VITAL RATES, THEIR VARIATION AND NATURAL SELECTION: A CASE FOR AN ATLANTIC FOREST MARSUPIAL

Abstract

Evolutionary theory predicts that viable wild populations confront environmental variation by maintaining the variance of the most important vital rates low, a phenomenon also known as demographic buffering. However, patterns diverging from the demographic buffering hypothesis have also been reported. Here we used Population Matrix Models to test the hypothesis of buffering in the early survival of an Atlantic Forest marsupial. We used sensitivity analyses, which allow quantifying the effects that perturbations in specific vital rates could have on the population’s growth rate. We obtained the sensitivities, elasticities, and second-order derivatives of the discrete population growth rate with respect to different vital rates for a population of the black-eared opossum ( Didelphis aurita , Marsupialia, Didelphidae) in a preserved Atlantic Forest, the state of Rio de Janeiro, Brazil. The second-order derivatives offer an insight into the selection forces occurring during the life cycle, clarifying the processes behind the common pattern of early-survival importance. We found evidence of buffering in early survival, however, in one of the cohorts, early survival displayed evidence of opposed behavior to the expected under demographic buffering, thus revealing that established life-history patterns can be quickly modified by a small change in one vital rate.