ABSTRACT
Simmons, 1999). Despite this acknowledged prob-
lem, the term remains a widely understood short-
hand for an evaluation of whether particular ways
of life and activities can be continued at their
present level in the short, medium and long term.
Among the perturbing influences which give rise
to non-equilibrium conditions, human agency is
one of the most important, as Chapters 6 and 7
have shown. People represent a key ecological
factor in almost all terrestrial ecosystems. Time-
depth must therefore include an understanding of
the contribution of people to the formation and
development of environments.
