Self‐Organization and Vegetation Collapse in Salt Marsh Ecosystems

Abstract

Complexity theory predicts that local feedback processes may strongly affect the organization of ecosystems on larger spatial scales. Whether complexity leads to increased resilience and stability or to increased vulnerability and criticality remains one of the dominant questions in ecology. We present a combined theoretical and empirical study of complex dynamics in mineralogenic salt marsh ecosystems that emerge from a positive feedback between clay accumulation and plant growth. Positive feedback induces self-organizing within the ecosystem, which buffers for the strong physical gradient that characterizes the marine-terrestrial boundary, and improves plant growth along the gradient. However, as a consequence of these self-organizing properties, salt marshes approach a critical state as the edge of the salt marsh and the adjacent intertidal flat becomes increasingly steep and vulnerable to wave attack. Disturbance caused, for instance, by a storm may induce a cascade of vegetation collapse and severe erosion on the cliff edge, leading to salt marsh destruction. Our study shows that on short timescales, self-organization improves the functioning of salt marsh ecosystems. On long timescales, however, self-organization may lead to destruction of salt marsh vegetation.

Salt-marsh evolution at Northwick and Aust warths, Severn Estuary, UK: a case of constrained autocyclicity.

“…This strip of halophytes represents the birth of a fourth marsh at Northwick Warth. As observed by van de Koppel et al (2005) and van der Wal et al (2008) from the relatively sheltered marshes they studied, and by Pedersen and Bartholdy (2007) from an exposed context, a narrow zone of unvegetated mud separated the new marsh from the bold scarp at the edge of the low marsh. In the case of the marshes of the Danish Wadden Sea, this zone, taking the form of a channel (landpriel), persists over the entire build-up of the new marsh (Jakobsen 1954;Pedersen and Bartholdy 2007).…”

“…The Aust-Northwick marshes exhibit many features expected of autocyclic marshes (Gao and Collins 1997;van de Koppel et al 2005) and observed from marshes claimed as autocyclic, but also give evidence for a degree of external control (Table 1). Could the Severn Estuary marshes be autocyclic but with the autocyclicity substantially locked into the pattern of extrinsic factors?…”

“…That of Gao and Collins (1997) is site-specific, and it is therefore hardly surprising that the results are consistent with their field data from microtidal Christchurch Harbour on the English Channel coast. Two field cases from the just-macrotidal Wadden Sea and Westerschelde were advanced by van de Koppel et al (2005) in support of their model, and van der Wal et al (2008) and Callaghan et al (2010) added another example from the Westerschelde. In reviewing salt marsh autocyclicity, Chauhan (2009) provided information for the Moricambe Basin of the Solway Firth in the northwest UK.…”

“…Gao and Collins (1997) suggested, but did not model, how a new marsh could develop on the sandflat below the cliff. The exploratory model proposed by van de Koppel et al (2005) emphasized the role of positive feedback in the biodynamics of a coupled mudflat-marsh. The plant-covered transitional zone becomes increasingly steep as the marsh-platform grows in elevation relative to the mudflat.…”

Long-Distance Interactions Regulate the Structure and Resilience of Coastal Ecosystems

“…Cordgrass modifies water flow and increases the deposition of sediments and the accumulation of peat substrate (Redfield 1972). Through this building process and scale-dependent interactions between vegetation growth, substrate building, and water flow, the developing marsh vegetation can generate hummocks (Bouma et al , 2009van de Koppel & Crain 2006), elevation gradients, and other landscape features (van de Koppel et al 2005b), which contrasts with the previous assumption that vegetation zonation is simply a product of elevation gradients (and associated local variations in abiotic and biotic drivers). Over time, however, this process can lead to the marsh being overbuilt-for example, facilitative interactions can stimulate local sediment buildup, increasing the height of the marsh.…”