Ecological thresholds: Concept,Methods and research outlooks

The concept of ecological thresholds was raised in the 1970s. However, it was subsequently given different definitions and interpretations depending on research fields or disciplines. For most scientists, ecological thresholds refer to the points or zones that link abrupt changes between alternative stable states of an ecosystem. The measurement and quantification of ecological thresholds have great theoretical and practical significance in ecological research for clarifying the structure and function of ecosystems, for planning sustainable development modes, and for delimiting ecological red lines in managing the ecosystems of a region. By reviewing the existing concepts and classifications of ecological thresholds, we propose a new concept and definition at two different levels: the ecological threshold points, i.e. the turning points of quantitative changes to qualitative changes, which can be considered as ecological red lines; the ecological threshold zones, i.e. the regime shifts of the quantitative changes among different stable states, which can be considered as the yellow and/or orange warning boundaries of the gradual ecological changes. The yellow thresholds mean that an ecosystem can return to a stable state by its self-adjustment, the orange thresholds indicate that the ecosystem will stay in the equilibrium state after interference factors being removed, whereas the red thresholds, as the critical threshold points, indicate that the ecosystem will undergo irreversible degradation or even collapse beyond those points. We also summarizes two types of popular Methods in determining ecological thresholds: statistical analysis and modeling based on data of field observations. The applications of ecological thresholds in ecosystem service, biodiversity conservation and ecosystem management research are also reviewed. Future research on ecological thresholds should focus on the following aspects: (1) methodological development for measurement and quantification of ecological thresholds; (2) emphasizing the scaling effect of ecological thresholds and establishment of national-scale observation system and network; and (3) implementation of ecological thresholds as early warning tools in ecosystem management and delimiting ecological red lines.