Impacts of resource abundance on populations of a tropical forest rodent
Populations of Proechimys semispinosus (Central American spiny rat) were provisioned with fresh native fruits for 6 mo to test the hypothesis that populations of frugivorous rodents in seasonal Neotropical forests are not limited by food during the season of greatest resource abundance. Islands in the Panama Canal were used as experimental systems so that results would not be confounded by individuals commuting from the fringes of the study areas. Populations isolated on four islands were censused by monthly live-trapping and served as unmanipulated controls, while populations isolated on four additional islands were provisioned with fruit at 315.2 kg/ha and censused by similar methods. Natural fruit abundance was concurrently censused by counting numbers of fruiting trees and lianas to account for naturally available resources. Means of overall spiny rat density and density of known births were compared between treatment groups using repeated-measures analysis of covariance, with the density of fruiting trees and lianas as the covariate. Both variables showed a treatment effect, with higher-than-expected densities and densities of births within experimental populations based on natural resource abundance. Numbers of known births per adult female were compared between treatment groups by constructing a log-linear model. This model also revealed a treatment effect. with per capita production of young being higher within all experimental populations. Monthly survival rates of young and adults were compared between treatment groups by constructing separate linear models for young and adults, and no treatment effect was evident. Adult male body mass was compared among islands by analysis of variance and similarly showed no treatment effect.Results showed that these spiny rat populations were limited by food even during the period of high resource abundance, and increased densities were due to increased production and recruitment of young rather than to increased immigration or survival.