Ecological thresholds and resilience in streams

Abstract

Ecological thresholds and resilience are powerful heuristics for understanding how lotic ecosystems change. Ecosystems may exist in self-organized states based on their taxonomic composition or the range of ecosystem functions, which are influenced by environmental drivers such as thermal or hydrologic regimes, channel morphology, and availability of nutrients. Changes in these underlying drivers may exceed an ecosystem’s ability to maintain its characteristic attributes and shift the system into alternative states of organization, which are often regarded as degraded or undesired. The boundaries where transitions occur are known as ecological thresholds and often show a rapid ecosystem response across a relatively small change in the environmental driver. Resilient ecosystems have the capacity to retain attributes in the face of disturbances. However, at some disturbance magnitude an ecosystem may become altered, and the magnitude necessary for a regime shift decreases as resilience declines. While ecological resilience remains largely metaphorical in lotic ecosystems, we describe some approaches for identification and assessment.