The rich get richer and the poor get poorer: commonness and rarity of arctic tundra plants diverge under warming and herbivore exclusion

Abstract While most species are rare, our understanding of how rare species persist remains limited. Consequently, little is also known about how the commonness and rarity of co-occurring species might be differentially impacted by direct and indirect effects of climate change. We report results of a 15-year field experiment investigating effects on commonness and rarity of 14 arctic tundra plant taxa to warming and exclusion of large herbivores, factors demonstrated to have important effects on plant community composition in many biomes. Across all taxa, pooled commonness was reduced by experimental warming, and more strongly under herbivore exclusion than under herbivory. However, taxon-specific analyses revealed that although warming elicited variable effects on commonness, herbivore exclusion disproportionately reduced the commonness of rare taxa. Over the course of the experiment, we also observed trends in commonness and rarity under all treatments through time. Sitewide commonness increased for two common taxa, the deciduous shrubs Betula nana and Salix glauca, and declined in six other taxa, all of which were rare. Across experimental treatments, rates of increase and decline in commonness (i.e., temporal trends over the duration of the experiment) were strongly related to baseline commonness of taxa early in the experiment. Hence, commonness itself may be a strong predictor of plant species responses to climate change in the arctic tundra biome, but large herbivores may mediate such responses in rare taxa, perhaps facilitating their persistence.