DOI: 10.5061/dryad.34tmpg4kh
title: Ecological and geographical marginality in rear edge populations of Palaearctic forest birds (data)

description: 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.
description: The first set of data ("Bird occurrences") provide
 geo-referenced observations of the study species, which result from
 extensive bird sampling carried out by the authors within the study area.
 These data were used to model species distribution (Maxent), and the
 occurrence probabilities provided by species distribution models were used
 to assess the spatial patterning of habitat suitability.
 The second set of data ("Transects") were used to
 estimate bird abundance. We counted all birds within a belt 500 m long and
 25 + 25 m wide. To control the effect of local habitat structure, we set
 two 25-m-radius circles distributed at 150 and 350 m of the starting point
 of each itinerary. In each point we visually assessed (%): grass layer
 (GRASS), shrub (vegetation between 0.5 and 2 m height; SHRUB) and tree
 (vegetation &gt;2 m height; TREE) covers, and tree and shrub
 (&gt;0.5 m) species numbers (SPP-SHRUB-TREE). In this data set, the
 acronyms of the species is: CERBRA: <i>Certhia
 brachydactyla</i>; CYATEN: <i>Cyanistes teneriffae</i>;
 ERIRUB: <i>Erithacus rubecula</i>; FRICOE: <i>Fringilla
 colebs</i>; PERATE: <i>Periparus ater</i>; PARMAJ:
 <i>Parus major</i>; PHYBON: <i>Phylloscopus
 bonelli</i>; REGIGN: <i>Regulus ignicapillus</i>;
 SYLATR: <i>Sylvia atricapilla</i>; TROTRO:
 <i>Troglodytes troglodytes</i>; TUERME: <i>Turdus
 merula</i>). These last data were used to perform
 a multivariate approach for a comprehensive view on the interacting
 effects of local forest cover, geographical location, and habitat
 suitability on bird abundance. After testing for multicollinearity, we
 controlled for the effect of spatial autocorrelation and, because of the
 scarcity of most forest birds within the study area, we coped with the
 effect of a zero-inflated structure in our data. To solve both problems,
 we performed a Zero-Inflated Poisson model with an intrinsic Conditional
 Autoregressive model (iCAR) process in which we accounted for the effect
 of the zero-inflated structure on abundance and controlled for the
 potential effects of spatial-autocorrelation.
description: The files ("Bird occurrences" and
 "Transects") contain the data on which all analyses have been
 carried out.