Population and life-history consequences of within-cohort individual variation

The consequences of within-cohort (i.e., among-individual) variation for population dynamics are poorly understood, in particular for the case where life history is density dependent. We develop a physiologically structured population model that incorporates individual variation among and within cohorts and allows us to explore the intertwined relationship between individual life history and population dynamics. Our model is parameterized for the lizard Zootoca vivipara and reproduces well the species' dynamics and life history. We explore two common mechanisms that generate within-cohort variation: variability in food intake and variability in birth date. Predicted population dynamics are inherently very stable and do not qualitatively change when either of these sources of individual variation is introduced. However, increased within-cohort variation in food intake leads to changes in morphology, with longer but skinnier individuals, even though mean food intake does not change. Morphological changes result from a seemingly universal nonlinear relationship between growth and resource availability but may become apparent only in environments with strongly fluctuating resources. Overall, our results highlight the importance of using a mechanistic framework to gain insights into how different sources of intraspecific variability translate into life-history and population-dynamic changes.