Ecological and geographical marginality in rear edge populations of Palaearctic forest birds

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Aim: The centre–periphery hypothesis predicts that habitat suitability will decrease at the edge of a species’ range, a pattern often questioned by empirical data. Here we explore if habitat suitability decreases southwards and shapes the abundance distribution of rear edge populations of forest birds within the restricted geographical setting of the south-western Palaearctic. We also test if birds endemic to the area fit more poorly to the latitudinal decrease in habitat suitability due to the putative effect of adaptations to regional conditions. Location: North-western Africa (Morocco). Taxon: Passerines (11 species). Methods: Bird occurrences were used to model species distribution and line transects were used to estimate bird abundance. Occurrence probabilities provided by species distribution models were used to display the spatial patterning of habitat suitability. Habitat suitability was employed to predict abundance after controlling for the effect of the distance to some regional source areas of forest birds (tree covered large areas). The species were classified as North African endemic according to an updated review of their taxonomic status. Results: Habitat suitability decreased southwards, supporting the predicted relationship between ecological and geographical marginality in most species. Abundance was positively correlated with habitat suitability and negatively correlated with distance to source areas. The taxonomic status of birds did not affect the patterns. Main conclusions: The southward decrease in habitat suitability predicted by the centre–periphery hypothesis shapes the distribution of rear edge populations of forest birds within the south-western Palaearctic. As most of these populations are endemic, the results suggest that they track the gradients in isolation within the geographical setting of north-western Africa. These results support the vulnerability of these isolated, peripheral populations of forest birds to large-scale environmental changes in a region under the effect of increasing drought and temperatura.