Density-dependent aggregation underlies spatial self-organization in coastal dunes

Across coastal ecosystems, habitat-forming organisms interact with wind and water to generate biogeomorphic feedbacks that produce spatially patterned landscapes. Such spatial patterns can emerge through different self-organizing mechanisms, with contrasting implications for ecosystem resilience. Here, we test whether spatial dynamics in developing coastal dunes are better explained by scale-dependent feedback or by density-dependent aggregation (DDA), the latter being a more recently proposed mechanism of spatial self-organization. Using high-resolution aerial imagery and coupled time series of elevation, we tracked the spatial dynamics of dune-grass patches and sand accumulation during early dune development. Rather than converging toward a dominant size or spacing, both dune-grass patches and the dune bodies they collectively form coarsened through time, with self-similar size distributions shifting toward larger scales. Sand accumulation was disproportionately concentrated in already-dense vegetation, with a density-growth relationship that changes sign at a critical threshold. Together, these findings are consistent with DDA rather than scale-dependent feedback as the dominant mechanism of spatial self-organization in developing coastal dunes. The resulting density-growth relationship implies threshold-dependent resilience to local disturbances. Once local vegetation density falls below a critical point, decline is self-reinforcing rather than self-dampening, with direct consequences for how dune disturbance and recovery are understood and managed.