Villellas, J., Doak, D. F., García, M. B., & Morris, W. F. (2015). Demographic compensation among populations: what is it, how does it arise and what are its implications? Ecology Letters, 18(11), 1139–1152. http://doi.org/10.1111/ele.12505
A common assumption in the analysis of species distributions across an environmental gradient are that vital rates decline across the gradient as they approach the range limit. Villellas et al. aimed to investigate an opposing theory, demographic compensation (DC), in which mean population vital rates change in opposing ways across spatial or environmental gradients. Using a randomization procedure, they tested for the presence of DC across 26 plant demographic studies, based on the assumption that DC would lead to more negative correlations among vital rates than expected by chance. They found both that there were more positive and more negative correlations than expected by chance, suggesting that the overall tendency is for vital rates to respond similarly, but that DC is a phenomenon that does exist. Investigating further, this pattern was driven by fecundity and recruitment rates, which had the highest proportion of negative correlation amongst vital rates and to which the population growth rate had high variance and sensitivity, respectively, evidence that DC acts by influencing vital rates as different life stages. For those studies that showed evidence of DC, another randomization procedure was done to compare the variation of population growth rates with a nd without DC, finding that DC halved the variance in population growth rates. This has important implications for SDM, as high growth rate variation at the range limits of species distributions can lead to a higher rate of stochastic extinctions, perhaps influence presence data and the probability of persistence in these limits. If we are to believe that DC is at play, however, there will be less variation at the range limits, implying that the ‘e-space’ may be closer to Hutchinson’s binary persistence-extinction demarcations than originally thought.
