Complex Ecosystem Changes: Thresholds and Regime Shifts

Introduction

Not all changes to ecosystems are gradual and reversible. Under pressure from human activities or natural forces, some ecosystems are resistant to change (e.g., they continue to be a grassland or clear lake) until the system reaches a “threshold.” At this point, the system changes dramatically and quickly (e.g, a shrubland or turbid, eutrophic lake).

For example, the graph below [from the work of Elwell and Stocking (1976), as reported by van de Koppel et. al. (1997)] shows a clear threshold in soil loss as plant cover decreases. As cover decreases from 70% to 40%, soil loss stays relatively constant, increasing only slightly with large decreases in cover. However, when vegetative cover decreases below about 30%, soil loss increases dramatically, leading to the steep incline in the soil loss line. The threshold in soil loss therefore occurs at approximately 30% of vegetative cover.

Once a system reaches a threshold, it may change to a new state. At this point, changes back to the original state are virtually impossible. Scientists use the term “regime shift” to refer to these long-lasting changes to ecosystems, such as a shift from grass-dominated to shrub-dominated landscapes in desert rangelands (Carpenter and Brock 2006).

Regime shifts can be difficult to study since they often have multiple causes and occur over large areas. Nonetheless, the goal is often to be able to predict a regime shift before it occurs, leaving managers time to avoid the shift to undesirable states. To this end, researchers have recently noticed that the variance of a system often increases as it reaches a critical threshold prior to the regime shift. For example, Carpenter and Brock (2006) found a detectible increase in the variability of lake-water phosphorous prior to a shift to eutrophic conditions (over-enrichment of water that results in excessive blue-green bacterial growth and depletion of oxygen).