HOW GENERAL ARE THE DETERMINANTS OF THE STOCHASTIC POPULATION GROWTH RATE ACROSS NEARBY SITES?

Abstract

A key question in both life history evolution and conservation biology is how much the contributions of different demographic processes to the rate of population growth vary from place to place. Using data from a six-year demographic study of five nearby populations, we asked, for the first time, how well the sensitivities and elasticities of the stochastic population growth rate (λs) to the means and standard deviations of underlying vital rates (survival, growth, reversion, and reproduction) can be generalized among populations. Relying on Tuljapurkar's approximation for λs, we used standard mathematical formulas for the sensitivities and elasticities of λs to the vital rate means, and we derived approximations for the sensitivities and elasticities of λs to the vital rate standard deviations. No single vital rate mean or standard deviation had the highest stochastic sensitivity or elasticity across all five populations. However, the summed sensitivities and elasticities of different types of vital rates were much more consistent: the mean survival rates had the highest summed sensitivity and elasticity, and the standard deviations of growth or reversion rates had the highest summed sensitivity or elasticity in all populations. The rank order of the individual sensitivities and elasticities of the vital rate means were, in general, also highly correlated among populations, with the exception of one population that showed a distinctive sensitivity/elasticity pattern, but the sensitivities and elasticities of the vital rate standard deviations were less correlated among populations. Environmental variables (geographical location, elevation, and geological parent material) did not predict the degree of similarity in sensitivity/elasticity patterns between population pairs. Our results show that overall patterns in the stochastic sensitivities and elasticities of mean vital rates can be extrapolated accurately across closely adjacent populations, but that caution is needed when extrapolating sensitivities/elasticities of vital rate standard deviations or when applying the single most important vital rate from one population to others.