Background Principles

Three principles apply to the assessment of the costs and benefits of extreme geophysical events to the nation's ecological systems. First, although the more tangible, quantifiable damages of extreme events to infrastructure and economies may be difficult to calculate precisely, the costs to and benefits for natural ecosystems—even from such apparently straightforward impacts as numbers of fish killed or trees destroyed—are even less tangible and may be nearly impossible to quantify precisely. Moreover, even if the physical effects can be measured, the monetary values of those impacts cannot be stated with precision.

Second, existing ecological systems have already adapted in many respects to the forces created by extreme events, such as floods or droughts. This process is lengthy, extending over thousands of years and involving the evolution of species and complex physical systems. The effects of geophysical extremes often are not undesirable. For example, major natural disturbances, such as fires or floods, rejuvenate old forests. The critical factors are the frequency, intensity, and extent of natural disturbances. If disturbances occur too frequently and over large areas, then only pioneering, short-lived, and opportunistic species survive. If disturbances occur too infrequently, then slower-growing, superior competitors for light, water, and nutrients replace the pioneers. Maximum diversity is maintained by an intermediate level of disturbance, so that patches of pioneers and superior competitors alike occur within the landscape. All of this suggests that attempts to eliminate natural disturbances (rather than attempts to mitigate their adverse impacts) can be counterproductive and in some cases, as in the 1927 and 1993 floods on the Mississippi River and the Yellowstone fires in 1988, can make a disaster worse.2

Third, precisely because many disasters are indeed "natural," they often produce mixed outcomes for the environment: benefits to some parts of the natural system and losses to others. For example, some thinning of tree branches caused by high winds or ice accumulations from winter storms can allow for subsequent stronger tree development, and studies of the 1993 flood in the Midwest revealed major ecological benefits in the immersed floodplains. To the average human observer, floodplain forests appear to change scarcely at all from year to year, and therefore the death of trees during or after a major flood seems


The severe damage from the 1927 flood on the lower Mississippi River and in 1993 in the Upper Mississippi basin could have been substantially reduced if levees bordering the river had neither failed nor been overtopped, and if other forms of mitigation had been adopted. Meanwhile, the Yellowstone fires that occurred during the 1988 drought were worsened because of "let-it-burn" and other forest management practices and because fire prediction models at the time were outmoded and inadequate for such an extreme event (Riebsame et al., 1991).

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