Saltmarsh resilience controlled by patch size and plant density of habitat-forming species that trap shells.

Habitat fragmentaion into small patches is regarded as a vital cause of biodiversity loss. Fragmentationof habitat-forming species is especially harmful, as patchiness of such species often controls ecosystem stability and resilience by density and patch size-dependent self-reinforcing feedbacks. Although fragmentation are expected to weaken or even break such feedbacks, it remains unclear how the resulting patchiness of habitat-forming species affect ecosystem resilience to environmental stresses. Here, using Spartian alterniflora, the habitat-forming species in saltmarshes as a model, we investigate how patch size, plant density, and shell aggregation interactively control the persistence of a degrading salt marsh that suffered from erosion induced by hydrodynamics. Our results demonstrate that large patches can trap more shells along the patch edge than the smaller ones, therefore significantly facilitating plant re-growth within the patch. Shell removal experiments further reveal that large patches trapping more shells along patch edges reinforce their own persistence by decreasing erosion and thus facilitating plant recovery. By contrast, small patches with lesser plants cannot persist as they trap less shells along patch edges but are able to accumulate more shells at interior locations where they hinder plant re-growth, indicating a critical threshold of patch size ~20 m below which ecosystem collapses. The current study highlights the importance to identify critical threshold of stress-resistant patch sizes in transition-prone ecosystems as early-warning to alert undesired ecosystem collapse and restoration practice.