Relationship between continental and regional distribution. Analysis with woodland birds of the Iberian peninsula

Abstract

We analyze the distribution patterns of bird species inhabiting deep forests and other woodland habitats in the Iberian Peninsula during the breeding season. The main goals are the description of the environmental factors responsible for their distribution, and the relationship between the distribution at the regional (Iberian peninsula) and higher geographical scales (Europe, Palearctic and biogeographical groups considering the world distribution of species -Voous’ 1960 ornitogeographic categories-). To overcome these objectives we use the most recent sources of bird distribution in Spain and in Europe (the Atlas de las Aves Reproductoras de España –Martí y Del Moral, 2003-, and the Atlas of European Breeding Birds -Hagemeijer y Blair, 1997-) and environmental data (geographical location, geomorphologic parameters, climatological variables and data on the extension of woodland habitats) derived from geographical information systems. The total number of bird species studied is 85. By means of tree regression analyses (Generalized Additive Models), the frequency distribution of the species in UTM squares of 10x10 km, within UTM blocks of 50x50 km (Figure 1), was explained to a large degree considering a small collection of 10 variables describing the main environmental gradients in the Iberian peninsula for forest and woodland species. We obtained very significant, simple models, explaining the distribution pattern of 83 out of the 85 studied species, that on average explained 74% of original deviance (Table 1). Tree regression analyses were also used to understand the main environmental factors underlying the geographical variation in the species richness of six ornitogeographical groups according to Voous (1960): Holarctic, Palearctic, Euroturkestan, European, Mediterranean species, and a group of birds with a wide distribution in Africa (Figure 2). High percentages of explained deviance were also obtained for these six biogeographical groups, ranging from 52 to 83% (Figures 3-5). The number of cloudless days per year (i.e., anticyclonic situations without rainfall and mild to warm temperature) was the most important variable negatively affecting the distribution pattern of species richness for the bird species with a wide geographical distribution in Europe and the whole Palearctic. The mountainous character of UTM blocks of 50x50 km was also very important, positively affecting species richness of Holarctic, Palearctic, Euroturkestan and European birds, while it had a clear-cut negative effect on the species richness of birds with a wide distribution in Africa (Ethiopian, Indo-African and Ancient World ornitogeographical groups). Geographical position had a significant but a limited effect on species richness of Holarctic, Palearctic, Euroturkestan and European species, being related positively with latitude (Figure 2). Nevertheless, latitude had a negative and prominent role in determining species richness of Mediterranean species and those widely distributed in Africa. Coniferous forest cover positively influenced species richness of Holarctic and Euroturkestan species. The distribution baricenter of the species in Iberia and Europe were closely related. The species on the Iberian Peninsula that had more northern and western distributions occupied more occidental and northern geographic baricenters in Europe. The species that in Spain are widely distributed in extensive forested mountainous areas, with a cool and wet climate, mostly occupy the northernmost regions in Europe (Table 4). Moreover, those species mainly distributed in Europe near the Mediterranean basin are more frequent within the Iberian Peninsula in warm valleys covered with little forest surface and great extensions of wooded agricultural formations (Figure 6). The possible effects of the past and future climatic changes are discussed considering the patterns obtained for each of the species and the six biogeographic groups analyzed. The small role of the geographic position (latitude, longitude) in determining the variation of species richness of the groups with broad distribution in the Western Paleartic (Holartic, Paleartic, Europeas sensu lato), does not support the role of the “peninsula effect” in explaining the geographic variation of species richness of this region. The specialization of the species in their environmental preferences on the Iberian Peninsula were significantly associated with the biogeographic groups of Voous (1960): Holartic and Paleartic species had more specialized requirements in terms of what was available on the peninsula, while Mediterranean species, on average, had smaller values of environmental specialization (Figure 7). The environmental specialization of the species on the Iberian Peninsula was also closely related to the geographic distribution in Europe: the species with less specialized environmental preferences in Iberia had a broader European distribution and were more scarce in northern European latitudes (Table 4). The distribution of species in the 50x50 km UTM blocks allowed a marked regionalization of the bird fauna, and characteristic species were identified in each of the 8 regions distinguished (Tables 5 and 6, Figure 9). The regions identified were closely associated with previously defined bioclimatic and fitogeographic areas. The frequency of appearance of the species belonging to the different biogeographic groups considered varied very markedly among these regions. The species that are widely distributed in Europe and in the Paleartic are mainly distributed in the Eurosiberian regions and in the mountains of northern Spain. Although the Mediterranean species and those widely distributed in Africa are spread throughout Spain, they are more frequent in the Mesomediterranean areas (southwestern part of the Iberian peninsula; Figure 8). The main biogeographic patterns derived from this regionalization are the large number of species that characterize the northernmost regions and those with the most oceanic climate, the low number of species characteristic of the southernmost Mediterranean regions, and the greater homogeneity of bird fauna of the southern part of Iberia, with a marked absence of species that are characteristic of sub-regions within this area. In summary, the patterns of geographic distribution and environmental preferences of woodland birds on the Iberian Peninsula reflect the distribution patterns on greater geographic scales (Europe, the whole range of world distribution).