On the potential of plant species invasion influencing bio-geomorphologic landscape formation in salt marshes

Species invasions are known to change biotic and abiotic ecosystem characteristics such as community structure, cycling of materials and dynamics of rivers. However, their ability to alter interactions between biotic and abiotic ecosystem components, in particular bio-geomorphic feedbacks and the resulting landscape configuration in tidal wetlands, such as tidal channels have not yet been demonstrated. We studied the impact of altered bio-geomorphic feedbacks on geomorphologic features (i.e. tidal wetland channels), by comparing proxies for channel network geometry (unchanneled flow lengths, fractal dimension) over time between non-invaded and invaded salt marsh habitats. The non-invaded habitats (the south of eastern Chongming Island, Yangtze estuary, China) show little change in network geometry over time with a tendency for an increased drainage density. The invaded site (salt marshes in the north of eastern Chongming Island invaded by the exotic plant species Spartina alterniflora) showed a decreasing tendency in channel drainage density throughout and after the species invasion. This suggests that species invasions might not only affect biotic ecosystem characteristics, but also their ability to change bio-geomorphic feedback loops, potentially leading to changes in existing geomorphologic features and therefore landscape configuration. Our results further suggest that the species invasion also altered sediment composition. Based on observations we propose a mechanism explaining the change in channel drainage density by an alteration in plant properties. The physical and physiological characteristics of the invading species Spartina alterniflora clearly differ from the native species Scirpus mariqueter, inducing different bio-geomorphic feedback loops leading to the observed change in salt marsh channel configuration.