On a larger scale of ecoregions of the United States, the Big Thicket is part of the Piney Woods. The Big Thicket is located in the southwest of the Piney Woods and transitions into the Western Gulf coastal grasslands immediately to the south. To the west are the Texas Blackland Prairies and East Central Texas forests (a.k.a. post-oak savanna). Within the Big Thicket, a major component in the high diversity of plants and animals is the variety of ecosystems present in a relatively small area. The National Park Service identifies eight plant communities in Big Thicket National Preserve. Some biologist and plant ecologists identify as many as nine, ten, or eleven ecosystems in the Big Thicket. In nature these ecosystems are not always found in pure, large tracts of land, and often occur in intertwined and checkerboard mixes and ecotones, transitioning into one another. Ecosystems do not have universal standardized names. Many ecologist, botanist, zoologist and others have studied and classified ecosystems in the Big Thicket region (and elsewhere), defining and naming these communities as relevant to their own disciplines and studies. Although the exact names, definitions, and boundaries might vary (subdivided, compounded, or overlapping), below are summaries of ten major ecosystems recognized by most who have studied and written about the Big Thicket, starting with higher elevations in the north, and in a generalized way, moving to lower elevations in the south.