Estimating latent individual demographic heterogeneity using structural equation models

Understanding the drivers of fitness is a key goal of population and evolutionary ecology. However, measuring individual variation in demographic components in imperfectly observed populations of wild organisms is extremely challenging. Recent research has demonstrated that estimates of fixed individual variation in Bernoulli variables (e.g., survival and breeding propensity) are often unreliable in the face of imperfect detection and small sample sizes. Thus, we demonstrate the use of structural equation modeling approaches to simultaneously estimate latent variation in demographic performance and link said variation to individual demographic components. We demonstrate the use of this approach with 30+ year capture-recapture datasets collected on two passerine species (white-throated dipper, Cinclus cinclus, and pied flycatcher, Ficedula hypoleuca) and simultaneously estimate latent variation in individual quality and age-specific variation in demographic components. We found senescent decline in survival and fecundity in both species and demonstrated strong among-individual heterogeneity in demographic performance. Notably, the residual reproductive value of older individuals of higher quality was greater than younger individuals of reduced and average quality. We demonstrate that this approach may be useful in a variety of situations, discuss its limitations, and make suggestions for future research.