Rural?urban gradients and the population genetic structure of woodland ground beetles

Abstract

Genetic diversity and differentiation of two carabid beetle species were examined in woodlands along rural–urban gradients in two cities (Brussels, Belgium, and Birmingham, UK), based on allozymes, studied in more than 1000 beetles. Compared to Abax ater, Pterostichus madidus showed higher levels of genetic diversity but lower genetic differentiation, probably because of its ability to survive in non-forest habitats. Higher genetic diversity in both species was observed in Brussels as compared to Birmingham. However, genetic differentiation among sites was higher in Birmingham corresponding to the more extreme degree of fragmentation and isolation between the Birmingham woodlands. The isolation-by-distance model did not explain genetic differentiation among sites within the two regions. Gene diversity in P. madidus Birmingham populations was higher in smaller urban forests with a small perimeter. A similar absence of genetic erosion in smaller and more highly modified (urban) populations was also obtained for A. ater populations from Brussels, with a higher genetic diversity in sites closer to woodland edges. This unexpected result is hypothesised to be the result of an adaptive increase of genetic diversity in more heterogeneous landscapes, closer to woodland edges and in smaller and more perturbed forests.