Arctic greening from warming promotes declines in caribou populations

The migratory tundra caribou herds in North America follow decadal population cycles, and browsing from abundantcaribou could be expected to counteract the current climate-driven expansion of shrubs in the circumpolar tundrabiome. We demonstrate that the sea ice cover in the Arctic Ocean has provided a strong signal for climate-inducedchanges on the adjacent caribou summer ranges, outperforming other climate indices in explaining the caribou-plantdynamics. We found no evidence of a negative effect of caribou abundance on vegetation biomass. On the contrary,we found a strong bottom-up effect in which a warmer climate related to diminishing sea ice has increased the plantbiomass on the summer pastures, along with a paradoxical decline in caribou populations. This result suggests thatthis climate-induced greening has been accompanied by a deterioration of pasture quality. The shrub expansion inArctic North America involves plant species with strong antibrowsing defenses.Our results might therefore be an earlysignal of a climate-driven shift in the caribou-plant interaction from a system with low plant biomass modulated bycyclic caribou populations to a system dominated by nonedible shrubs and diminishing herds of migratory caribou