old are rare in the area now. Mixed-grass prairies: Prairies (small patches are sometimes called coves), are sometimes included as a Big Thicket ecosystem. These are isolated fragments of coastal prairie more typical of Chambers and Jefferson counties to the south, in areas mostly found where the forest and coastal prairies transition. Prairies are dominated by grasses and herbaceous plants, some are the same as those found longleaf pine savannas, but others are distinctive like Indian grass (Sorghastrum nutans) and prairie bluebell (Eustoma grandiflorum). Common grasses of these areas include Indian grass (Sorghastrum avenaceum), eastern gama-grass (Tripsacum dactyloides), and tall dropseed (Sporobolus asper). One author distinguished prairies from longleaf pine savannas by the presence of pimple mounds or mima mounds. Prairies and coves were once found in the Big Thicket with some consistency, particularly in higher elevations in the south, and specifically in higher southwestern areas between the Pine Island Bayou and the Trinity River drainages. However, natural wildfires are an important factor in maintaining these areas and when fire is suppressed, trees and shrubs quickly grow in and fill in these areas. The Chinese tallow tree (Triadica sebifera) is a particularly aggressive and problematic invasive species in prairie habitat in the region. Prairies are also often the first areas to be developed for farming and residential housing. Unfortunately, prairies in the Big Thicket are now rare. Roadsides and river edge: Although not a natural or even stable plant assemblage, some have recognized roadsides as an ecosystem in the Big Thicket. Proponents of the concept have identified a number of distinguishing characteristics of roadsides from the other ecosystems. Sunlight and heat can reach the ground along the clear highway easements, unlike most areas where the light is filtered by the forest canopy before it reaches the ground. Roadsides receive more rainwater, diverted from the pavement and often retained for a time in roadside ditches. Soil types not normally found at the surface are often exposed with the construction of roads, particularly deep road cuts in the low hills in the north that expose calcareous soils or where the formations are thin near transition zones supporting the growth of plants not normally occurring in the surrounding habitat, allowing for more diverse plants species to grow. The "corridor effect" of traffic, both human (vehicles, tourists, and road crews) and birds and other animals, inadvertently dispersing seeds and roots along roadsides from other