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).


Regime Shift Example: Mesquite Invasion Into A Desert Grassland

Many desert grasslands have been invaded in the last century by mesquite (Prosopis glandulosa), often leading to a shift from a grassland to a shrubland. This regime shift results in numerous negative effects, including soil degradation, decreased air quality, losses of plants and animals associated with grasslands, and decreased livestock production.

Drought and poor grazing management can lead to loss of grasses, and once enough grasses are lost, we see threshold-like increases in soil movement and loss. This sequence can lead to large-scale changes in the environment, as shown in the photographs below (from Brandon Bestelmeyer, USDA-ARS Jornada Experimental Range). In 1984, the site was dominated by grasses, which still exist at the same site in 1997. Just six years later, however, the site looks completely different. Soil has buried the grasses which were once dominant in the area, leaving considerable bare soil and some persistent shrubs. In this case, grasses in the photo were not disturbed, but because the surrounding landscape had already shifted to shrubland, remaining grasslands were lost due to soil movement from adjacent areas. Thus, regime shifts may involve changes that spread once thresholds are crossed in a large enough area.

1984   1997
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References

Carpenter, S.R. and W.A. Brock. 2006. Rising variance: a leading indicator of ecological transition. Ecology Letters 9:311-318.

Elwell, H.A. & M.A. Stocking. 1976. Vegetal cover to estimate soil erosion hazard in Rhodesia. Geoderma 15:61-70.

Van de Koppel, J.; M. Rietkerk; F.J. Weissing. 1997. Catastrophic vegetation shifts and soil degradation in terrestrial grazing systems. Trends in Ecology and Evolution 12(9):352-356.