Where within a geographical range do species survive best? A matter of scale


Chris D Thomas, Department of Biology (Area 18), University of York, PO Box 373, York YO10 5YW, UK. E-mail: cdt2@york.ac.uk



  • 1Opinions differ as to whether declining species are most likely to survive in central or peripheral parts of their distributions. The former pattern is likely to be driven by high extinction risks in peripheral areas; the latter by gradients of extinction risk.
  • 2At a continental scale of analysis, the declining butterfly Euphydryas aurinia survived best in southern and eastern countries within Europe. This was statistically associated with geographical variation in agricultural intensification. At this scale of analysis, there was a gradient of survival, caused by a gradient of agricultural intensification.
  • 3Within England and Wales, survival was greatest in population concentrations, or core areas; that is in 10-km grid squares that were surrounded by other 10-km grid squares that also contained populations of E. aurinia. In the English county of Dorset, populations were also most likely to be found in core areas; that is in habitat patches that were close to other populated habitat patches.
  • 4In this system, there is support for two patterns of decline. At a coarse scale, there is a geographical gradient of habitat degradation, associated with agricultural intensification. But within a region where decline has taken place, populations survive best in core areas, where aggregations of habitat support viable metapopulation dynamics.
  • 5Large-scale geographical patterns of decline towards the periphery (or other locations within) the distribution of a species do not negate the validity of conservation strategies based on core-margin population dynamic principles. Core areas within each country or region represent appropriate targets for conservation action.