By William Godwin
Texas A&M Department of EntomologyTEXAS: Wood Co. Hawkins, 12 mi.N.
near county line. 29/IV/2000
32°48’42″N; 95°10’04″W
ENTOBLITZ participants spent one afternoon exploring and collecting in a drastically different habitat on the outcrop zone of the Sparta Sand geological formation. This habitat is characterized by sand jack oaks, yuccas, short leaf pines and little bluestem grass. It is the highest and driest section of the surrounding countryside and stands out in contrast to the bottomlands that we concentrated on otherwise. The Sparta Sand was deposited in a layer as the shore of the Gulf of Mexico transgressed inland during the Eocene, about 50 million years ago. It dips roughly 100 feet per mile toward the gulf. It outcrops in a relatively narrow band with other similar formations that parallel the gulf shore.
Sparta Sand and its sisters, the Carrizo and Queen City sands, have remarkable, subtle and often counter-intuitive effects on biogeography. The Sparta looks dry and most plants have moisture conserving mechanisms like thick roots or hairy tough leaves. In extended drought or with drastic climate drying, many would expect Sparta forest to dry-up and die first while forest on clay or loamy soil persists. Exactly the opposite occurs. Coarse sand in big piles holds its charge of water because a layer of dry sand on the surface acts as an evaporation insulator. Water is loss is through transpiration of plants mostly, while clay soils wick-up moisture on their own to be lost to the wind. Then when some rain does come, the coarse sand replentishes immediately while a dry clay surface looses water to run-off because absorption is slower. Confirmation of these phenomena may be seen in any vegetative map of Texas. Mesic eastern plant formations and individual species ranges commonly stretch westward like tongues following the sandy formations. Early Texas settlers recognized this. They prefered to travel on the open string prairies that ran between sand formations because travel was easier there than in the thickly wooded and soft sand. Just follow the Camino Real to see for yourself.
Eocene sands outcrop across Texas from the Rio Grande to the Louisiana border
Sandy habitats like this in Texas represent the antithesis of bottomland forests or wetlands. Wetlands defined broadly act as corridors in the state with a generally NW/SE orientation. Wetland species have greater powers of dispersal because the flow of water is intrinsically distributive and continuous. You may begin in the bottoms of the upper Sabine and proceed down river in wet habitat to the gulf shore. Then go southwestward hopping bayous, swamps and sloughs to the mouths of the Trinity, San Jacinto, Brazos, Colorado, San Bernard, etcetera, etcetera, etcetera. Take a right turn on any and go upstream as far as possible. You have just dispersed like so many wetland species have always done. Also, when the climate has fluctuated from wet to dry in the past mesic species have probably expanded and contracted with it.
Sand adapted species exist on a narrow archipelago. As the Sparta and other Eocene sands outcrop from the Rio Grande to the Louisiana border they have been chopped into pieces by rivers and their wide floodplains. The sandy segments have many species with low tolerance for other soil types and consequently high incidence of endemism. There are examples of species that are endemic to one or a few sandy segments.
