Using individual-based demographic modelling to estimate the impacts of anthropogenic mortality on territorial predators

Abstract

Wildlife anthropogenic mortality is increasing worldwide, yet there is limited understanding regarding its population-level impacts. Territorial species stand out in this context, as they possess distinctive characteristics that are often overlooked but may significantly affect their vulnerability. In particular, population impacts may depend on the level and spatial distribution of additional mortality risk across territories, and on the extent to which exposure to increased mortality varies across life stages (i.e., territorial and non-territorial individuals). In this study, we developed an Individual-Based Model (IBM) to explore these issues, using the Bonelli's eagle (Aquila fasciata) and electrocution in powerline pylons as a model system. We used declines in annual population growth rates as a proxy for negative impacts, and conducted simulations to estimate the relative impacts of different levels of mortality risk, the spatial pattern of such risks, and the risk exposure of different life stages. Population-level impacts greatly increased with the mortality risks simulated, and they were lower when exposure to mortality risks was concentrated versus spread across territories. Impacts were highest when both territorial and non-territorial individuals were exposed to anthropogenic mortality risks, and they were higher when such exposure only affected non-territorial versus territorial individuals. Our results underscore that each breeding territory should be considered as a unit, where all existing pylons should be intervened whenever mitigation actions are put in place. Results also highlight the importance of considering both the territorial and non-territorial fractions of the population to prevent and mitigate the impacts of increased mortality. More generally, our study illustrates the value of IBM frameworks such as ours to explore population-level impacts resulting from anthropogenic mortality in territorial species, and to inform the development of conservation strategies to mitigate such impacts.