Introduced reindeer, Rangifer tarandus, overexploited lichen-rich plant communities on St. Matthew Island in the Bering Sea. A die-off of the reindeer followed, exacerbated by extreme weather in 1964, resulting in extirpation of the reindeer. A similar pattern of removal of lichens as major components of plant communities has occurred following introductions of reindeer to other islands at high latitudes. By 1985, two decades following die-off of the reindeer, total lichen biomass was only 6% of that in similar plant communities on adjacent Hall Island, not reached by the reindeer. By 2005, 41 y after the reindeer die-off, lichen regrowth on St. Matthew was only 12% of lichen biomass in the Hall Island communities. A warmer, drier climate and decreased fog in recent decades contributed to deterioration of conditions favoring lichen growth on St. Matthew Island.