Understanding interactions between animal populations and their environment is fundamental to the discipline of ecology and vital for conservation efforts of ecosystems.  Long-term monitoring studies of animal populations give ecologists a greater ability to identify patterns of interactions between animal populations and their environment that may not be detectable by short-term studies.  Here we present the results of a long-term study monitoring two small mammals, Sigmodon hispidus and Oryzomys palustris, in hardwood hammock islands within the Everglades National Park, Florida, USA.  We used mark-recapture trapping to monitor small mammals on 17 hammock islands for nearly 109 consecutive months between February 1994 and December 2005.  We examined the relationships among animal abundance, density, reproductive condition, movement distance, movement frequency and how each of these factors are influenced by island size and water depth. The small mammal community within Everglades’ tree islands is primarily composed of the cotton rat (S. hispidus) and the marsh rice rat (O. palustris,) with cotton mice (Peromyscus gossypinus) representing less than 1 % of captures. Tree-islands were arbitrarily classified into three size classes by area.  Animals were classified as adults, juveniles and sub-adults by weight.

Results/Conclusions

The results of this analysis revealed three interesting insights into the natural history of these species:  1) The highest densities of cotton rats were on the largest islands, whereas the marsh rice rats had the highest densities on medium to small islands: 2) Large islands appear to be the source of cotton rat recruits. Sixty-five percent of reproductively active adults and 82 % of juveniles were captured on large islands. 3) Contrary to our expectations, water-levels did not significantly influence the average movement distance of hispid cotton rats or marsh rice rats, and movement distances did not differ between the species.  To examine the possible influence of the small mammal community on the ecosystem properties of the Everglades we used the population density estimates from our long-term study and data on the elemental composition of the small mammal species.  From this we estimated the changes in small mammal biomass within the Everglades’ and how changes in the number and area of tree islands could potentially influence the standing stock of phosphorus and nitrogen within the small mammal community.