Herbivore decline switches a high Arctic plant community from top‐down to bottom‐up control

Abstract In the nutrient‐limited Arctic, plant communities are shaped by the dynamic interplay between herbivores and climate. The continued warming of the Arctic is, however, also associated with shifts in herbivore abundances and distributional ranges, and there is an urgent need to understand the interplay to predict the combined impacts on Arctic plant communities and potential consequences for ecosystem processes. Using a long‐term (13 years) experimental exclusion of muskoxen ( Ovibos moschatus ) from their key foraging habitat, we examined the responses of the plant community in a wet fen to changes in large herbivore abundance in a high Arctic area exposed to continued warming and concomitant increases in permafrost thaw and vegetation productivity. Excluding muskoxen initially (after 5 years) resulted in marked increases in plant biomass and nitrogen pools. However, muskox abundance declined during the study period, and after 13 years, the treatment effect was no longer evident. Instead, both vegetation biomass and nitrogen pools had increased markedly both inside and outside the exclosures. Particularly strong increases were observed for moss biomass, resulting in a state transition in the plant community from being dominated by graminoids into being dominated by mosses. Neither the species number nor the Shannon diversity of vascular plants changed with treatment nor during the study period. Over time, the vegetation in both Control and Exclosure plots became increasingly complex structurally with more canopy layers. Synthesis : Declining abundance of large herbivores in the Arctic loosens the top‐down control on the vegetation, which now responds primarily to the changing climatic conditions and follows the general greening pattern observed in a warming Arctic. These changes may impact terrestrial ecosystem element cycling, with knock‐on effects on the dynamics in adjacent aquatic and marine ecosystems.