Ecological thresholds and riparian wetlands: an overview for environmental managers

Ecological thresholds represent the point at which an ecological process or parameter changes abruptly in response to relatively small changes in a driving force. Although the non-linear nature of ecological dynamics is widely recognised, the concept of threshold responses has only recently received the warranted attention in conservation and management. In this short review we synthesise current knowledge on ecological thresholds and alternative stable states and review examples of threshold responses in riparian wetlands. The review shows that changes in the hydrologic regime often represent the trigger that causes abrupt shifts in riparian species composition. Also, inputs of nutrients over critical loads can cause drastic changes in wetland biogeochemical functions. We then discuss the implications of threshold relationships for the management of riparian wetlands. Critical steps needed to embody threshold models in adaptive management include the assessment of wetlands' predisposition to threshold responses and the adaptation of monitoring schemes to facilitate the statistical identification of nonlinear relationships. Future research should also aim at assessing how climate change may influence the likelihood of ecological thresholds. Also, as the dynamics of degraded ecosystems often differ substantially from those of pristine systems, a better understanding of human modified systems is necessary for the development of a predictive framework.