Effects of rainfall on population dynamics of Atlantic Forest rodents
Understanding reproduction is central to better understand population fluctuations because, besides its effects on population size, its energetic requirements can affect body condition, survival probability, and/or future reproduction of the parent generation. Physiological costs of reproduction are the most significant component underlying vertebrate life-history trade-offs and thus reproduction is frequently associated with food availability, which in turn is highly correlated to rainfall in tropical environments. Using a monthly capture-recapture dataset, comprising three two-ha grids sampled for almost two years, we investigated the influence of rainfall on reproduction and population dynamics of four small rodents in the Atlantic Forest. More specifically, we investigated the effects of rainfall on survival and recruitment rates, on the frequency of reproductive females and on individual body condition aiming at testing for the existence of live-history trade-offs. We implemented Pradel and Pollock’s robust design models in program MARK to estimate population parameters and its relation to rainfall. The influence of rainfall on reproduction activity and body condition were investigated through a selection of generalized linear models.
The frequency of reproductive females was positively related to rainfall (with no time lag) in most species. Given that pregnancy takes around one month in small rodents, the absence of a time lag suggests that litters are born mainly during the period of highest rainfall, and the phase of highest energy demand (lactation and weaning) will occur during the period of highest food availability. Although we expected body condition to improve in periods with higher resource availability, especially given observed results on reproduction, body condition of all species did not change in relation to rainfall. In turn, survival rates were negatively related to rainfall, and the relationship included one and/or two month time lag in most cases. These results suggest that the energy provided by increased food availability was allocated to increase reproductive output and not accumulated as body reserves to improve body condition and survival prospects, resulting in a trade-off between reproduction and survival. Thus, trade-offs between survival rates and reproduction can have a strong effect in the dynamics of small rodents in Atlantic Forest and future studies consider this trade-off together with predation and density-dependent factors to understand the dynamics of rodents at Atlantic Forest.