The Big Thicket is the name given to a somewhat imprecise region of a heavily-forested area of Southeast Texas in the United States. It is a highly biodiverse area for a temperate region, which has been described as "America's Ark" and the "Biological Crossroads of North America". The National Park Service established the Big Thicket National Preserve (BTNP) within the region in 1974 and it is recognized as a biosphere reserve by UNESCO. Although the diversity of animals in the area is high, with over 500 vertebrates, it is the complex mosaic of ecosystems and plant diversity that is particularly remarkable. Biologists have identified at least eight, and up to eleven, ecosystems in the Big Thicket area. More than 160 species of trees and shrubs, 800 herbs and vines, and 340 types of grasses are known to occur in the Big Thicket, and estimates as high as over 1000 flowering plant species and 200 trees and shrubs have been made, plus ferns, carnivorous plants, and more. The Big Thicket has historically been the most dense forest region in what is now Texas.