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