Are edge bird populations doomed to extinction? A retrospective analysis of the common guillemot collapse in Iberia

Munilla, I., Díez, C. & Velando, A. Are edge bird populations doomed to extinction? A retrospective analysis of the common guillemot collapse in Iberia. Biological Conservation 137, 359–371 (2007).


In the first half of the XXth century, the common guillemot (Uria aalge) was the seabird with the largest breeding population in Atlantic Iberia (ca. 20,000 individuals), the low-latitude limit of the species breeding range. However, this population suffered a dramatic decline and is quasi-extinct at present. The decline was believed to be associated with reduced availability of pelagic prey fish due to climate change. In this study, we analyzed the population change of Iberian guillemots in the second half of the XXth century by means of a retrospective analysis. Our study showed that between 1960 and 1974 the guillemots in Iberia suffered a dramatic population crash (33.3% annual decline) and that subsequently, the population continued to decline at a slower annual rate (13.4%). Simulation models indicated that the factors driving the population crash should be related to adult survival, rather than reproduction. The analysis of environmental and fishery data suggested good climate conditions and higher or sustained availability of pelagic prey fish when the Iberian guillemots crashed. In contrast, relevant human-related factors were affecting adult mortality in that period, specially a rapid and large increase in the number of synthetic fishing nets. During the collapse, no conservation measures were undertaken to mitigate anthropogenic threats and it was assumed, in some extent, that this low-latitude edge population was somehow prone to extinction as a consequence of climate change. This study high- lights that to carelessly attribute the decline of rear edge populations to climate change could be highly misleading if the population is suffering from other, particularly human, threats.

Are edge bird populations doomed to extinction: A response to Munilla et al.

Martínez Abraín, A. Are edge bird populations doomed to extinction: A response to Munilla et al. Biological Conservation 191, 843–844 (2015).


Munilla et al. (2007) defended the idea that the population crash of common guillemots (Uria aalge) in Atlantic Spain, between 1960 and 1974, was not due to food scarcity associated to climate change, but to decreased adult survival caused by human-related factors, such as the generalization of the use of synthetic fishing nets, illegal shooting and oil spills. I strongly agree with them regarding the relevance of not using climate change as a default scapegoat for conservation problems. Often, proximate ecological factors are responsible for population declines, and can be managed if identified. However, regarding the quasi-extinction of common guillemots in NW Iberia the authors did not rule out convincingly the possibility that the local population crash was not due to massive adult emigration, rather than to reduced adult survival. In fact they admitted in their discussion that local adult survival in their population models also included adult emigration, and that they could not discard at all this possibility, although they considered it unlikely.