We have studied a small isolated population of black grouse (Tetrao tetrix) in the Netherlands to examine the impact of isolation and reduction in numbers on genetic diversity. We compared the genetic diversity in the last extant Dutch population with Dutch museum samples and three other black grouse populations (from England, Austria and Norway, respectively) representing isolated and continuous populations. We found significantly lower allelic richness, observed and expected heterozygosities in the present Dutch population compared to the continuous populations (Austria and Norway) and also to the historical Dutch population. However, using a bottleneck test on each population, signs of heterozygosity excess were only found in the likewise isolated English population despite that strong genetic drift was evident in the present Dutch population in comparison to the reference populations, as assessed both in pairwise F(ST)and STRUCTURE analyses. Simulating the effect of a population reduction on the Dutch population from 1948 onwards, using census data and with the Dutch museum samples as a model for the genetic diversity in the initial population, revealed that the loss in number of alleles and observed heterozygosity was according to genetic drift expectations and within the standard error range of the present Dutch population. Thus, the effect of the strong decline in the number of grouse on genetic diversity was only detectable when using a reference from the past. The lack of evidence for a population reduction in the present Dutch population by using the program bottleneck was attributed to a rapidly found new equilibrium as a consequence of a very small effective population size.

We studied microsatellite genetic variation in 14 different geographic populations of black grouse (Tetrao tetrix) across the European range. Populations were grouped in three different fragmentation categories: isolated, contiguous and continuous, respectively. Genetic diversity, measured as observed heterozygosity (H O ), expected heterozygosity (H E ) and allelic richness, were lower in isolated populations as compared to the other two categories that did not differ amongst one another. These results imply that lowered genetic variability in black grouse populations is negatively affected by population isolation. Our results suggest that the connectivity of small and isolated populations in Western Europe should be improved or else these face an increased risk of extinction due to genetic and demographic stochasticity.

Black grouse in Britain have faced contraction of their range and have declined in numbers during the recent decades. As such, the species is a conservation concern in the UK. In order to aid conservation decisions, the terms Evolutionary Significant Unit (ESU) and Management Unit (MU) have been proposed. An ESU is an independently evolving evolutionary lineage defined by being reciprocally monophyletic for mitochondrial alleles, and which is significantly different from other lineages with regard to nuclear alleles, whereas an MU is operationally defined by only the latter criterion. Using mitochondrial sequences and nuclear microsatellite loci, we failed to find evidence that British black grouse is an ESU. However, British black grouse are sufficiently different from continental black grouse both with respect to mitochondrial and nuclear data to regard them as a separate MU. Furthermore, we present genetic data which suggest that British black grouse presently occur in what are probably three demographically independent units (roughly corresponding to Wales, northern England/southern Scotland and northern Scotland), which are genetically differentiated. The two southern units (Wales and northern England/southern Scotland) have lower genetic diversity and show signs of having lost genetic variability.

Genetic variation in a Chinese grouse Bonasa sewerzowi population was assessed using five microsatellite markers. The mean number of alleles and allelic richness were comparable with what has been observed in other grouse, e.g. black grouse Tetrao tetrix and capercaillie T. urogallus, populations studied with the same markers regardless of whether these populations were isolated or from within the continuous range of the respective species. However, the proportion of heterozygotes and FIS observed in Chinese grouse were more similar to isolated than to continuous grouse populations suggesting that the Chinese grouse population may show genetic signs of habitat fragmentation and relative isolation. Furthermore, demographic analyses indicated that the studied Chinese grouse population would not persist without immigration. We suggest that excess heterozygosity is a sign of a sink deme within a metapopulation system.