Recruitment bottlenecks and reinforcing environmental degradation drive marsh collapse

Man Qi , Keryn Gedan

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 found that plant occupancy decreased along the inundation gradient, but the decline did not differ significantly between flood-tolerant and flood-sensitive functional groups. Using transplants of the flood-tolerant rush Juncus roemerianus, we show that recruitment limitation arises from multiple, context-dependent mechanisms that vary across sites and performance metrics. In the less saline marsh, competition from established species suppressed clonal growth (shoot production) but had little effect on flowering or survival, whereas in the more saline marsh, competitive effects were weak across all metrics. In dieback patches, transplants performed similarly to source populations in shoot production, flowering and survival, indicating that absence from these patches is driven by dispersal or colonization limitation rather than habitat unsuitability. In contrast, in ponded areas where peat collapse reduced elevation, transplants exhibited reduced shoot production and survival, while reproductive output was less affected, demonstrating strong physical constraints on establishment. Together, these results reveal that marsh collapse arises from compounded recruitment bottlenecks—including dispersal limitation, context-dependent competition, and post-dieback physical degradation—rather than regional species exhaustion. This finding challenges models that infer future species distribution from current occupancy without accounting for recruitment failure. Effective restoration must therefore target multiple stages of recruitment, including propagule delivery, competitive release, and substrate stabilization, before physical degradation renders habitats unsuitable.