In spite of their highly patchy distribution, the effects of landscape configuration on specialist biodiversity has been little studied in salt marshes. We investigated the impact of patch size on the abundance of specialist arthropods in two contrasted salt-marsh environments. Dominant ground-active species were sampled by pitfall traps in increasing areas of natural vegetation (dominated by Atriplex portulacoides) along two transects surrounded by either grazed (dominated by Puccinellia maritima) or invasive (dominated by Elymus athericus) vegetation. Spatially- and temporally-replicated sampling took place in the Mont Saint-Michel Bay (Western France) during 2012. Three dominant species, the wolf spider Pardosa purbeckensis (Araneae, Lycosidae), the ground beetle Pogonus chalceus (Coleoptera, Carabidae) and the beach-hopper Orchestia gammarellus (Amphipoda, Talitridrae), constituted 96% of all arthropods caught (N = 66,299). Patch size only had an effect on the carabid and on the amphipod, with large patches more populous than small ones, reinforcing the idea that the effects of fragmentation are stronger for species with limited mobility. Environment had a significant effect on the population density of all species, with systematically more individuals in patches surrounded by invaded than by grazed salt marshes, which confirms the particularly negative impact of over-grazing on salt-marsh biodiversity. This study finally suggests that both invasive species and grazing impact salt-marsh biodiversity also at a landscape scale.
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