Dictionary Definition
ascendance n : the state that exists when one
person or group has power over another; "her apparent dominance of
her husband was really her attempt to make him pay attention to
her" [syn: dominance,
ascendence, ascendancy, ascendency, control]
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Extensive Definition
wikt ascendency
Ascendency is a quantitative attribute of an
ecosystem, defined as
a function of the ecosystem's trophic
network. Ascendency is derived using mathematical tools from
information
theory. It is intended to capture in a single index the ability
of an ecosystem to prevail against disturbance by virtue of its
combined organization and size.
One way of depicting ascendency is to regard it
as “organized power”, because the index represents the magnitude of
the power that is flowing within the system towards particular
ends, as distinct from power that is dissipated willy-nilly. Almost
half a century earlier, Alfred J.
Lotka (1922) had suggested that a system’s capacity to prevail
in evolution was related to its ability to capture useful power.
Ascendency can thus be regarded as a refinement of Lotka’s
supposition that also takes into account how power is actually
being channeled within a system.
In mathematical terms, ascendency is the product
of the aggregate amount of material or energy being transferred in
an ecosystem times the coherency with which the outputs from the
members of the system relate to the set of inputs to the same
components (Ulanowicz
1986.) Coherence is gauged by the average
mutual information shared between inputs and outputs (Rutledge
et al. 1976.)
Originally, it was thought that ecosystems
increase uniformly in ascendency as they developed, but subsequent
empirical observation has suggested that all sustainable ecosystems
are confined to a narrow “window of vitality” (Ulanowicz 2002.)
Systems with relative values of ascendency plotting below the
window tend to fall apart due to lack of significant internal
constraints, whereas systems above the window tend to be so
“brittle” that they become vulnerable to external
perturbations.
Sensitivity analysis on the components of the
ascendency reveals the controlling transfers within the system in
the sense of Liebig
(Ulanowicz and Baird 1999.) That is, ascendency can be used to
identify which resource is limiting the functioning of each
component of the ecosystem.
It is thought that autocatalytic feedback is the
primary route by which systems increase and maintain their
ascendencies (Ulanowicz 1997.)
References
- Lotka, A.J., 1922. Contribution to the energetics of evolution. Proc. Natl. Acad. Sci. U.S.A., 8: 147-150.
- Rutledge, R.W., B.L. Basorre and R.J. Mulholland. 1976. Ecological stability: an information theory viewpoint. J. theor. Biol. 57: 355-371.
- Ulanowicz, R.E. 1986. Growth & Development: Ecosystems Phenomenology. Springer-Verlag, NY. 203 p.
- Ulanowicz, R.E. 1997. Ecology, the Ascendent Perspective. Columbia University Press, NY. 201p.
- Ulanowicz, R.E. 2002. The balance between adaptability and adaptation. BioSystems 64:13-22.
- Ulanowicz, R.E. and D. Baird. 1999. Nutrient controls on ecosystem dynamics: The Chesapeake mesohaline community. J. Mar. Sys. 19:159-172