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 areas. And finally, regular cycles of mowing and cutting, as well as the use of herbicides can both inhibit and benefit various wildflowers and plant species growing there. Most plant species at any given roadside represent the association immediately adjacent to that locality, but often have a few odd of species from other areas. Roadsides are a convenient and accessible places to view wildflowers, regardless of whether they are true ecosystems. In the past some (including the National Park Service) have discussed the river edge as an ecosystem in the Big Thicket however, most ecologist do not recognize river edges as an ecosystem distinct from the one a river runs through.