Long-term responses of snail populations and communities to disturbance in the Luquillo Experimental Forest, Puerto Rico

Large-scale natural disturbances, such as hurricanes, have extensive effects on the structure and dynamics of populations and communities. Studies of large-scale, infrequent disturbances are few compared to other types of disturbance and have tended to focus on a single disturbance event. A general understanding of the effects of disturbance requires more comparative work on disturbance events of different intensities. Additionally, a long-term perceptive is necessary, in that responses to disturbance may change in direction or magnitude over time.

In this dissertation, I used a long-term data set to examine population- and community-level responses of terrestrial gastropods in Puerto Rico to two major hurricanes. 1 first examined responses of 17 taxa in search of a general hurricane effect on populations. Then, I assessed the degree to which gastropod assemblages exhibit compositional stability in response to disturbance. Third, I used morphological characters to evaluate the importance of competition in structuring gastropod assemblages in a disturbance-mediated environment.

No single species typified the response of snail populations to disturbance. Some species appear to benefit from disturbance, whereas others do not. Likewise, no consistent hurricane effect emerged, as several species responded differently to the two different storms. Population responses probably hinge on the relative sensitivities of species to changes in microclimate and resource availability, both of which are affected to differing degrees by hurricanes of different intensity.

Irrespective of the severity of hurricane effects, successional replacement of species was not characteristic of gastropod assemblages. Instead, species composition remained fairly constant. Moreover, variability in population density was low, and rank abundances were consistent over time. Patterns of temporal change in community structure were driven primarily by changes in density of a single species, Nenia tridens, which nonetheless remained one of the three most abundant taxa throughout the study.

Competition for resources probably is not an important mechanism producing patterns of abundance and distribution of terrestrial snails. Snails do not partition niches via body size, strength of interspecific interactions does not increase with morphological similarity, and growth of Caracolus caracolla, a common and widespread species, is not inhibited by high densities of conspecifics or other species of snails. Instead, stochastic population fluctuations and the effects of habitat modification by disturbance may be agents of primary importance in structuring populations and communities of terrestrial snails.