External disturbances have a significant impact on nature, such as population declines, changes in prey dynamics (Gliwicz & Jancewicz, 2016) or range shifts due to newly introduced species (Strauss et al., 2006). To effectively manage and conserve biodiversity, it is essential to understand population baselines and trends. Although numerous studies have thoroughly collected abundance data for population estimation, it remains challenging to estimate the baseline stable population for some species. Major pitfalls leading to miscalculations include missing existing functional groups within complex age structures exhibiting cryptic behaviour (Penteriani et al., 2005) and neglecting the confounding influence of predation shift in fluctuating environments.
Building on the previously described groups, the “floaters” are individuals who delay their first breeding while waiting for a territorial vacancy (Smith, 1978). The influence of floaters on future population trend has been demonstrated both theoretically (Law, 2024) and empirically (Penteriani et al., 2006; Robles & Ciudad, 2017). However, due to their elusive nature, population size after the external impact may often be underestimated (Piper et al., 2020), and conservation efforts might fall short because they do not fully encompass the areas where floaters may be residing (Penteriani et al., 2005). Furthermore, delayed breeding may occur due to resource limitations (Lenda et al., 2012) or early life experiences (Both et al., 2017) or the combinations of both, creating a knowledge gap regarding fundamental movement patterns. This, in turn, hampers our understanding of the actual dynamics of population abundance.
What further complicates population dynamics is the interaction within the ecological community. It is well known that prey abundance influences both behaviour and the demographic trends of predators. However, within the intraguild system, where both top- and meso-predator target the same prey (Polis & Holt, 1992), identifying the main drivers of change can be difficult due to cascading effects on behaviour (Mueller et al., 2016) and population trends (Russell et al., 2009), as well as indirect effects on demographic rates (Creel & Christianson, 2008). To fully understand these changes, a sufficient amount of high-quality data on species within intraguild is essential (Nicoll & Norris, 2010). Nevertheless, such data often require long-term, intensive surveys, which are usually impractical in real-world conditions.
One solution to data limitations is to develop a sufficient model by integrating various data sources and leveraging past knowledge (Buckland et al., 2000). The Integrated Population Model (IPM) is a particularly promising approach for incorporating elusive subpopulations (Iijima, 2020). IPM is a powerful modelling framework that links demographic models with the likelihood of existing but fragmented datasets, allowing for robust estimates of population trend (Schaub & Abadi, 2011). To account for predation contexts, the robustness of these models can be further enhanced through Bayesian inference, which integrates results from previous studies as probabilistic priors (Banner et al., 2020; Ellison, 2004). This framework offers an opportunity to integrate comprehensive ecological knowledge and maximize the value of existing data, ultimately supporting more informed conclusions in conservation decision-making (Ellison, 1996). This project aims to quantify the behaviour of invisible populations using various statistical frameworks and to understand their role in overall population stability, taking advantage of a 45-year-long-term survey of tawny owls conducted in Kielder Forest.
#References * Joanna Gliwicz & Elżbieta Jancewicz. (2016). Cascade Effect of Climate Warming: Snow Duration—Vole Population Dynamics—Biodiversity. British Journal of Environment & Climate Change, 6(1), 43–52. * Law, P. R. (2024). Equilibrium population dynamics of site-dependent species. Theoretical Ecology, 17(2), 107–119. https://doi.org/10.1007/s12080-024-00578-4 * Smith, S. M. (1978). The ‘Underworld’ in a Territorial Sparrow: Adaptive Strategy for Floaters. The American Naturalist, 112(985), 571–582. https://doi.org/10.1086/283298 * Strauss, S. Y., Lau, J. A., & Carroll, S. P. (2006). Evolutionary responses of natives to introduced species: What do introductions tell us about natural communities? Ecology Letters, 9(3), 357–374. https://doi.org/10.1111/j.1461-0248.2005.00874.x * Penteriani, V., Otalora, F., Sergio, F., & Ferrer, M. (2005). Environmental stochasticity in dispersal areas can explain the ‘mysterious’ disappearance of breeding populations. Proceedings of the Royal Society B: Biological Sciences, 272(1569), 1265–1269. https://doi.org/10.1098/rspb.2005.3075 * Piper, W. H., Grear, J., Hoover, B., Lomery, E., & Grenzer, L. M. (2020). Plunging floater survival causes cryptic population decline in the Common Loon. The Condor, 122(4). https://doi.org/10.1093/condor/duaa044