Ecoregions of Texas, AS-199 - Texas Commission on Environmental ...

savannah ecosystem. On the west, the Broken Red Plains meet the smooth topography <strong>of</strong> the Red Prairie(27h), the broken badlands <strong>of</strong> the easternmost portions <strong>of</strong> the Caprock Canyons, Badlands and Breaks (26c),recognized as the Pease River Formation, and the more finely dissected, thinly soiled, remnant erosionalridges <strong>of</strong> the Limestone Plains (27j) composed <strong>of</strong> Cretaceous, Permian, and Pennsylvanian limestones.As in Ecoregion 27h, the prairie type is transitional between tallgrass and shortgrass growth forms. Besideshoney mesquite, lotebush (Zizyphus obtusifolia), wolfberry (Lycium berlandieri), sand sagebrush (Artemisiafilifolia), yucca (Yucca spp.), and pricklypear cacti (Opuntia spp.) may be mixed with the grasses. Riparianvegetation includes cottonwood (Populus spp.), hackberry (Celtis spp.), cedar elm (Ulmus crassifolia), pecan(Carya illinoensis), and little walnut (Juglans microcarpa). In contrast to the predominance <strong>of</strong> cultivation inEcoregion 27h, the Broken Red Plains are used mainly for grazing.Rivers flowing across the Broken Red Plains are meandering and generally turbid except near the outflow<strong>of</strong> regional dams. River levels follow drought and rainy cycles, but flow may be particularly reduced inriver segments below dams, depending on the timing <strong>of</strong> regulated water releases. Particularly during lowflow episodes, rivers and their tributaries may experience problems with high total dissolved solids, lowdissolved oxygen, algal growth, and bacteria due to the proximity <strong>of</strong> oil fields and livestock grazing (RedRiver Authority 2003).The prairie dog town was once the ecological focus <strong>of</strong> the prairie ecoregions (26b, 26c, 27h, 27i, 27j);colonies once covered tens <strong>of</strong> thousands <strong>of</strong> square miles <strong>of</strong> prairie in <strong>Texas</strong> and ranged east as far as thewestern boundary <strong>of</strong> the Western Cross Timbers (29c) (Schmidley 2002). Black-tailed prairie dogs (Cynomysludovicianus) were a plentiful food source for predators, such as the black-footed ferret (Mustela nigripes),rattlesnake (Crotalus spp.), badger (Taxidea taxus), and ferruginous and Swainson’s hawks (Buteo regalis,B. swainsoni). Their extensive system <strong>of</strong> underground burrows housed other animals, such as kit fox (Vulpesvelox macrotis), burrowing owl (Athene cunicularia), and horned lizard (Phrynosoma cornutum). Grasslandbirds, such as the mountain plover (Charadrius montanus), that today are in serious decline, depended on themanicured short grass areas near prairie dog towns for foraging (Doughty 2004). Ranchers saw the prairiedog as a competitor for cattle forage, and poisoning crews used strychnine during the 20 th century to reducethe prairie dog population by 98% (Schmidley 2002). Many associated species were poisoned as well, andother species dependent on the prairie dog for food have disappeared along with the prairie dog. There is somedebate about whether the reduction in competition for forage by the elimination <strong>of</strong> the prairie dog, is balancedtoday by the reduction in forage area caused by the expansion <strong>of</strong> brush which was controlled in part by theprairie dog (Doughty 2004).Riparian vegetation in Ecoregion 27i can include hackberry,cottonwood, elms, and willows. Tree density is sometimes greaterto the east near the Cross Timbers (29), as shown here along theClear Fork <strong>of</strong> the Brazos River near Ft. Griffin.Photo: <strong>Texas</strong> Archeological Research Laboratory, University <strong>of</strong> <strong>Texas</strong>The burrowing owl uses prairie dog burrowsfor nesting and shelter. Burrowing owlnumbers are seriously declining with thedestruction <strong>of</strong> prairie dog towns as well asthe conversion <strong>of</strong> their south <strong>Texas</strong> winteringground to rowcrop agriculture. Photo: TPWD<strong>Ecoregions</strong> <strong>of</strong> <strong>Texas</strong> 35