The interactive effects of press/pulse intensity and duration on regime shifts at multiple scales

Abstract

Abstract. Regime shifts are difficult-to-reverse transitions that occur when an ecosystem reorganizes around a new set of self-reinforcing feedbacks. Regime shifts are predicted to occur when the intensity of some exogenous driver variable, such as temperature, annual harvest rate, or nutrient addition rate, gradually approaches and crosses a threshold value, initiating a transition to an alternative state. However, many driver variables now change rapidly as presses or pulses, not gradually, requiring new conceptual frameworks for understanding and predicting regime shifts. We argue that identifying and controlling regime shifts in response to presses and pulses will require a greater focus on the duration, not just the intensity, of changes in driver variables. In ecosystems with slower dynamics, transitions to an alternative state can take years to decades and as a result, a driver press with an intensity capable of resulting in a regime shift over long time spans may fail to cause a regime shift when applied for shorter durations. We illustrate these ideas using simulations of local-scale alternative stable state models and preliminary evidence from long-term grazing and eutrophication experiments. The simulations also suggest that small changes in the duration of driver presses or pulses can determine whether an ecosystem recovers to its original state. These insights may extend to larger scales. In spatially extended simulations that included patchiness, spatial heterogeneity, and spatial connectivity, all patches recovered to their original state after shorter presses. However, once press duration exceeded a threshold, growing proportions of the landscape shifted to an alternative state as press duration increased. We observed similar patchy transitions in a catchment-scale experiment that reinstated frequent fires approximately halfway through a regime shift from grassland to shrubland, initiated by fire suppression. In both the local-and larger-scale models, the threshold duration needed to elicit regime shifts decreased as press intensity increased or when factors counteracting regime shifts weakened. These multiple lines of evidence suggest that conceptualizing regime shifts as an interactive function of the intensity and duration of driver changes will increase understanding of the varying effects of driver presses, pulses, and cycles on ecosystem dynamics.

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“…For example, it could indicate a state change to an undesirable state (Ratajczak et al. ) or it could indicate increased stability of ecosystem processes (Brown , Isbell et al. , Hector et al.…”

Regional grassland productivity responses to precipitation during multiyear above‐ and below‐average rainfall periods

“…For example, both semiarid warm grasslands and mesic grasslands-which exist at opposite ends of low and high annual PPT in grasslands, respectively-are vulnerable to encroachment by woody plant species and the ecological transition of grasslands to shrublands. An increase in the frequency of multiyear above-and below-average periods would increase the probability that these periods co-occur with additional site-specific factors and land management decisions, potentially increasing grassland vulnerability to degradation and ecological transitions (Ratajczak et al, 2017). An increase in the frequency of multiyear above-and below-average periods would increase the probability that these periods co-occur with additional site-specific factors and land management decisions, potentially increasing grassland vulnerability to degradation and ecological transitions (Ratajczak et al, 2017).…”