Stuut, J.-B. | bioRxiv Science

bioRxiv2025

Urbanisation drives non-linear, guild-specific restructuring of fungal communities in coastal dune habitats

Coastal dunes form dynamic ecosystems where fungal communities play critical roles in nutrient cycling and plant establishment. Urban expansion increasingly fragments these habitats through trampling, beach grooming, and nutrient inputs, yet the effects of urbanization on below{square}ground fungal diversity remain poorly understood. We used eDNA metabarcoding on 294 soil cores from 49 transects spanning 35 km of the Dutch coast to test how urban proximity influences total fungal diversity and composition in different functional guilds (litter saprotrophs, plant pathogens, mycorrhizal fungi) across incipient (early-successional) and established dunes. Urban effects were habitat-specific and generally strongest in incipient dunes. In incipient dunes, total alpha diversity showed a U-shaped relationship with city distance, with minima at intermediate distances, whereas established dunes showed weak responses. Fungal guilds diverged in their responses as predicted: litter saprotroph diversity increased with distance from cities in both habitats (consistent with greater organic inputs by vegetation), plant pathogens showed U-shaped diversity in incipient dunes, suggesting distinct urban-associated and native-associated assemblages, and mycorrhizal diversity increased with distance in both habitats, consistent with limited suitable hosts and higher disturbance near cities. Partitioning of beta diversity revealed that turnover, rather than nestedness, accounted for almost all variation in community composition, indicating taxon replacement rather than directional loss. Community uniqueness was higher near cities, particularly in incipient dunes and for most guilds, implying that urban environments foster compositionally atypical assemblages relative to the regional pool. Overall, urbanization acts as a trait-based environmental filter, restructuring fungal communities without necessarily reducing fungal diversity. Stronger responses in early-successional dunes highlight their vulnerability to combined natural and urban stressors and the need to prioritize protection of incipient coastal habitats under intensifying urban pressure.

ecology↗

bioRxiv2025

Urbanisation reshapes community assembly differently across coastal beach and dune habitats

Urbanisation can reduce biodiversity through environmental filtering, but its effects may differ among neighbouring habitats within ecotones. Beach-dune systems provide a useful test because strong environmental gradients occur over short distances and overlap with gradients in human pressure. We collected 660 sediment samples from 55 transects along the Dutch North Sea coast, covering established dunes, incipient dunes, the upper intertidal zone, and the lower intertidal zone. Using environmental DNA metabarcoding and habitat-specific joint species distribution models, we tested how distance-to-city was associated with community composition, operational taxonomic unit (OTU) richness, and OTU-level occurrence. We found compositional differentiation along the urban-proximity gradient in both dune habitats, particularly in incipient dunes, but not in the intertidal habitats. Across all habitats, communities far from cities were more similar to one another than communities near cities, indicating greater among-site compositional variation near urban areas. OTU richness and mean OTU occurrence probabilities increased with distance-to-city in the lower intertidal, upper intertidal, and established dunes. Incipient dunes showed no clear richness gradient or directional OTU-level response despite the strong compositional turnover, suggesting that the main response was community replacement rather than loss of OTUs. Overall, habitat position within the beach-dune ecotone influenced whether biodiversity responses to urban proximity led to directional filtering, compositional turnover, or increased community heterogeneity. Our findings show that biodiversity and impact assessments based on a single habitat may miss important ecological change in coastal habitats.

ecology↗