Arctic ecosystems are changing fast, and these changes involve both plant and herbivore communities. Poleward shifts of species distributions, termed borealization when referring to northward movements of boreal species into tundra, have been observed and are expected to continue. Increasing greenness, shrubification and treeline advance in Arctic ecosystems have been associated with warming trends. Vertebrate herbivores have the potential to limit greening and shrub advance and expansion on the tundra, posing the question of whether changes in herbivore communities could partly mediate the impacts of climate warming on Arctic tundra. Therefore, future changes in the herbivore community in the Arctic tundra will depend on whether the community tracks the changing climates directly (i.e. occurs in response to temperature) or indirectly, in response to vegetation changes (which can be modified by trophic interactions).
We used biogeographic and remotely sensed data to quantify spatial variation in vertebrate herbivore communities across the boreal forest and Arctic tundra biomes and assess whether borealization of vertebrate herbivore communities is a direct response to warming temperatures, or an indirect response through changing vegetation. We then tested whether present-day herbivore community structure is determined primarily by temperature or vegetation.
We found that vertebrate herbivore communities are significantly more diverse in the boreal forest than in the Arctic tundra in terms of species richness, phylogenetic diversity and functional diversity. A clear shift in community structure was observed at the biome boundary, with stronger northward declines in diversity in the Arctic tundra. Interestingly, important functional traits characterizing the role of herbivores in limiting tundra vegetation change, such as body mass and woody plant feeding, did not show threshold changes across the biome boundary. The composition of herbivore communities was mainly driven by temperature rather than by vegetation productivity or woody plant cover. Thus, our study does not support the premise that herbivore-driven limitation of Arctic tundra shrubification or greening would limit herbivore community change in the tundra. Instead, borealization of tundra herbivore communities is likely to result from the direct effect of climate warming.
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Reference: Speed, J.D., Chimal‐Ballesteros, J.A., Martin, M.D., Barrio, I.C., Vuorinen, K.E. and Soininen, E.M., 2021. Will borealization of Arctic tundra herbivore communities be driven by climate warming or vegetation change?. Global Change Biology, 27(24):6568-6577.
Photo: reindeer grazing (photo by Hannes Skarin)