A refugium is a locality that has escaped drastic alteration following climatic change, in contrast to the region as a whole. A refugium usually forms a center for relic species. Refugia exist widely over the Earth's surface because although, owing to glacial advance and recession, changes of climate affecting the survival of plants and animals have occurred, no general climatic change affects a large area uniformly, with the result that relic colonies of species or communities, are left behind.