Lichens account for approximately 8 percent of the vegetation covering Earth’s surface and are extremely important ecologically. In certain environments, such as regions of tundra, they cover vast areas of land. When they cover the ground, they prevent soil from drying out; in desert areas they are able to capture and conserve the moisture present in fog and dew. Lichens also release nutrients, such as nitrogen and phosphorus, which are important for tree growth in many regions. Lichens are also an important food source for many species of animals, including wild turkeys and reindeer of the Arctic tundra. Birds such as the olive-headed weaver of Madagascar and the goldfinch of Europe use lichens to build their nests. It is thought that lichens have played a part in delaying global warming by consuming significant amounts of carbon dioxide (CO2) during photosynthesis, but other factors may negate their effect.