Poleward shifts in species distributions are expected and frequently observed with a warming climate. In Arctic ecosystems, the strong warming trends are associated with increasing greenness and shrubification. 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). In this study, we used biogeographic and remotely sensed data to quantify spatial variation in vertebrate herbivore communities across the boreal forest and Arctic tundra biomes. We then tested whether present-day herbivore community structure is determined primarily by temperature or vegetation. We demonstrate 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. Temperature was a more important determinant of herbivore community structure across these biomes than 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.

中文翻译：

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北极苔原食草动物群落的北极化会受到气候变暖或植被变化的驱动吗？

随着气候变暖，预计并经常观察到物种分布的两极变化。在北极生态系统中，强烈的变暖趋势与日益增加的绿化和灌木化有关。脊椎动物食草动物有可能限制苔原上的绿化和灌木的生长和扩张，这就提出了一个问题，即食草动物群落的变化是否可以部分调节气候变暖对北极苔原的影响。因此，北极苔原草食动物群落的未来变化将取决于该群落是直接跟踪气候变化（即响应温度而发生）还是间接跟踪气候变化，以响应植被变化（可通过营养相互作用进行修改）。在这项研究中，我们使用生物地理和遥感数据来量化整个北方森林和北极苔原生物群落中脊椎动物食草动物群落的空间变化。然后我们测试了当今的食草动物群落结构是否主要由温度或植被决定。我们证明，就物种丰富度、系统发育多样性和功能多样性而言，北方森林中的脊椎动物食草动物群落的多样性明显高于北极苔原。在生物群落边界观察到群落结构发生明显变化，北极苔原的多样性向北下降幅度更大。有趣的是，表征食草动物在限制苔原植被变化中的作用的重要功能特征，如体重和木本植物摄食，并未显示跨生物群落边界的阈值变化。与植被生产力或木本植物覆盖相比，温度是这些生物群落中食草动物群落结构的一个更重要的决定因素。因此，我们的研究不支持草食动物驱动的北极苔原灌木化或绿化限制将限制苔原中食草动物群落变化的前提。相反，苔原草食动物群落的北方化很可能是气候变暖的直接影响造成的。