The biogeography of the magnesian limestone grassland flora of east Durham and Tyne & Wear

Abstract

1. The remaining fragments of Magnesian Limestone Grassland (MLG) in East Durham and Tyne & Wear were surveyed. Biogeographical theory and climate modelling techniques are applied in order to provide insights for conservation policy 2. Either Area or Perimeter is the most important variable in predicting species richness at all three scales. The Degree of isolation of a habitat isolate from other MLG habitat is also a significant predictor. Micro-habitat variation is of most importance at the smallest scale.3. A Schematic model is presented which summarises the main processes determining MLG species richness at three interconnecting scales. 4. There is an exponential Species/Area relationship across the studied MLG fragments. It is very difficult to specify the exact mechanisms underlying this relationship however it appears that random and biological processes play a role. 5. Habitat sub-division appears not to depress species richness of MLG fragments. However this may be a result of smaller fragments supporting higher percentages of ecotonal species and also that these patches may be in historical disequilibrium. 6. There is a high degree of heterogeneity within the overall dataset. As different combinations of variables predict species richness within species' subsets representing different dispersal mechanisms. 7. There is a moderate nested structure across all studied MLG fragments 8. Few species occur in a high proportion of fragments and those that do tend to be non-characteristic MLG species. Conservation policy should focus on maximising the amount of habitat which is managed and protected, as opposed to saving specific species. Self-supporting metapopulations should be encouraged by viewing individual fragments as part of a wider network. 9. The distributional ranges of Bromus erectus and Thesium humifusum are highly correlated with three bioclimatic variables: MTCO, GDD5 and AET/PET. Sesleria albicans' distribution is also correlated with these bioclimatic variables but other non- climatic may also be important. 10. The distributional ranges of all three species change dramatically under the two future climate scenarios. There is a general pattern that the climate optimum for any species migrates in Northwards and Eastwards. 11. The present MLG community will disintegrate as species respond individualistically to climate change. It is likely that Sesleria albicans will no longer be the dominant grassland species