Ireland is an island in the North Atlantic separated from Great Britain to its east by the North Channel, the Irish Sea, and St George’s Channel. It is the second-largest island of the British Isles, trailing only Great Britain, the third-largest in Europe, and the twentieth-largest on Earth. Politically, Ireland is divided between the Republic of Ireland (officially also named Ireland), which covers five-sixths of the island, and Northern Ireland, a part of the United Kingdom, which covers the remaining area and is located in the northeast of the island. In 2011 the population of Ireland was about 6.4 million, ranking it the second-most populous island in Europe. Just under 4.6 million live in the Republic of Ireland and just over 1.8 million live in Northern Ireland. The island’s geography comprises relatively low-lying mountains surrounding a central plain, with several navigable rivers extending inland. The island has lush vegetation, a product of its mild but changeable oceanic climate, which avoids extremes in temperature. Thick woodlands covered the island until the Middle Ages. As of 2013, the amount of land that is wooded in Ireland is about 11% of the total, compared with a European average of 35%. There are 26 extant mammal species native to Ireland.