Ecosystem engineers and physical forces in synergy

Seed dispersal is a critical process in the life history of flowering, often involving multiple steps both horizontally and vertically.  For many plant species, seed burial i.e. vertical seed movement can have mighty consequences for seed fate, plant recruitment and vegetation dynamics despite being only a tiny step when compared with horizontal seed dispersal.

Traditionally, ecosystem engineers (e.g. earthworms, marine worms) have been credited as the being the most important drivers for seed burial in both terrestrial and aquatic plants. The role of physical forces, such as wind or water flow, has however been largely overlooked. Using tidal habitats as a model system, and a combination of flume and mesocosm experiments, we investigated the effects of currents, benthic animals with different engineering activities and their interplay on seed burial of a common salt marsh pioneer plant, Spartina anglica. Four benthic species commonly found in north-west European tidal flats were employed in our manipulative experiments.  Ragworm A. virens functioned as an active seed burying engineer, while clam M. balthica, cockle C. edule and lugworm A. marina influenced seed burial mainly through seed entrapment and sediment mixing, and are, thus, viewed as passive seed burying engineers.

Our results show that in such systems, water flow can be of equal or higher importance than ecosystem engineers for seed burial. For passive seed-burying engineers, coupling their actions with currents produced synergistic seed burial effects, whereas the interactive effects were only additive for active seed-burying engineers. This paper extends current understanding of seed burial and seed bank formation by revealing the need to incorporate physical forces into seed burial mechanisms. We provide the first empirical evidence that physical forces influence seed burial by synergistically interacting with ecosystem engineers, thus highlighting the role of biophysical interactions as important drivers for vertical seed movement.    

The authors through Zhenchang Zue

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