From local monitoring to a broad‐scale viability assessment: a case study for the Bonelli's Eagle in western Europe

Abstract

Abstract. Population viability analysis (PVA) has become a basic tool of current conservation practice. However, if not accounted for properly, the uncertainties inherent to PVA predictions can decrease the reliability of this type of analysis. In the present study, we performed a PVA of the whole western European population (France, Portugal, and Spain) of the endangered Bonelli's Eagle (Aquila fasciata), in which we thoroughly explored the consequences of uncertainty in population processes and parameters on PVA predictions. First, we estimated key vital rates (survival, fertility, recruitment, and dispersal rates) using monitoring, ringing, and bibliographic data from the period 1990-2009 from 12 populations found throughout the studied geographic range. Second, we evaluated the uncertainty about model structure (i.e., the assumed processes that govern individual fates and population dynamics) by comparing the observed growth rates of the studied populations with model predictions for the same period. Third, using the model structures suggested in the previous step, we assessed the viability of both the local populations and the overall population. Finally, we analyzed the effects of model and parameter uncertainty on PVA predictions. Our results strongly support the idea that all local populations in western Europe belong to a single, spatially structured population operating as a sourcesink system, whereby the populations in the south of the Iberian Peninsula act as sources and, thanks to dispersal, sustain all other local populations, which would otherwise decline. Predictions regarding population dynamics varied considerably, and models assuming more constrained dispersal predicted more pessimistic population trends than models assuming greater dispersal. Model predictions accounting for parameter uncertainty revealed a marked increase in the risk of population declines over the next 50 years. Sensitivity analyses indicated that adult and pre-adult survival are the chief vital rates regulating these populations, and thus, the conservation efforts aimed at improving these survival rates should be strengthened in order to guarantee the long-term viability of the European populations of this endangered species. Overall, the study provides a framework for the implementation of multi-site PVAs and highlights the importance of dispersal processes in shaping the population dynamics of long-lived birds distributed across heterogeneous landscapes.

Effects of rearing conditions on natal dispersal processes in a long‐lived predator bird

“…The study population consisted of 85–90 pairs during the 1970s but decreased in number until it stabilized at 63 pairs in 2000; nevertheless, in recent years, numbers have risen to 73 pairs (DTS 2016; Real, Tintó, Boran, Beneyto, & Parellada, 2004). However, this increase is not a response to improvements in the principal vital rates—which have in fact worsened in recent years—but are, rather, the consequence of the net entry of immigrant eagles from neighboring populations (Hernández‐Matías et al., 2013, 2015). …”

Electrocution threatens the viability of populations of the endangered Bonelli's eagle ( Aquila fasciata ) in Southern Europe

“…The study population had 85-90 pairs during the 1970s but decreased in number until it stabilized at 63 pairs in 2000; nevertheless, in recent years, numbers have timidly increased up to 70 pairs (Real et al, 2004;DARPAMN, 2015). However, this increase is not a response to any improvement in main vital rates, which have actually worsened recently (see below), and seems, rather, to be the consequence of the net entry of immigrant eagles from neighboring populations (Hernández-Matías et al, 2013).…”

“…This species has undergone a dramatic decline in recent decades and is now listed as endangered in Europe (2009/147/EC;BirdLife International, 2004). Previous analyses have highlighted high levels of adult and pre-adult mortalities (Real and Mañosa, 1997;Soutullo et al, 2008;Hernández-Matías et al, 2013), mainly caused by electrocution and direct persecution .…”

“…Our model was based on one of the models used by Hernández-Matías et al (2013) that assumes closed local populations. Local populations in Western Europe are connected by dispersal (Hernández-Matías et al, 2013) and so our assumption that the population is closed was not realistic. Nevertheless, by assuming that our study population was isolated we were able to simplify the interpretation of the results, since our focus was on estimating the electrocution mitigation effort required to achieve self-sustainability, a reasonable conservation goal in populations at risk.…”

Importance of Long-Term Studies to Conservation Practice: The Case of the Bearded Vulture in the Pyrenees

“…Even when the underlying mechanisms may be of great interest in understanding the evolution of life histories, it is tough to correctly separate one mechanism from another (Tavecchia et al 2001;Sanz et al 2008). All in all, consequences for population dynamics are similar: population growth rates among long-lived species are expected to be highly sensitive to changes in adult survival (Real and Mañosa 1997;Oro et al 2008;Ortega et al 2009;Hernández-Matías et al 2013;Margalida et al 2015), so natural selection might have minimised variation in this parameter to ensure population stability (Saether and Bakke 2000). However, our results show how survival of bearded vultures changes through a bird's lifespan in an unnatural way, with non-adult birds (<5 years old) having higher and more constant estimates than adults (0.944 and 0.878, respectively, Fig.…”