Global distribution of extinction risk

Proc. R. Soc. B (2006) 273, 2127–2133 doi:10.1098/rspb.2006.3551 Published online 8 June 2006

Human impacts and the global distribution of extinction risk
Richard G. Davies1, C. David L. Orme2, Valerie Olson3, Gavin H. Thomas4, Simon G. Ross1, Tzung-Su Ding5, Pamela C. Rasmussen6, Ali J. Stattersfield7, Peter M. Bennett3, Tim M. Blackburn4, Ian P. F. Owens2,8 and Kevin J. Gaston1,*Biodiversity & Macroecology Group, Department of Animal & Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK 2 Division of Biology, Imperial College London, Silwood Park, Ascot, Berkshire SL5 7PY, UK 3 Institute of Zoology, Zoological Society of London, Regent’s Park, London NW1 4RY, UK 4 School of Biosciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK 5 School of Forestry &Resource Conservation, National Taiwan University, 1, Sec 4, Roosevelt Road, Taipei 106, Taiwan, ROC 6 Michigan State University Museum, West Circle Drive, East Lansing, MI 48824-1045, USA 7 BirdLife International, Wellbrook Court, Girton, Girton Road, Cambridge CB3 0NA, UK 8 NERC Centre for Population Biology, Imperial College London, Silwood Park, Ascot, Berkshire SL5 7PY, UK Understanding theglobal geographical distribution of extinction risk is a key challenge in conservation biology. It remains controversial, however, to what extent areas become threat hotspots simply because of high human impacts or due to predisposing ecological conditions. Limits to the taxonomic and geographical extent, resolution and quality of previously available data have precluded a full global assessment of therelative roles of these factors. Here, we use a new global database on the geographical distributions of birds on continents and continental islands to show that, after controlling for species richness, the best predictors of the global pattern of extinction risk are measures of human impact. Ecological gradients are of secondary importance at a global scale. The converse is true for individualbiogeographic realms, within which variation in human impact is reduced and its influence on extinction risk globally is therefore underestimated. These results underline the importance of a global perspective on the mechanisms driving spatial patterns of extinction risk, and the key role of anthropogenic factors in driving the current extinction crisis. Keywords: extinction risk; globalbiodiversity; human population; species richness; threatened species
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1. INTRODUCTION Understanding the geographical distribution of extinction risk and its causes are key challenges in conservation biology, and are central to determining spatial priorities for the focus of conservation responses. Major determinants of extinction risk across space include not only anthropogenic environmental impacts butalso variation in predisposing ecological conditions (Forester & Machlis 1996). The former include human population density, agricultural and urban land-use, species exploitation, introduced species and disease, and anthropogenic ´ climate change (Soule 1991; Forester & Machlis 1996). Predisposing ecological conditions include, but are not exclusive to, the availability of ambient environmentalenergy which is thought to influence speciation rates and thus the occurrence of neoendemics (Rohde 1992), the availability of productive environmental energy which is

* Author for correspondence (k.j.gaston@sheffield.ac.uk). The electronic supplementary material is available at http://dx.doi. org/10.1098/rspb.2006.3551 or via http://www.journals.royalsoc.ac. uk. Received 28 November 2005 Accepted16 March 2006

thought to limit overall and individual species population numbers ( Wright 1983), absolute species numbers which may influence food web structure and thus the likelihood of extinction cascades (Gaston 2002), and surface topography which influences the occurrence of narrowly distributed species (Richerson & Lum 1980). Despite a number of valuable regional studies, restrictions to...