Biomedical subjects

C Rahbek

Publications and source records attributed to C Rahbek.

4 recordsLinked to original sources

PubMed2001

Geometric constraints explain much of the species richness pattern in African birds.

The world contains boundaries (e.g., continental edge for terrestrial taxa) that impose geometric constraints on the distribution of species ranges. Thus, contrary to traditional thinking, the expected species richness pattern in absence of ecological or physiographical factors is unlikely to be uniform. Species richness has been shown to peak in the middle of a bounded one-dimensional domain, even in the absence of ecological or physiographical factors. Because species ranges are not linear, an extension of the approach to two dimensions is necessary. Here we present a two-dimensional null model accounting for effects of geometric constraints. We use the model to examine the effects of continental edge on the distribution of terrestrial animals in Africa and compare the predictions with the observed pattern of species richness in birds endemic to the continent. Latitudinal, longitudinal, and two-dimensional patterns of species richness are predicted well from the modeled null effects alone. As expected, null effects are of high significance for wide ranging species only. Our results highlight the conceptual significance of an until recently neglected constraint from continental shape alone and support a more cautious analysis of species richness patterns at this scale.

Africa↗

PubMed2001

Multiscale assessment of patterns of avian species richness.

The search for a common cause of species richness gradients has spawned more than 100 explanatory hypotheses in just the past two decades. Despite recent conceptual advances, further refinement of the most plausible models has been stifled by the difficulty of compiling high-resolution databases at continental scales. We used a database of the geographic ranges of 2,869 species of birds breeding in South America (nearly a third of the world's living avian species) to explore the influence of climate, quadrat area, ecosystem diversity, and topography on species richness gradients at 10 spatial scales (quadrat area, approximately 12,300 to approximately 1,225,000 km(2)). Topography, precipitation, topography x latitude, ecosystem diversity, and cloud cover emerged as the most important predictors of regional variability of species richness in regression models incorporating 16 independent variables, although ranking of variables depended on spatial scale. Direct measures of ambient energy such as mean and maximum temperature were of ancillary importance. Species richness values for 1 degrees x 1 degrees latitude-longitude quadrats in the Andes (peaking at 845 species) were approximately 30-250% greater than those recorded at equivalent latitudes in the central Amazon basin. These findings reflect the extraordinary abundance of species associated with humid montane regions at equatorial latitudes and the importance of orography in avian speciation. In a broader context, our data reinforce the hypothesis that terrestrial species richness from the equator to the poles is ultimately governed by a synergism between climate and coarse-scale topographic heterogeneity.

Animals↗

PubMed2001

Conservation conflicts across Africa.

There is increasing evidence that areas of outstanding conservation importance may coincide with dense human settlement or impact. We tested the generality of these findings using 1 degree-resolution data for sub-Saharan Africa. We find that human population density is positively correlated with species richness of birds, mammals, snakes, and amphibians. This association holds for widespread, narrowly endemic, and threatened species and looks set to persist in the face of foreseeable population growth. Our results contradict earlier expectations of low conflict based on the idea that species richness decreases and human impact increases with primary productivity. We find that across Africa, both variables instead exhibit unimodal relationships with productivity. Modifying priority-setting to take account of human density shows that, at this scale, conflicts between conservation and development are not easily avoided, because many densely inhabited grid cells contain species found nowhere else.

Africa South of the Sahara↗

PubMed2000

Detection of macro-ecological patterns in South American hummingbirds is affected by spatial scale.

Scale is widely recognized as a fundamental conceptual problem in biology, but the question of whether species-richness patterns vary with scale is often ignored in macro-ecological analyses, despite the increasing application of such data in international conservation programmes. We tested for scaling effects in species-richness gradients with spatially scaled data for 241 species of South American hummingbirds (Trochilidae). Analyses revealed that scale matters above and beyond the effect of quadrat area. Species richness was positively correlated with latitude and topographical relief at ten different spatial scales spanning two orders of magnitude (ca. 12,300 to ca. 1,225,000 km2). Surprisingly, when the influence of topography was removed, the conditional variation in species richness explained by latitude fell precipitously to insignificance at coarser spatial scales. The perception of macro-ecological pattern thus depends directly upon the scale of analysis. Although our results suggest there is no single correct scale for macro-ecological analyses, the averaging effect of quadrat sampling at coarser geographical scales obscures the fine structure of species-richness gradients and localized richness peaks, decreasing the power of statistical tests to discriminate the causal agents of regional richness gradients. Ideally, the scale of analysis should be varied systematically to provide the optimal resolution of macro-ecological pattern.

Animals↗