Recruitment bottlenecks and reinforcing environmental degradation drive marsh collapse

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Abstract
Ecosystem collapse is often attributed to the absence of stress-tolerant species, yet many climate-stressed systems fail even when these species remain regionally present. This raises a critical, unresolved question: does collapse result from species pool depletion, or from failed recruitment? We addressed this by combining vegetation surveys and a three-year transplant experiment in sea level rise–threatened brackish marshes of Chesapeake Bay, USA. We first showed that replacement failure (i.e. suitable species exist in the regional species pool but failed to colonize) intensified along a gradient of increasing inundation, where flood-tolerant species exhibited lower occupancy and colonization of dieback patches compared to flood-sensitive species. By transplanting a dominant flood-tolerant species, Juncus roemerianus, into marsh zones of different stages of degradation, we found three main constraints on colonization: (1) competition from established species such as the sedge Schoenoplectus americanus, (2) dispersal inefficiency or establishment failure in dieback areas, and (3) post-dieback peat collapse, which rapidly renders unoccupied habitats physically uninhabitable. Our findings reveal that marsh collapse arises from compounded recruitment bottlenecks—not regional species exhaustion. This finding challenges species distribution assumptions that infer future presence from current occupancy without accounting for failed recruitment. Restoration must intervene early to reopen recruitment pathways—by managing competitors, stabilizing substrate, and enhancing propagule delivery—before physical degradation locks ecosystems into collapse.

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