​​​​​​Increasing evidence shows that facilitative interactions between species play an essential role in coastal wetland ecosystems. However, there is a lack of understanding of how such interactions can be used for restoration purposes in salt marsh ecosystems. We, therefore, studied the mechanisms of reciprocal facilitative interactions between native annual plants, Suaeda salsa, and burrowing crabs, Helice tientsinensis, in a middle-elevation salt marsh (with generally high plant density and moderate tides) in the Yellow River Delta of China.

We investigated the relationship between the densities of the plants and crab burrows in different seasons. Then, we tested whether and how salt marsh plants and crabs indeed facilitate each other in a series of field and laboratory experiments. Finally, we applied the results by creating a field-scale artificial approach for microtopographic modification to restore a degraded salt marsh.

We found that the density of plant seedlings in spring was positively correlated with the density of crab burrows in the previous autumn; moreover, the density of crab burrows was correlated with the density of plants in summer. The concave-convex surface microtopography created by crabs promoted seed retention and seedling establishment of salt marsh plants in winter and spring. These plants in turn facilitated crabs by inhibiting predators, providing food, and reducing physical stresses for crabs in summer and autumn. The experimental removal of salt marsh plants decreased crab burrow density, while both transplanting and simulating plants in bare patches promoted crabs. The microtopographic modification, inspired by our new understanding of the interactions between salt marsh plants and crabs, showed that these degraded salt marsh ecosystems can be restored by a single ploughing intervention.

Synthesis. Our results suggest a reciprocal facilitation between annual plants and burrowing crabs in a middle-elevation salt marsh ecosystem. This knowledge yielded new restoration options for degraded coastal salt marshes through the one-time ploughing initiation of microtopographic variation, which could promote the re-establishment of ecosystem engineers and lead to the efficient recovery of pioneer coastal vegetation and associated fauna.