Other disciplines such as ecology use thresholds in a similar manner, but the public may be more familiar with the analogous phrase, tipping point, thanks to Malcolm Gladwell’s 2002 book “The Tipping Point.” Gladwell described a tipping point as the point in time when change in a parameter or system is no longer progressive or linear but instead becomes exponential. In the context of the critical zone

and geomorphology, we can focus on thresholds that are relatively easy to identify, such as exceeding a regulatory level for a specified substance. Examples include mandated total maximum daily load for a river, permissible nitrate concentrations in drinking water, or standards for particulate matter in the atmosphere. Understanding and manipulating the factors that cause a substance to exceed a regulatory level, or learn more predicting the consequences of that exceedance, are typically more difficult, but at least the exceedance is relatively easy to identify. Identification of thresholds that cause the critical zone to move between alternative stable states is more difficult. Obeticholic Acid cost Ecologists define alternative stable states as different

stable configurations that an ecological community can adopt and that persist through at least small perturbations (Beisner et al., 2003). A community can move from one stable state to another by a sufficiently large perturbation applied to state variables such as population density (in this scenario, different states can exist Vitamin B12 simultaneously), or via a change in the parameters that determine the behavior of state variables and the ways they interact with each other (Beisner et al., 2003). As with ecological integrity, the definition of ecological alternative stable states implicitly includes physical and chemical processes, and can easily be broadened to include geomorphic process and form. Wohl and Beckman (in press), for example, describe wood-rich and wood-poor states in forested mountain streams, and quantify thresholds of instream wood load that can cause a stream to move from one persistent, stable state to another. Arguably the most difficult thresholds

to identify, but also the most important, are those that define the limits of sustainability for a species, a biotic community, or a specific resource use by humans. As noted earlier, sustainability is most effectively defined within a specified time interval, but implies the ability to maintain existing conditions during that time interval. Thresholds associated with exceeding sustainability limits unfortunately seem to be most commonly identified once they have been crossed and a species has gone locally or globally extinct, a biotic community has disappeared locally or globally, or a human community can no longer use a resource such as agricultural soils that have eroded or become saline, fisheries that have collapsed, or ground or surface waters that are no longer potable.