Snails Population Dynamics related data sets

PROJECT DESCRIPTION: Diadromous fauna, migrating between marine and coastal streams is dominant in the neotropics. However, the factors controlling their population distribution and size are poorly understood. The migratory snail Neritina virginea (Gastropoda: Neritidae), abundant in estuaries and coastal rivers in the Caribbean, was studied to understand those factors. The objectives of the study were three-fold: 1) to identify the factors controlling the distribution of diadromous fauna at different spatial scales, 2) to determine patterns and causes of massive upstream migrations and their importance for population persistence, and 3) to integrate this knowledge into conservation strategies for migratory species and stream management plans.

After a survey of 32 coastal streams around Puerto Rico (Greater Antilles), it was evidenced that physical variables operating at different levels of the spatial hierarchy controlled longitudinal distribution of N. virginea. The hierarchically organized variables were river-ocean connectivity (regional scale), instream barriers and water chemistry (stream-network scale), habitat hydraulics (reach scale) and nearbed-flow roughness and depth (habitat scale). However, michohabitat scale distribution depended upon habitat and reach scale context. At habitat scale, water depth in pools, and nearbed flow roughness in riffles were the most important controls. At the reach scale, spatial heterogeneity among and within streambed patches was related to flow refugia, therefore controlling sensitivity of N. virginea density to flooding disturbances.

In addition, the role of structures (bridges) and maintenance activities (downstream channel realignment) associated to road crossings over streams was assessed. By splitting the stream channel and deflecting the flow, bridge pilings altered upstream migration routes. In particular, migratory individuals used more frequently boulder-and-cobble riffle reaches and avoided gravel-bed run reaches. Impact assessment of channel realignment over a 70 m reach in lower Rio Mameyes (NE Puerto Rico) showed that increase in frequency of fine sediment patches contributed to local population depletion and high mortality of juvenile individuals migrating upstream. As a consequence a population located 100 m upstream became isolated and did not recover from a crash caused by a major storm flood occurred after the channel realignment. In conclusion, distribution and size of populations of diadromous fauna such as N. virginea in coastal streams is influenced by hierarchically organized variables. By affecting variables at microhabitat to reach scales, road crossings over streams contribute to reduce longitudinal connectivity, and isolate upstream populations of N. virginea.