How to Keep Fit in the Real World: Elasticity Analyses and Selection Pressures on Life Histories in a Variable Environment

Most life-history theory assumes the environment is invariant. For the first time, analytical and numerical techniques were employed to investigate the impact of environmental variability on selection pressures (elasticities = proportional sensitivities) on a range of life histories. We find that the impact of variability is influenced significantly by the amount of variability an organism experiences (more variability affects selection pressures more), the correlations between variations among the vital rates (negative correlations are more likely to relax selection on fecundities and increase it on survival rates), and the life history in question (shorter life histories are more affected). In addition, the impact of a variable environment on the elasticities of life histories is sensitive to the sampling distribution used to generate the variability, and it is particularly sensitive to extreme values, such as those caused by occasional catastrophic events. The elasticities of life histories in highly variable environments may bear little relationship to those in a constant environment. In detailed optimality or evolutionarily stable strategy (ESS) modeling, variability in vital rates as small as a standard deviation being 10%-15% of the mean may appreciably alter the conclusions. Thus, it may be very important to consider the possible impact of environmental stochasticity and not to assume that it has no effect.