Genetic population structure of the vulnerable bog fritillary butterfly

摘要

Habitat destruction, the leading cause of species extinction (Pimm and Raven 2000), is becoming increasingly common as human population growth continues. Habitat loss poses the greatest threat to the long term survival of species on earth and has three major components: straightforward destruction of habitat, increasing fragmentation and deterioration of habitat quality. Habitat fragmentation, i.e. the reduction of continuous habitat into several smaller spatially isolated remnants, decreases species richness, increases edge effects, decreases density and abundance of species, alters interspecific interactions and ecological processes, and decreases connectivity (Debinski and Holt 2000). Following the theory of island biogeography, species richness in habitat fragments is expected to be a function of island size and degree of isolation. Smaller, more isolated fragments are expected to retain fewer species than larger, less isolated patches. Comparable to the effects of area on species richness, one might expect to observe area effects on genetic diversity within species; smaller fragments having a lower genetic diversity compared to larger fragments. Genetic diversity is essential in conservation biology for at least two reasons. First of all, the fundamental theorem of natural selection (Fisher 1930) tells us that “the rate of increase in fitness of any organism at any time is equal to its genetic variance in fitness at that time”, i.e. the rate of evolutionary change in a population is proportional to the amount of genetic diversity available. Consequently, loss of genetic diversity reduces future evolutionary options (Frankel et al. 1995; Lande 1995). Secondly, decreasing genetic diversity (partly measured by heterozygosity, or the state of having one or more pairs of non-identical alleles) increases the extinction risk of populations due to a decline in fitness of individuals (Meffe and Carroll 1994; Oostermeijer et al. 1995; Saccheri et al. 1998; Shikano and Taniguchi 2002). That genetic diversity is inversely proportional to isolation and that it is directly correlated with population size has been proven empirically numerous times. A review by Frankham (1996), including 23 allozyme studies, showed that in 22 of these studies genetic variation within a species was correlated with population size. A study by Schmitt and Seitz (2002) on the butterfly Polyommatus coridon, detected a tendency of genetic erosion in smaller populations and found that gene flow was much higher in regions with high habitat densities than in areas with very fragmented habitats, i.e. populations where isolation was greatest had significantly reduced amounts of gene flow which is though to be a trigger for genetic erosion in isolated populations. Another butterfly study, using microsatellite markers, found that larger populations support significantly higher levels of genetic diversity than do small populations, but did not detect an influence of isolation on genetic diversity (Harper et al. 2003). The authors believe that this was largely due to the closed population structure, i.e. very little gene flow occurring even between closely situated habitats.In this study, we use RAPDs (random amplified polymorphic DNA) to analyse the genetic diversity and population structure of a butterfly species in which dispersal and demography are exceptionally well known (Petit et al. 2001; Schtickzelle et al. 2002; Schtickzelle and Baguette 2003, 2004). In the Belgian Ardennes, the bog fritillary butterfly, Proclossiana eunomia Esper, is present in several clusters of suitable habitat patches. The current distribution of favourable habitats for this vulnerable species is influenced by two factors. First, since soil conditions required for the species’ presence are particular (seasonal alternation of flooding and dry periods), its habitat (wet hay meadows) is naturally patchy along river valleys or in peat bogs on uplands. Secondly, important changes in agricultu