Influence of historic coffee cultivation on terrestrial snail communities in the Luquillo Experimental Forest, Puerto Rico

Background/Question/Methods

Regular cenusing of terrestrial snails in the Luquillo Experimental Forest, Puerto Rico, has been ongoing since 1989.  These data have routinely shown significant spatiotemporal variation in the distribution of species in this area.  The influence of habitat factors, such as land use history, underlying this variation has not been fully explored.  This study investigated the influence of historic coffee cultivation on extant snail communities.  Snail diversity was surveyed in an area near the El Verde Research Station known to support coffee cultivation until 1930.  Snails were sampled during the summer for three consecutive years (2009-2011) along three linear transects running perpendicular to known plantation boundaries.  Each transect consisted of 10 plots (3 m radius x 3 m height) with 5 plots in the historical plantation and 5 plots in adjacent undisturbed forest.  Surveys were conducted at night to coincide with peak snail activity.    

Results/Conclusions

Overall snail abundance was significantly higher on the old coffee plots.   Caracolus caracolla and Nenia tridens were the most common snails in both land uses.  These species, however, were 2-5x more abundant on the old coffee.  Coffee C. caracolla populations also supported a greater proportion of juveniles. Platysuccinea portoricensis was also much more abundant on old coffee.  No difference in species diversity was found, but richness was greater on the undisturbed forest.  Differences in habitat, arising from land-use history, may underlie these differences.  Previous surveys of these same areas found distinct differences in tree species and soil properties between land uses.  Notably, calcium, nitrogen, and pH were elevated in old coffee soils.  Limestone, applied to coffee fields to raise pH, is still present is high concentration.  Calcium is needed by snails for shell growth, so limestone soil amendments would benefit resident snails.  Higher soil pH would accelerate litter decomposition and increase available food for detritivorous snails.  Elevated soil nitrogen likely arose from leguminous Inga vera trees planted to shade coffee plants.  The resulting increase in soil fertility would promote faster forest growth and greater litterfall production.  These increased detrital inputs coupled with faster decomposition would act to enhance snail habitat.