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

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 of habitat suitability due to the
putative effect of adaptations to regional conditions. Location:
North-western Africa (Morocco) Time period: Present day Major taxa
studied: 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 to habitat suitability
and negatively correlated to distance to source areas. The taxonomic
status of birds did not affect the patterns. Main conclusions: The
southward decrease of 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
temperature.