Influence of habitat fragmentation on the genetic structure of Polyommatus coridon (Lepidoptera: Lycaenidae): implications for conservation
Introduction
The maintenance of at least constant levels of genetic diversity is generally considered essential for the long-term conservation of taxa (Frankel & Soulé, 1981, Simberloff, 1988). Many examples are known where populations with reduced genetic diversity are suspected to lack sufficient capacity to adapt to their environment (O’Brien et al., 1985, O’Brien & Evermann, 1988, Ellstrand & Elam, 1993, Milligan et al., 1994, Frankham, 1995, Lacy, 1997). In the long term, reduced gene flow is thought to be a trigger for genetic erosion in isolated populations (e.g. Packer et al., 1991). The fragmentation of habitats is one factor responsible for the decline of genetic exchange between populations (e.g. Margules et al., 1994, Brookes et al., 1997, Lewis et al., 1997, van Dongen et al., 1998, Clarke & O’Dwyer, 2000).
One habitat type that has suffered severe fragmentation in Europe during the last few decades is semi-natural calcareous grassland (Ebert and Rennwald, 1991). These grasslands are extremely rich in plant and animal species, and no other habitat type in central Europe is inhabited by so many different butterfly species (van Swaay, 2002). Therefore, calcareous grasslands are of high conservation interest. However, the consequences of fragmentation on the genetic composition of typical species of this habitat type are scarcely known. Thus, we studied the population genetics of the chalk-hill blue (Polyommatus coridon), which is one of the six most characteristic butterfly species of European calcareous grasslands (van Swaay, 2002). For the study area we chose the Rhineland-Palatinate and the Saarland in western Germany where the degree of fragmentation varies considerably between areas and extensive limestone areas alternate with non-limestone areas often with acid soils.
These conditions allow us to study the following questions that are relevant for conservation: (1) Do the patterns of primary immigration influence local and regional genetic composition in P. coridon populations? (2) Is the genetic heterogeneity in large P. coridon populations higher than in small populations? (3) Is genetic heterogeneity in large and stable populations of P. coridon maintained without gene flow? (4) Is gene flow higher in a region with a high density of habitats than in one with a lower density? (5) Are the observed densities of calcareous grasslands in the different studied limestone areas sufficient for high gene flow rates between the habitats and the maintenance of high genetic diversity? (6) Are the non-limestone regions between the studied limestone areas barriers that reduce gene flow? (7) What are management units (MU) for P. coridon in our study area: the single populations, the different limestone areas or the whole study area?
Section snippets
Study species
The ecology of P. coridon (Poda, 1761) is well studied in western Germany (e.g. Ebert and Rennwald, 1991). The species is widespread on calcareous grasslands and can reach very high densities in suitable sites. It is very rare outside these typical habitats so little interchange takes place between populations. In western Germany, the caterpillars feed only on Hippocrepis comosa. The young larvae overwinter within the egg and develop during spring and early summer. The flight period of this
Results
Eighteen of the 20 loci analysed were polymorphic and showed banding patterns consistent with known quarternary structures (Richardson et al., 1986). Gpd and Fum were monomorphic. The results for four estimated parameters of genetic diversity are presented in Table 2. Complete allele frequencies are available on request from the authors. Twenty of the 22 samples were in Hardy Weinberg equilibrium. After Bonferroni correction, only the Gersheim sample (Bliesgau) was significant due to strong
Discussion
The majority of German calcareous grasslands are not older than a few hundred years (Ellenberg, 1963, Pott, 1998). Therefore, major spread of P. coridon in the study area has only been possible since the middle ages (Malicki, 1970). According to Ibrahim et al. (1996), stepping stone and leptokurtic (i.e. founding of populations by single long distance dispersers) colonisation of these developing calcareous grasslands may result in stochastic genetic structures that do not necessarily reflect
Acknowledgments
We thank Andreas Gießl (Mainz) for performing the electrophoresis of five populations, our colleagues Marc Meyer (Luxembourg), Dr. Harald Schreiber (Saarbrücken), Dr. Hartmut Steiniger (Trier), Roland Summkeller (Völklingen), Rainer Ulrich (Wiesbach), Andreas Werno (Nunkirchen) and Dr. Norbert Zahm (Schmelz-Hüttersdorf) for very friendly help in finding many of the sampling localities, and the Ministry of Environment of the Saarland for providing the map of calcareous grasslands. Special thanks
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