The environment of Andorra was once heavily forested. One explanation for the name of the country is that it came from the Moorish word aldarra, meaning place thick with trees. Andorra’s mountainous environment attracts 12 million tourists each year. In recent decades, however, the forested area has been decreasing steadily. Overgrazing of mountain meadows by sheep, with consequent soil erosion, is another environmental problem. According to a 2006 report issued by the International Union for Conservation of Nature and Natural Resources (IUCN), threatened species included 1 type of mammal: the common otter. The Apollo butterfly and the lesser horseshoe bat are vulnerable species.