A Unified Framework for Assessment and Application of Ecological Thresholds
Abstract
he goal of this synthesis is to initiate development of a unified framework for threshold assessment that is able to linkecological theory and processes with management knowledge and application. Specific objectives include the investigation ofthreshold mechanisms, elaboration of threshold components, introduction of threshold categories and trajectories, andpresentation of an operational definition of ecological thresholds. A greater understanding of ecological thresholds is essentialbecause they have become a focal point within the state-and-transition framework and their occurrence has critical con-sequences for land management. Threshold occurrence may be best interpreted as a switch from the dominance of negativefeedbacks that maintain ecosystem resilience to the dominance of positive feedbacks that degrade resilience and promotethe development of post-threshold states on individual ecological sites. Threshold categories have been identified to serve asecological benchmarks to describe the extent of threshold progression and increase insight into feedback mechanisms thatdetermine threshold reversibility. Threshold trajectories describe the developmental pathway that post-threshold states mayfollow once a threshold has been exceeded. These trajectories may produce a continuum of potential post-threshold states, butthe majority of them may be organized into four broad states. This framework lends itself to management application byproviding an operational definition of thresholds that is based on a probabilistic interpretation. Probabilities associated with1) the occurrence of triggers that initiate threshold progression, 2) the trajectory of post-threshold states, and 3) thresholdreversibility will provide an operational procedure for threshold assessment and application. If thresholds are to play a centralrole in rangeland ecology and management, then the rangeland profession must accept responsibility for their conceptualdevelopment, ecological validity, and managerial effectiveness.