Assessing Recovery Following Environmental Accidents: Environmental Variation, Ecological Assumptions, and Strategies

Gauging whether or when a population, species, or community recovers from an environmental accident or disturbance, such as an oil spill or forest fire, is complicated by environmental variation in time and space, and therefore depends on the assumptions one makes about equilibrium. These ecological assumptions about equilibrium affect how one designs and interprets studies to assess recovery from environmental accidents or disturbances. We use examples from studies conducted following the Exxon Valdez oil spill to illustrate several approaches to assessing recovery and their sensitivity to the form of equilibrium one assumes. Baseline study designs, which compare levels of a resource after the disturbance to pre-disturbance levels for impact data only, are generally inadequate because they rest on the unrealistic assumption of steady-state equilibrium. Since data for the impact area only are used, recovery and temporal variation are confounded. Unlike baseline designs, before-after control-impact (BACI) designs use impact and reference data, and relax this sensitivity by incorporating both temporal and spatial variation. Studies that compare impacted with reference areas in a single year following the disturbance assume spatial equilibrium and therefore may confound recovery with systematic spatial differences between the areas. Sampling and analytical strategies such as stratified random sampling or the use of environmental measures as covariates may lessen the sensitivity to this assumption. Multiyear studies that include comparisons between impacted and reference areas or that sample areas along a gradient of disturbance rest on the more realistic assumption of dynamic equilibrium. Understanding the underlying assumptions and how they relate to the approach one uses must be part of assessing the recovery of biological resources from an environmental accident. Because the dynamics of different populations, species, and communities and the environments they occupy vary and exhibit different dependencies on the scale of disturbance (and the scale of analysis), there is no single "best" approach to assessing recovery. Discussions about recovery should include an explicit and honest consideration of the underlying ecological assumptions, the likelihood that they hold in the system being studied, and the consequences if the assumptions are violated.