Found in polar regions and on high mountains, ice sheets, glaciers and perennial snowfields make up around 10% of the earth’s surface. They have very low productivity and diversity in extreme cold conditions. Nutrients are in short supply, and generally come from glacial debris, seawater or guano. At the base of simple foodwebs, micro-organisms such as bacteria, viruses and algae are the dominant life forms, although itinerant vertebrates make important contributions to nutrient and carbon subsidies. Micro-organisms are often dispersed by wind, and accumulate organic matter at the surface, fuelling microbial activity both at the surface, and below the ice. Productivity is restricted to summer months, when migratory birds and mammals visit.