How many Bearded Vultures live in the Pyrenees? A recently published article answers this question comprehensively by using an integrated population model to quantify the population, including the non-breeding wandering, or “floater”, individuals too. The study estimates that around 937-1119 Bearded Vultures live in the Pyrenees. It also reveals the number of individuals, their population dynamics and the factors that condition them, which is critical to plan and implement effective conservation actions to improve the status of this threatened species.
Overview
The Pyrenees is the stronghold of the Bearded Vulture population in Europe. It is the only substantial population of the species that did not go extinct in Europe. The only other populations, which did not go extinct, are in Corsica and Crete and are small island population with less than 10 breeding pairs each. Scientists knew how many breeding pairs lived in the Pyrenees mountain range from direct counts or monitoring of various demographic parameters. But these methods are not capable of providing complete information on a population, including non-breeding individuals, which is critical for understanding how populations function or the factors that drive population changes. Younger non-breeding Bearded Vultures travel vast distances, with their range often reaching 10,000 km2, making it almost impossible to determine their numbers, until now.
Methods
An investigation led by Instituto de Investigación en Recursos Cinegéticos (IREC) developed an Integrated Population Model (IPM) that has determined, for the first time, the total size of the Bearded Vulture population in the Pyrenees, distinguishing by age classes and estimating the main parameters that influence its population dynamics.
The research team used information on the reproduction and productivity rates of the species in the Pyrenees collected in Spain, Andorra and France through a long-term study that began more than 30 years ago, between 1987-2016. They then combined this information with data from monitoring 150 individuals of known age class that were captured and marked with rings and wing tags during the same period, enabling the estimation of population sizes and demographic parameters such as survival rates, from re-sightings of those marked individuals analysed in synergy with the long-term breeding monitoring data.
Results
The estimated total population size reached 937-1119 individuals: 57 juveniles (39-77), 220 subadults (179-282) and 748 adults (677-826). Within the adult age-class, only 365 were breeding individuals (48.8% of the adult population, and 35.6% of the total population).
The study also revealed that the adult population of the species has increased over the period examined, from 61% to 73%. Although this is good news, the breeding productivity of Bearded Vultures has declined in some areas, suggesting a possible saturation of the high quality territories as the population reaches its carrying capacity (i.e. the maximum population size that the area can sustain indefinitely). The analysis also found that as the population continues to increase there could be a negative effect on adult and juvenile survival rates and further reductions in breeding productivity as more individuals compete for limited resources, particularly breeding territories (termed negative density dependence). Ultimately this may lead to the paradox that as the population size increases, the population growth rate will decline. Also, the large number of adults and scarcity of suitable nesting sites has likely resulted in a higher average age of first reproduction than previously estimated and to the fact that 35% of the active territories were occupied by breeding trios — reproductive units of two males or more and one female.
Recommendations
Ideally, to increase the Bearded Vulture population beyond the Pyrenees, more individuals will start to leave the Pyrenees and colonise other suitable sites in the Iberian Peninsula and across Europe’s mountain ranges. But for the moment, this is not happening frequently, perhaps because it is difficult for the species to disperse across an increasingly human-modified landscape, or even because an abundant food supply reduced the incentive to forage further afield. However, there is some hope as a pair recently recolonised the Moncayo Mountains and attempted to breed in 2019/20. They did not manage to breed successfully, but nonetheless, this is a historical milestone for the conservation of the Bearded Vulture in the Iberian Peninsula. This reproductive attempt of the Bearded Vulture, in a natural way and outside the Pyrenean environment, is excellent news for the conservation of the species, especially given the limited records of natural colonisation of isolated breeding sites by the species.
One of the proposals now being discussed by a working group to encourage the species to recolonise the Iberian Peninsula is the possibility of reducing the supplementary food provided to the species in different feeding stations. This would provide them with greater incentive them to look for new territories. This measure must be managed sensitively, however, because it could affect survival rates and inhibit the ongoing recovery of the species where much effort and resources have already been invested.
Some initiatives try to attract the species away from the Pyrenees to other regions like our own LIFE GypConnect project, which aims to establish a breeding population of Bearded Vultures in the French Massif Central, as well as in the Pre-Alps, through reintroduction and promoting dispersal movements between the Alps and the Pyrenean populations, like the released Bearded Vulture Cardabelle who travelled to the Pyrenees. Expanding the distribution range of this species by promoting dispersal movements is important for different reasons. A wider distribution area of the Pyrenean birds minimises the risk of extinction of otherwise isolated populations. Further to this, such movements will facilitate the restoration of the species in Western Europe, as they promote gene flow and increase genetic diversity within sub-populations, ultimately increasing the resilience of the population.
The findings show the importance of long-term studies, something that is difficult to achieve and sustain. Even though the same model could be used for other species, it would require the same quality of information with homogeneous and integrated parameters where a long series of data are needed. In this case, the Pyrenees is a relatively closed population, making the necessary standardized data easier to attain.
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Source
Margalida, A., Jiménez, J., Martínez, J. M., Sesé, J. A., García-Ferré, D., Llamas, A., Razin, M., Colomer, M. A., Arroyo, B. 2020. An assessment of population size and demographic drivers of the Bearded Vulture using integrated population models. Ecological Monographs (2020).
