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

Abstract

AimWe 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. MethodsWe record the presence of each species in 1961 1°×1°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) being dominated by richness gradients. Main conclusionsThe 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 s...

Biogeographical patterns of endemic terrestrial Afrotropical birds

“…These transitional zones are characterized by both high scores of species range-edges and species replacement (Williams et al, 1999). The major subregional divisions suggested by the cluster analysis agree well with the results from a divisive classification technique (Williams et al, 1999), indicating that results presented in this study are robust to different hierarchical algorithm techniques. These results are also well supported by fieldbased studies conducted at localized scale, such as those for Ethiopia (Urban & Brown, 1971), Kenya (Muriuki et al, 1997), the Brachystegia belt (Benson & Irwin, 1966), Angola (Hall, 1960;Traylor, 1963), the Guinea-Congolian subregion (Diamond & Hamilton, 1980) and the Southwestern subregion (Winterbottom, 1978).…”

“…replacement of one avifauna by another, sensu Whittaker, 1960). These transitional zones are characterized by both high scores of species range-edges and species replacement (Williams et al, 1999). The major subregional divisions suggested by the cluster analysis agree well with the results from a divisive classification technique (Williams et al, 1999), indicating that results presented in this study are robust to different hierarchical algorithm techniques.…”

“…This has also been noted by Moreau (1935), Crowe & Crowe (1982), and Williams et al (1999) and may be due largely to a number of species that occur widely in the Afrotropics except in the driest parts (e.g. Ispidina picta, Halcyon senegalensis, Oxylophus levaillantii, Myioparus plumbeus and Ploceus cucullatus), which increase the similarity of the mesic subregions, while simultaneously resulting in species 'drop outs' (i.e.…”

Conservation Biogeography: assessment and prospect

“…He was writing about an attempt to test the validity of Bailey's foundational ecoregions scheme, but the same case can be made for the WWF scheme. Magnusson suggests a simple approach involving looking for natural breaks in species distributions across ecoregion boundaries, and a more sophisticated approach to the problem is illustrated by Williams (1996) and Williams et al (1999).…”

Efecto de la estacionalidad y lluvias en la avifauna del Chaco Serrano Austral, Argentina

“…La adecuada conservación, es aún un reto tanto para las aves como para sus hábitats y la tarea se dificulta más si se tratan las variables que las afectan de forma aislada (Williams, de Klerk, & Crow, 1999;Kark & Van Rensburg, 2006). De este modo, las variaciones de la riqueza y la abundancia de aves del Chaco Serrano Austral observadas entre las diferentes estaciones e hidroperíodos anuales (excepto las lluvias mensuales), contribuirán con las tareas de manejo y conservación.…”