The influence of interdecadal variation of temporally variable demographic drivers on population dynamics

Understanding how variation in vital rates contributes to variation in population growth rates is critical yet remains unexplored. Examining the interconnected nature of weather and biotic interactions may answer the question: what is the role of extrinsic factors in regulating population dynamics? Weather may drive population dynamics either through direct density-independent effects or indirect density-dependent effects (via alterations in biotic interactions). The drivers of vital rates vary across time. This variation cannot be captured by short-term demographic studies. Here, we use a 35+ year demographic dataset for an introduced population of Euphydryas gillettii (Lepidoptera: Nymphalidae) at the Rocky Mountain Biological Laboratory. The population was introduced in 1977 and remained at low numbers until the 2000s. Following the first explosion (2002), the population greatly fluctuated in size until 2011. Since then, the population settled into a relatively stable state with lower variance. We ask if the drivers of vital rates differ across three time periods: before the initial explosion (1979 – 1988), the period of instability (2002 – 2011), and post explosion (2012 – 2023). We model E. gillettii population dynamics using a Bayesian approach that underlies an integrated population model (IPM). We then quantify the relative contribution of vital rates on the population growth rate using perturbation analyses. We use a transient life table response experiment (LTRE) to estimate the contribution of vital rate drivers across each time period. Combining LTREs with an IPM may result in a more mechanistic understanding of how extrinsic factors perturb population dynamics.