The Great Smoky Mountains is home to an amazing diversity of plants, animals, and invertebrates with over 17,000 species documented and possibly an additional 30,000-80,000 species. The diversity is the result of climate and mountain terrain. Elevation levels range from 875 to 6,643 feet. Plants and animals common in the southern United States thrive in the lower elevations while species common in the northern United States thrive at the higher elevations. The amount of rainfall and the high summertime humidity also provide excellant growing conditions.The average annual rainfall varies from about 55 inches in the valleys to over 85 inches in the highest elevation mountain tops.