Are
Large, Infrequent Disturbances
Qualitatively Different from
Small,
Frequent Disturbances?
Romme, W.H.,
E.M. Everham, L. E. Frelich, M.A. Moritz, R.E. Sparks. 1998. Are large, infrequent
disturbances qualitatively different from small frequent disturbances? Ecosystems 1: 524-534
Abstract: In this article, we develop a heuristic model of ecosystem-disturbance
dynamics that illustrates a range of responses of disturbance impact to
gradients of increasing disturbance extent, intensity, or duration. Three general kinds of response are
identified and illustrated: (a) threshold response, (b) scale-independent
response, and (c) continuous response. Threshold
responses are those in which the response curve shows a discontinuity or a
sudden change in slope along the axis of increasing disturbance extent,
intensity, or duration. The response
threshold occurs at a pint where the force of the disturbance exceeds the
capacity of internal mechanisms to resist disturbance, or where new mechanisms
of recovery become involved. Within
this conceptual framework, we find that some unusually large or intense
disturbances, but not all, produce qualitatively different responses compared
with similar disturbances of lesser magnitude.
If disturbance impact does not increase with increasing disturbance
extent, intensity, or duration, or if the response curve changes monotonically,
then large disturbances are not qualitatively different from small
ones. For example, jack pine tends to
become reestablished after stand-replacing fire is boreal forests, regardless
of fire size, because its serotinous cones provide an adequate seed source
throughout the burned area. Thus, large
fires are not qualitatively different from small fires in terms of jack pine
reproduction. However, if disturbance
impact does increase abruptly at some point with increasing disturbance extent,
intensity, or duration, often because of thresholds in the capacity of internal
mechanisms to resist or respond to disturbance impact, then large disturbances are
qualitatively different from small ones, at least for some parameters of
ecological response. For example,
balsam fir and white cedar can recolonize a small burned patch of boreal forest
in close proximity to surviving individuals of these species, but they will be
eliminated from a large burn because of their susceptibility to fire-caused
mortality and their inability to disperse their seeds over long distances. The conceptual framework presented here
permits some new insights into the dynamics of natural systems and may provide
a useful tool with which managers can assess the potential for catastrophic
damages resulting from large, infrequent disturbances.