence of stabilizing mechanisms,  these average fitness differences  predict competitive exclusion (Chesson 2006).  Fundamentally, the average fitness differences specify how species differ in their degree of adaptedness to their common environment.  Equalizing mechanisms are means by which average fitness differences are reduced. Tradeoffs often have the effect of reducing aveage fitness differences, for example, the often discussed tradeoff between competitive ability and susceptibility to predation (Chesson and Huntly 1997).  Stabilizing mechanisms are essential for stable species coexistence.  We say that species coexist stably if each species is able to recover from perturbations to low density. Recovery of all species from low density requires significant differences in their ecology of the sort that would generally be called niche differences (Chesson 1991). In general, stabilizing mechanisms allow stable species coexistence in the presence of average fitness differences that are not too large.  If species differ a lot in average fitness differences, then they require very strong stabilizing mechanisms to allow them to coexist stably.  On the other hand, if average fitness differences are small, weak stabilizing mechanisms are sufficient for coexistence. Thus, stable coexistence arises jointly from the action of stabilizing and equalizing mechanisms, but by definition, stabilizing mechanisms are essential for species coexistence. A given mechanism need not be purely stabilizing or equalizing.  I.e., it might both affect average fitness differences (perhaps making more inequality rather than less) and have stabilizing aspects (Chesson 2000, Snyder et al 2005) . 


Chesson, P. 1991.  A need for niches?  Trends in Ecology and Evolution 6, 26-28.

Chesson, P. 2000. Mechanisms of maintenance of species diversity. Annual Review of Ecology and Systematics 31, 343-66

Chesson, P., Huntly, N. 1997. The roles of harsh and fluctuating conditions in the dynamics of ecological communities.  American Naturalist 150, 519-553. 

Chesson, P. 2006. Quantifying and testing species coexistence mechanisms.  In F. Valladares, ed. “Unity in Diversity.”

Snyder, R.E., Borer, E.T. and Chesson, P. 2005. Examining the relative importance of spatial and nonspatial coexistence mechanisms.  American Naturalist 166, E75-E94