Alarming global-scale declines of bird numbers are occurring under the changing climate(1), and species belonging to alpine and Arctic tundra are particularly affected(2,3). Increased nest predation appears to be involved(4), but the mechanisms linking predation to climate change remain to be shown. Here we test the prediction from food web theory that increased primary productivity (greening of tundra) in a warming Arctic leads to a higher risk of nest predation in tundra ecosystems. By exploiting landscape-scale spatial heterogeneity in areas of primary productivity across alpine tundra ecotones and supplied with experimental nests in sub-Arctic Scandinavia, we found that predation risk indeed increased with primary productivity. The productivity-predation risk relation was independent of the simultaneous effects of rodent population dynamics and vegetation cover at nest sites. Predation risk also increased steeply with altitude, implying that species at the high-altitude end of the alpine tundra ecotones are particularly vulnerable. Our study contributes to an improved understanding of how climate change may affect Arctic and alpine ecosystems and threaten endemic biodiversity through a trophic cascade.