Applying landscape ecology to conservation biology: Spatially explicit analysis reveals dispersal limits on threatened wetland gastropods

Three gastropods in the UK red data book (RDB) occupy drainage ditches on threatened grazing marshes. Segmentina nitida, Anisus vorticulus and Valvata macrostoma are affected by habitat loss, ditch management and eutrophication, but dispersal and fragmentation effects have also been postulated. We used landscape ecological approaches to examine such effects on these and 17 other gastropods on four English marshes, specifically combining ordination to identify suitable habitat with spatially-explicit analysis of occupancy. Among all gastropods, the occupancy of suitable habitat declined significantly as the distance to the nearest occupied site increased. S. nitida and A. vorticulus were among the species most affected, with median nearest distances from occupied to unoccupied suitable sites significantly greater by 3 − 4X than distances between occupied sites. V. macrostoma was not limited locally by dispersal, but was absent from three out of four marshes with suitable habitat. Eutrophication (elevated N) had no effects on distances between occupied and unoccupied sites and did not contribute to fragmentation. Although four non-threatened species were apparently also limited by dispersal, only two (Armiger crista; Gyraulus albus) showed some combination of the dispersal effects, low occupancy of suitable habitat (<50%) and small niche extent that characterized RDB species. While dispersal alone cannot explain unfavourable conservation status in these wetland snails, our data support the hypothesis that limited dispersal between (all species) and within marshes (S. nitida and A. vorticulus) affect all three RDB snails in their remaining UK range. Action is required to better understand the population and genetic consequences; to better understand dispersal mechanisms; and to evaluate re-introduction and reinforcement as aids to recovery. The latter could double the existing site occupancy.