Not exodus, but population increase and gene flow restoration in Cantabrian brown bear (Ursus arctos) subpopulations. Comment on Gregório et al. 2020

Abstract

In a genetic study on brown bears ( Ursus arctos ) in the Cantabrian Mountains, Gregório et al. (2020) interpreted the asymmetrical gene flow they found from the eastern subpopulation towards the western one as an exodus of bears forced to flee from the eastern nucleus “with higher human disturbance and poaching”, concluding that connectivity may be operating as a means for eastern Cantabrian bears to find more suitable territories. In this reply, we maintain that the explanations of Gregorio et al. contradict the source-sink theory and we also present demographic data not considered by these authors showing that the eastern subpopulation is not declining, but persistently increasing. After reviewing the demographic and genetic studies published during the last 20 years, we conclude that the connectivity between the two subpopulations is operating as a route which allows the regular movement of males and the restoration of the gene flow across the whole Cantabrian population.

And yet, migration, population growth or mortality are not balanced among Cantabrian brown bear subpopulations. Reply to Blanco et al (2020)

“…Based on our results and on those of previous studies, we called the attention to the fact that Eastern Cantabrian brown bears might be taking advantage of increased connectivity to avoid higher human pressure and direct persecution in the areas occupied by the eastern Cantabrian subpopulation. In reply, Blanco et al (2020) [ 11 ] have criticized our ecological interpretation of the data presented in our paper. Namely, Blanco and co-authors criticize: (1) the use of the exodus concept in the title and discussion of the paper; (2) the apparent contradiction with source-sink theory; (3) the apparent overlooking of historical demographic data on Cantabrian brown bear and the use of the expression of population decline when referring to eastern subpopulation.…”

“…Major criticisms of Blanco et al [ 11 ] to our paper [ 8 ] are focused on: (1) the use of the concept of exodus applied to asymmetric migration on Cantabrian brown bears; (2) the interpretation of an asymmetric migration itself; (3) an apparent contradiction with classical source-sink theory; (4) and the apparent disregard of long-term and abundant information on the demography of brown bear. We start by re-focusing the use of the concept of exodus (and colonization), as well as the references to source-sink theory by Gregório et al (2020) [ 8 ], that are different from the interpretation of Blanco and co-authors [ 11 ] Secondly, we present a compilation of data on brown bear mortality going back several decades and discuss these figures at the light of available demographic data on brown bears. In third place, we discuss the potential role of this asymmetric flow of bears from Eastern to Western Cantabrian subpopulations at the light of these mortality data and on previous literature on Cantabrian and other brown bear populations.…”

“…While acknowledging the misuse of the expression “population decline”, we reply to Blanco et al [ 11 ], with the main aim of refocusing the discussion on the most relevant issue, which is to understand the reasons behind the distinct paces of demographic increase in the two subpopulations and the “counterintuitive” migration and gene flow patterns, to inform the Cantabrian brown bear conservation efforts. We also place in context the use of the “exodus” concept and of the reference to source-sink theory in our original paper.…”

“…The non-monophyly of the Cantabrian populations and the relation of the extant eastern Cantabrian haplotype with the historical Pyrenean lineage is supported by the comparison with extinct haplotypes recovered by Valdosiera and co-authors [ 10 ]. More recently, gene flow among Cantabrian brown bear subpopulations is increasing, as well as population size [ 4 , 5 , 11 , 12 ] even if this increase has been more pronounced in the western Cantabrian subpopulation. We also found solid evidence of migration of males and asymmetrical gene flow from eastern to western Cantabrian subpopulation.…”

“…This hypothesis was initially presented by Lamamy and co-authors [ 13 ], after finding that habitat alone could not explain the differences in population trends, numbers, and fecundity among the two subpopulations. Blanco and co-authors [ 11 ] contested our interpretation of the data, as well as the use of the concept of exodus, the apparent disregard for well-established demographic data and presented anecdotal data on mortality to refute our interpretation.…”