Example - Bare vs. Vegetated Sand Dune Alternative Stable State and Regime Shift

Free-blowing sand dunes (blowouts) existing alongside vegetated sand dunes illustrate alternative stable states (Figure 4). Alternative stable states are different arrangements of an ecosystem’s functions, processes, and relationships (Holling 1973, May 1977, Scheffer et al. 2001). Vegetated dunes and blowouts show how two stable states can exist side by side, each with their own reinforcing processes that contribute to their existence. Plants spread via roots and seeds into open sand, which stabilizes blowing sand over time and creates vegetated dunes. Simultaneously, wind erosion scours the edges of the blowouts, gradually weakening plants and eroding around their roots, creating a blowout (Hesp 2002). These two processes create a constantly changing landscape affected by many factors, but specific environmental changes such as dry or wet years that favor either vegetation or blowing sand can trigger a shift between states at any given point in space and time.

Figure 4. Bare and vegetated sand dunes.

Left: A Sandhills blowout, home to an unusual community of plants in McKelvie National Forest, Nebraska, by the U.S. Forest Service, 2012, Wikimedia Commons. In the public domain. 

Right: Blowout on the dunes at Harlech, by Williams, K. circa 2005, Geograph. CC BY-SA 2.0.

References

Hesp P. 2002. Foredunes and blowouts: initiation, geomorphology and dynamics. Geomorphology 48:245–268.

Holling, C. S. (1973). Resilience and Stability of Ecological Systems. Annual Review of Ecology and Systematics, 4, 1–23. https://doi.org/10.1146/annurev.es.04.110173.000245

May RM. 1977. Thresholds and breakpoints in ecosystems with a multiplicity of stable states. Nature 269:471–477.