Interpreting biogeographical boundaries among Afrotropical birds: spatial patterns in richness gradients and species replacement

Aim We analyse the geographical distribution of 1911 Afrotropical bird species using indices of three simple biogeographic patterns. The first index, the frequency of species with range edges (T-e), is formulated to map directly the density of species distribution limits, for comparison with the results of traditional biogeographical classification and ordination procedures, in order to show variations in the strength and breadth of transition zones. The other two indices are formulated to seek to distinguish as directly as possible between two components within these transition-zone patterns: contributions from gradients in species richness (T-g); and contributions from replacements among species (T-r). We test the ability of these indices to discover the same boundaries among Afrotropical bird faunas as one popular procedure for classifying areas (TWINSPAN) and then use them to look for geographical trends in the different kinds of transition zones. Location The analysis is restricted to the sub-Saharan or Afrotropical region, excluding the Arabian Peninsula, Madagascar and all offshore islands. Methods We record the presence of each species in 1961 1 degrees x 1 degrees grid cells of the map. To apply the three indices, each (core) grid cell in turn is compared with its neighbouring eight cells in the grid. The range edges index (T-e) counts the number of species with range edges between the core cell and the surrounding cells. The richness gradients index (T-g) counts the largest difference in species richness measured diametrically across the core cell in any direction when there is a consistent trend in richness along this line of three cells. The species replacements index (T-r) counts the number of species pairs recorded within a nine-cell neighbourhood that are not corecorded within any of the cells. Values for each of the 1961 grid cells are calculated and used to produce colour-scale maps of transition zones. Results Large-scale spatial patterns of variation in density of range edges (T-e) are consistent with classifications of the same data and with most previous biogeographical classifications proposed for the region. Variation in richness gradients (T-g) and species replacements (T-r) explain different parts of this pattern, with transition zones around humid forests in the equatorial region being dominated by species replacement, and transition zones around deserts (most extensive in the north and south) bring dominated by richness gradients. Main conclusions The three indices distinguish the spatial arrangement and intensity of different kinds of transition zones, thereby providing a first step towards a more rigorous mechanistic understanding of the different processes by which they may have arisen and are maintained. As an example of one such pattern shown by our analyses of Afrotropical birds, there is evidence for a broad latitudinal trend in the nature of transition zones in faunal composition (following the latitudinal distribution of the different kinds of habitat transitions), from being dominated by species replacements near the equator to being dominated by richness gradients further from the equator.