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. The Apollo butterfly, the European otter, and the lesser horseshoe bat are vulnerable species.