The concept of threshold and its potential application to landscape planning

The concept of threshold can potentially be applied to conservation planning of species, habitats, and ecosystems. It also has significance in managing social–ecological systems for resilience. However, our understanding and use of threshold has been scattered among various disciplines, and the link to conservation planning and social–ecological system management has not been strongly established. The review of the use of threshold in various disciplines reveals that the term is used in a similar manner in both natural and social sciences: a threshold is a point or a zone on an independent variable, and if it is crossed, a sudden, large change in the state of a dependent variable occurs. Even a small change in the independent variable brings this drastic change; nonlinear relationship characterizes the threshold response. Thresholds also separate alternative regimes in a social–ecological system. The discussion of the application of threshold concept to watershed planning concludes that although using one threshold value of impervious surfaces in a watershed to regulate new developments and retrofit old ones is a cost-effective method, a more integrated approach is needed. The use of habitat amount threshold to conserve species promotes proactive planning that would prioritize areas for protection before the threshold is reached and would restore habitat based on the threshold target. However, species-specific data to decide on the threshold is often lacking, and the identification of thresholds is not straightforward. Nonetheless, the concept of threshold is appealing for proactive planning and significant in managing social–ecological systems for resilience.