Herbivores in the tundra interact with vegetation through several mechanisms, especially defoliation, trampling and nutrient addition through urine and faeces. Through these mechanisms, herbivores drive shifts in plant species composition, richness and diversity. As reindeer effects on vegetation accumulate over time, they might cascade to other trophic levels, but how and when this happens is poorly understood. Since it is methodologically demanding to measure biodiversity across spatial gradients, an alternative approach is to assess it indirectly via biodiversity indices of vascular plants. Values from the Index of Biodiversity Relevance were coupled with vegetation data from a network of 96 fenced and paired grazed plots across Fennoscandia. We analysed the role herbivory has on plant richness and diversity, and on the number of organisms that depend on the vegetation according to the index values. We also explored how herbivores affect the competitive effects of shrubs on other plants since the dominance of a vegetation type links directly to biodiversity. Vegetation richness and diversity did not present any differences between treatments, yet reindeer had an increasing effect on plant diversity when testing the interaction between grazing and herbaceous vegetation. Three out of six biodiversity indexes were higher in fenced plots indicating a higher number of interactions between plants and organisms from other trophic levels. Finally, herb abundance was negatively related to shrubs in both treatments but with a faster decline in the absence of herbivores, suggesting that herbivory increases plant diversity and decreases the diversity of other taxa by reducing shrub abundance. This study highlights the importance of maintaining herbivore populations in the Arctic to prevent the expansion of climate-driven biodiversity into the tundra. The effect of herbivores on ecological communities is not merely a product of plant diversity but can be quantitatively and qualitatively different.

Cultivation and naturalization of plants beyond their natural range can bring previously geographically isolated taxa together, thereby increasing the opportunity for hybridization and inter-specific gene flow, the outcomes of which are not predictable. These anthropogenic events therefore allow us to study how hybridization and inter-specific gene flow affect genetic and phenotypic diversity. Here, we explore the phenotypic and genomic effects of increased inter-specific gene flow following the re-introduction of the cultivated Mentha spicata (spearmint) into the ranges of the native mints M. longifolia and M. suaveolens. Using morphological analyses, we show that the cultivated M. spicata has altered trichome characters, which is likely a product of human imposed selection for a more palatable plant or a byproduct of selection on essential oil production. Using whole genome sequencing, we then show that there is extensive genetic admixture between the morphologically defined mint taxa that to some extent is mediated by the cultivated M. spicata. This has, at least partially, resulted in a breakdown of the species barriers. However, despite this breakdown, we find that genetic variants associated with the cultivated trichome morphology continue to segregate in cultivated, naturalized, and wild populations and we identify three genes that may function in the production of the characteristic aromatic oils of mints. Although hybridization can increase species richness by forming new hybrid taxa, we here show that unless reproductive barriers are strong it can also merge species into population/coalescent complexes over evolutionary time.