Demographic heterogeneity, cohort selection, and population growth

Abstract

Abstract. Demographic heterogeneity-variation among individuals in survival and reproduction-is ubiquitous in natural populations. Structured population models address heterogeneity due to age, size, or major developmental stages. However, other important sources of demographic heterogeneity, such as genetic variation, spatial heterogeneity in the environment, maternal effects, and differential exposure to stressors, are often not easily measured and hence are modeled as stochasticity. Recent research has elucidated the role of demographic heterogeneity in changing the magnitude of demographic stochasticity in small populations. Here we demonstrate a previously unrecognized effect: heterogeneous survival in long-lived species can increase the long-term growth rate in populations of any size. We illustrate this result using simple models in which each individual's annual survival rate is independent of age but survival may differ among individuals within a cohort. Similar models, but with nonoverlapping generations, have been extensively studied by demographers, who showed that, because the more ''frail'' individuals are more likely to die at a young age, the average survival rate of the cohort increases with age. Within ecology and evolution, this phenomenon of ''cohort selection'' is increasingly appreciated as a confounding factor in studies of senescence. We show that, when placed in a population model with overlapping generations, this heterogeneity also causes the asymptotic population growth rate k to increase, relative to a homogeneous population with the same mean survival rate at birth. The increase occurs because, even integrating over all the cohorts in the population, the population becomes increasingly dominated by the more robust individuals. The growth rate increases monotonically with the variance in survival rates, and the effect can be substantial, easily doubling the growth rate of slow-growing populations. Correlations between parent and offspring phenotype change the magnitude of the increase in k, but the increase occurs even for negative parent-offspring correlations. The effect of heterogeneity in reproductive rate on k is quite different: growth rate increases with reproductive heterogeneity for positive parentoffspring correlation but decreases for negative parent-offspring correlation. These effects of demographic heterogeneity on k have important implications for population dynamics, population viability analysis, and evolution.

Variance in animal longevity: contributions of heterogeneity and stochasticity

“…The impact of heterogeneity on demographic outcomes, eco-evolutionary processes, and population dynamics has been the topic of several studies (e.g., Kendall and Fox 2002;Robert et al 2003;Kendall et al 2011;Vindenes and Langangen 2015;Cam et al 2016). However, the extent to 1 3 which variance in fitness components can be accounted for by individual stochasticity is still open (Cam et al 2016).…”

Successful by Chance? The Power of Mixed Models and Neutral Simulations for the Detection of Individual Fixed Heterogeneity in Fitness Components

“…Given their skewed and heterogeneous nature, LRS distributions are unlikely to be solely shaped by unstructured environmental stochasticity. Instead, individuals seem to differ in their probability of surviving or reproducing (Kendall et al 2011). …”

Consequences of Dispersal Heterogeneity for Population Spread and Persistence

“…It has been shown to affect demographic stochasticity Fox 2002, 2003;Vindenes et al 2008), extinction risk of both density-independent populations (Conner and White 1999;Fox 2005;Lloyd-Smith et al 2005) and density-dependent populations (Robert et al 2003), population growth rate (Kendall et al 2011), and equilibrium population size (Stover et al 2012). We aim to expand this literature to include dispersal heterogeneity's effect on spread rate and persistence in advective environments.…”