[Extinction of large herbivore mammals: niche characteristics of musk ox Ovibos moschatus and reindeer Rangifer tarandus coexisting in isolation].

The extinction of large northern herbivores is a puzzle for many biologists. It is long debated whether climate change or human activity was the main factor of the extinction. The survival of the weak trophic competitors should reject the climatic hypothesis. Extant species of Pleistocene communities allow testing this explicitly. Up to date, reindeer and musk ox coexist in the Arctic territory. Their island populations provide a unique natural experiment to assess the role of competition. On Wrangel Island, their population sizes show the opposite trends and the same situation recurs on other Arctic islands--the reindeer population size decreases with the muskoxen population increasing. We have shown that the trends are defined by food-web structure. Niche overlap between species is found to .be considerable and cannot be facilitated by habitat partitioning. The number of plant species in the muskoxen diet was higher than in the reindeer. The exclusive part of the muskoxen diet was higher as well. Food webs in all of the habitat types showed the same relation. However, the changes in herbivores distribution during the Pleistocene demonstrate the opposite pattern. Therefore, the competitive advantage could not save the Palaearctic musk ox, and the extinction seems to be a result of selective overkill. Conclusively, the human activity may be considered as the main factor of the Late Pleistocene herbivore extinctions, and the musk ox reintroducing should be coupled with extensive conservational measures.

[Microsatellite DNA variability in the populations of muskoxen Ovibos moschatus acclimated to the Russian North].

The muskoxen populations introduced to the Taimyr Peninsula and Wrangel Island in 1974 to 1975 were examined for sequence variation at seven microsatellite loci. Donor material originated from the populations of Banks Island (Canada) and Eastern Greenland. Relative to the allele frequencies, both introduced populations demonstrated rather strong deviation from the populations of the native range. At the same time, population allelic structures evidenced that they were closer to the Greenland populations. Estimates of genetic diversity at microsatellite loci (expected heterozygosity and the allele number) in the introduced muskoxen were found to be high for populations originating from a small number of founder individuals. In the immigrants, linkage disequilibrium and deviation of the genotype frequencies from the Hardy-Weinberg proportions were observed, which was mainly caused by the deficit of heterozygotes. The same pattern was also typical of native populations and was explained in terms of specific population structure and demographic processes. The latter were manifested as a periodic decline of the effective population size, resulting in the prevailing influence of genetic drift and inbreeding. The consequences of genetic drift were not as dramatic, as could be expected, which may be explained by a high mutation rate of neutral microsatellite loci and fast growth of the new populations.