Tropical tree diversity, forest structure and the demography of a frugivorous rodent, the spiny rat (Proechimys semispinosus)

Populations of the Central American spiny rat Proechimys semispinosus isola ted on eight small islands in Panama were sampled monthly for 1 year to exa mine the relationships of demography to tree species composition and forest structure. Demography was characterized by calculating seven variables fro m the monthly census data. Tree species composition was determined by marki ng, measuring and identifying all trees greater than or equal to 10 cm in d iameter at breast height (dbh) on each island. Forest structure (tree statu re, density and species richness) was quantified by measuring seven variabl es at each of the 470 sampling stations. Bivariate correlations were comput ed between spiny rat demography and tree species composition and forest str ucture. To examine whether islands with more similar tree species compositi on or forest structure supported P. semispinosus populations with more simi lar demography, dendrograms based on complete-linkage cluster analysis were constructed. Spiny rat densities were correlated positively with the densi ty of large-fruited fig trees and non-linearly to an index of forest age ba sed on tree species composition. Spiny rat densities were not related to an y structural variable. Dendrograms based on cluster analysis showed that sp iny rat demography did not map onto forest structure, overall tree species composition or any group of selected tree species except figs Ficus spp. Th us, islands with more similar fig tree composition contained spiny rat popu lations with more similar demography. Tree species composition, which partl y determines resource abundance, is apparently more important than forest s tructure in influencing spiny rat demography. Proechimys semispinosus has a temporally and spatially variable demography, and this flexible demography , in concert with an ability to use diverse resources, presumably promotes abundance and persistence in heterogeneous tropical forests and may contrib ute to the ubiquity of this rodent.