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.

Long-term studies of cyclic rodent populations have contributed fundamentally to the development of population ecology. Previous research has shown macroecological patterns of population dynamics in relation to latitude, but without disentangling the role of underlying ecological and climate drivers. We collected 26 rodent time-series from the tundra biome and assessed how population dynamics characteristics of the most prevalent species varied with latitude and environmental variables. While we could not find a relationship between latitude and population cycle peak interval, other characteristics of population dynamics had latitudinal patterns. The environmental predictor variables provided insight into causes of these patterns, as i) increased proportion of optimal habitat in the landscape led to higher population cycle amplitudes in all species and ii) mid-winter climate variability had negative impacts on cycle amplitude in Norwegian lemmings and grey-sided voles. These results indicate that biome-scale climate and landscape change can be expected to have profound impacts on rodent population cycles and that the macro-ecology of such functionally important tundra ecosystem characteristics is likely to be subjected to transient dynamics.

Herbivore grazing is an important determinant of plant community assemblages. We used data from a multi-year experimental study in Northern Fennoscandia to analyze the effect of reindeer (Rangifer tarandus) grazing on plant community diversity including its phylogenetic structure. Similar to previous studies on low productivity ecosystems in this region, we found no effect of grazing on plant biodiversity and a transition from dwarf shrubs to graminoids with pulse (i.e. grazed every other year) and press (i.e. grazed every year) grazing. Interestingly, communities with pulse grazing were more phylogenetically dispersed than communities with press grazing and lightly grazed communities. We argue that the indirect effects of reindeer grazing (e.g. interspecific competition and soil nutrient availability) may work in opposition of the direct effects (i.e. physical removal of species) and that the phylogenetic structure of the resulting community will depend upon the relative strengths of the direct and indirect effects. This interpretation suggests that the indirect effects of reindeer grazing in our study region are more important than the direct effects in driving community assembly for plant communities that are exposed to pulse grazing. Our results have important implications for the management of reindeer in Fennoscandia if pulses of grazing can increase the phylogenetic diversity of plant communities.