1. Throughout the most recent glacial period (Weichsel), the mountain hare Lepus timidus had a continuous distribution in the tundra habitat south of the ice‐rim. When the ice retreated, mountain hares colonized deglaciated land, and spread over northern Europe. 2. Since the Weichsel, the mountain hare's distribution in Europe has been gradually reduced and at present comprises Ireland and the Scottish Highlands, high altitudes in the Alps, isolated forests in eastern Poland, most of Fennoscandia and from the Baltic countries eastwards through Russia. Declines during the last century have been observed in Sweden and Russia. 3. This review defines and evaluates causes for this gradual reduction and fragmentation of the mountain hare's distribution, with special focus on interactions with brown hares Lepus europaeus. The relative importance of diseases, predation, cultivation and interactions with other herbivores than brown hares are discussed. 4. A plausible cause of the possible permanent disappearance of mountain hares in Europe appears to be exclusion by interspecific competition and hybridization with, and/or epidemic diseases mediated by, the congeneric brown hare.

If interspecific hybrids are fertile and backcross to either parental species, transmission of mitochondrial DNA over the species barrier can occur. To investigate if such transmission has occurred between the brown hare Lepus europeus Pall and the mountain hare L. timidus L. in Scandinavia, an analysis of genetic variation in mitochondrial DNA from 36 hares, collected from 15 localities, was performed. Sequence divergence of mtDNA between species was estimated at 8 +/- 1% (SD). Intraspecific mtDNA sequence divergence varied between 0.09 and 0.38% in brown hares and 0.10 and 1.44% in mountain hares. In six out of 18 brown hares examined, two different haplotypes of mountain hare origin were detected, demonstrating a transmission of mtDNA haplotypes from mountain hares to brown hares. The results indicate that interspecific hybridization between the two species occurs in wild populations.

Through interspeci®c hybridization and subsequent backcrossing, genes and genomes may be transferred over the species barrier. In Sweden, the introduced brown hare Lepus europaeus hybridizes with the native mountain hare L. timidus. To investigate the direction and the extent of transfer of mitochondrial DNA (mtDNA) between the species, the mtDNA haplotypes were screened in 522 brown hares and 149 mountain hares from areas of sympatry and allopatry. A total of 51 brown hares with mountain hare mtDNA, but no mountain hares with brown hare mtDNA were detected. Thus, mtDNA transfer over the species barrier is directed from mountain hares to brown hares. We argue that frequency-dependent hybridization and/or interspeci®c male competition mediates this directionality. Further, the percentage of brown hares with transmitted mountain hare mtDNA was lower in areas of former species sympatry (0.6%) compared to areas of current sympatry (15%). Thus, the transferred mtDNA may disappear from brown hare populations if there is no continuous input through hybridization. We suggest that specimens with an alien mtDNA experience a ®tness reduction as a result of a functional incompatibility between the cytoplasmic mitochondrial genomes and the cell nucleus.

Semi‐natural grasslands are rich in biodiversity and thus important habitats for conservation, yet they are experiencing rapid declines due to agricultural intensification and abandonment. Promoting a more diverse mammalian herbivore community, including large and megaherbivores, may result in positive cascade effects for biodiversity and ecosystem functioning. Therefore, reintroducing an ecologically functional substitute of an extinct large herbivore could mitigate current biodiversity declines and foster semi‐natural grassland conservation. To test this hypothesis, we set up a 3‐year rewilding experiment where 12 feral horses were introduced in three 10‐hectare enclosure replicates (four horses per enclosure). We used community‐weighted mean plant functional traits to elucidate plant community changes induced by grazing through time. We also investigated the effects of this experimental treatment on insect pollinated plants and on pollinator habitat use. The grassland community exerted a mixed tolerance/avoidance response to grazing. This resulted in plant functional compositional changes which favoured prostrate plant species with higher specific leaf area, characteristic of ruderal communities. Plant species richness was higher in grazed compared to ungrazed areas. Butterfly and bumblebee habitat use, as well as feeding and resting activities were also higher in grazed areas. Moreover, the number of pollinators increased with plant species richness. Synthesis and applications. This study demonstrates that, to enhance the diversity of a given herbivore community with ecological replacements of extinct wild horses can have significant effects on the functional composition of grasslands. It can also mitigate plant species declines, in particular bee‐dependent plants, and boost pollinator habitat use. Novel management alternatives are urgently needed to reverse the negative effect of land abandonment in European agricultural landscapes. Thus, rewilding interventions with large mammalian herbivores may offset current biodiversity declines by maintaining important functional links between plants and pollinators in grassland ecosystems.

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