Identifying ecological thresholds for regulating human activity: Effective conservation or wishful thinking?

Increasingly, conservation professionals and regulatory agencies are attempting to identify ecological thresholds that define a point at which populations or communities demonstrate a marked response to human disturbance. Such thresholds may serve as the foundation for recognizing and then imposing limits on the levels of disturbance or types of land use. Although an intuitive concept, I argue that there are numerous scientific and practical limitations to applying ecological thresholds to conservation decision making. First, the concept is limited by the lack of a clear and universally accepted definition. Often, a threshold is defined as the point at which a population demonstrates a nonlinear ecological or biological response to some critical level of disturbance. Alternative definitions are premised on different trajectories of response or a direct recognition of conservation risks when prioritising socioeconomic values. Second, there is no clear or consistent method for identifying ecological thresholds including the appropriate response variables. Third, there is little evidence to suggest that ecological thresholds generalize to other species or populations; this limits the efficiency of thresholds as a decision making tool. Fourth, even where ecological thresholds are identified for the purpose of regulation or conservation planning, there are few examples of effective implementation. In comparison, regulatory limits are premised on the ecological response of populations to disturbance, but also consider the socioeconomic realities of conservation decision making including the risk associated with greater levels of development. Limits can be identified through participatory decision making processes that allow a consideration of potential future conditions within the context of current ecological and economic circumstances. When followed with long-term monitoring, regulatory limits can improve existing or provide new avenues for conservation planning and regulation.