Demographic heterogeneity – variation in propensities to survive and reproduce – is ubiquitous in natural populations. Most population models disregard heterogeneity other than those reflecting differences in age, size, or major developmental stages. In a series of earlier papers, we showed that this heterogeneity can have important effects on demographic stochasticity, and by implication, on the risk of extinction. Here we present a theoretical framework that explains why there is so much heterogeneity.  

Results/Conclusions

Sources of heterogeneity include genetic variation, spatial heterogeneity in the environment, maternal effects, and differential exposure to stressors, especially early in life. Natural selection can reduce heterogeneity caused by genes with additive effects, but not the other sources. Moreover, many demographic traits are expected to have small heritability (even if there is substantial genetic variation for the underlying organismal traits) because there is large variation in demographic outcomes – put simply, some individuals survive or reproduce in spite of their (relatively) bad genes. Consequently, while selection on these traits can be strong, they evolve slowly, and selection tends to have a weak effect on reducing heterogeneity. Temporal environmental variation can also cause selection for the maintenance of heterogeneity, through mechanisms like bet-hedging. The other sources of heterogeneity are inherent in ecological processes, with their intensity depending on ecological context and spatial scale.  The various sources of heterogeneity create different types of population structure (e.g., heterogeneity within or between families) and different potential levels of parent-offspring correlation, both of which influence how the heterogeneity affects population dynamics and extinction risk. This theory will help us develop predictive models of the biological circumstances under which demographic heterogeneity is likely to be important in population dynamics.