Polar-alpine rocky outcrops occur in permanently ice-free areas of polar regions and high mountains. Productivity and biomass are limited by extreme cold, rocky substrate and strong winds. Algae, lichens, mosses and bacteria support a short and simple foodweb in the summer months, with cold-tolerant invertebrates such as tardigrades. Some rocky sites provide nesting sites for birds in summer. Substrate weathering and guano are major nutrient inputs. These systems are periodically disturbed as accumulated snow and ice collapses down steep slopes.