Mangroves in the Caribbean and particularly in the Uraba Gulf (Colombia) are strongly threatened by selective logging and conversion to pastures and croplands. Specifically, extensive Avicennia germinans- basin stands were converted to pastures during the twentieth century, thus exposing benthic fauna to an edge effect. We measured this effect on the population of a numerically dominant gastropod (Neritina virginea). Despite its resistance to natural disturbances, it is sensitive to extreme anthropogenic disturbances, and it would therefore be a good biological indicator of basin-mangrove conversion to pastures. Forest structure variables, soil texture, porewater properties and snail density and size were measured in quadrats placed in pastures, pasture-mangrove edges, and mangrove interiors. Snail abundance sharply decreased from the mangrove interior to the edge and then gradually towards the pastures. Individuals in the pasture were predominantly >10mm, and they frequently exhibited shell corrosion compared to individuals found in the interior. There were increases in soil temperature and pH (but oxygen) from interior to pasture consistent with canopy openness. The occurrence of the mangrove edges has led to a marked ecosystem-wide deterioration; however, N. virginea (abundance, size, shell corrosion) could be used as a reliable short to midterm indicator of microhabitat and microclimatic differences observed across mangrove-pasture edge.