Terrestrial Mammals of the Riparian Corridor in Big Bend National Park 1 William J. Boeer and David J. Schmidly2 Abstract.--Thirty species of terrestrial mammals inhabit riparian habitats in Big Bend National Park (BBNP), but only one species (the beaver, Castor canadensis) is restricted to these areas. Major changes in the vegetation during the past 30 years, involving an increase in basal and canopy cover, have resulted in the elimination of at least one species (Dipodomys ordii) from the river corridor as well as increased abundance and distribution for two other species (Sigmodon hispidus and Peromyscus leucopus). Compared to the other major plant communities in BBNP, the rodent fauna of the riparian community has lower evenness, richness, and diversity indices (based on the Shannon-Weaver Index). Humqn use and trespass livestock grazing are the major impacts acting upon the natural riparian communities in BBNP today. INTRODUCTION DESCRIPTION OF THE RIPARIAN CORRIDOR Mammalian studies of the Big Bend area began Floodplain or riparian vegetation exists with general surveys (Bailey 1905; Johnson 1936; wherever periodic flooding occurs along the Borell and Bryant 1942; and Taylor et al. 1944) Rio Grande in BBNP. These riparian communities designed to identify and document the varied vary from areas a few meters (m) wide to areas fauna of the area. After the park was esta­ extending inland a distance of one kilometer blished, the perspective of mammalian research (km); furthermore, adjacent arroyos and creeks changed somewhat and in recent years studies may carry enough surface or ground water to have concentrated on mammalian autecology and produce a similar floodplain environment. Top­ synecology (Porter 1962; Dixon 1958; and Easter­ ography along the river includes (1) sheer wall la 1973). Most mammalian studies have focused canyons (i.e., Santa Elena and Mariscal canyons on the mammals of the montane woodland and which rise to elevations of 366 m) with few desert grassland habitats. There have been areas of alluvial deposits; (2) long deep canyons no comprehensive studies of riparian mammals. (i.e., Boquillas Canyon) where the walls do not Baccus (1971) investigated the distribution rise abruptly and where larger areas of alluvial of rodents in the park with respect to the major deposits occur; and (3) areas of broad flat physical features, focusing on the effects of floodplain with extensive alluvial deposits. the elimination of grazing on the vegetation and the rodent populations. He also described Denyes (1956) recognized three plant associ­ the similarities and dissimilarities of the ations along the Rio Grande floodplain: (1) the rodent faunas of the woodland, grassland. and riverbank association, consisting of mesquite desert shrub communities; however, he divided (Prosopis juliflora) , seep willow (Baccharis the fauna of the Rio Grande floodplain between sp.), willow (Salix gooddingi) , or cottonwood the desert shrub and grassland communities and (Populus palmeri), located adjacent to areas of did not consider the riparian corridor as a exposed silt and coarse gravel at the water's unique habitat. edge; (2) the Baccharis association, composed of dense stands of seep willow; and (3) the mesquite association, consisting of a thin line 1 Contributed paper. Symposium on the of extensive mesquite trees or an extensive area Importance, Preservation and Management of the of several different plant forms. We have found Riparian Habitat, July 9, 1977, Tucson. Arizona. these three associations to be somewhat altered 2 . from Denyes'description and, although difficult Texas Agricultural Experiment Station, Department of Wildlife and Fisheries Sciences, to document, our general impression is that significant vegetation changes have occurred Texas A&M University, College Station, Texas. 212 \~ .­ l, I in the riparian habitats of BBNP over the past Southern Plains l\'oodrat Neotoma 30 years. The major change is associated with miaropus - C the tremendous increase of salt cedar (Tamarix New World Porcupines - Family ahinensis) along the river. The Baaaharis asso­ Erethizontidae ciation, mentioned by Denyes (1956) as common in Porcupine Erethizon dorsat:wi1 "" R the fine sandy loam soils along the river, is Carnivores - Order Carnivora recognizable today only at a few places (for Dogs and Relatives - Family Canidae example, Black Dike) and appears to have been Coyote Canis Zatrans - C replaced by a mixed mesquite-salt cedar-bermuda Gray Fox Uroayon cinereoargenteus - U grass (cynodon daatyZon) association. Similarly, Raccoons - Family Procyonidae salt cedar also appears to be replacing native Ringtail Bassariscus astutus - U cottonwood and willow trees at many places. Raccoon Proayon Zotor - C Weasels and Relatives - Family Mustelidae Striped Skunk Mephitis mephitis - U MAMMALIAN FAUNA OF THE RIPARIAN CORRIDOR Western Spotted Skunk SpiZogaZe graailis - R Thirty species of terrestrial mammals have Hog-nosed Skunk Conepatus mesoZeuaus - R been either collected or observed in the rip­ Cats - Family Felidae arian habitats of BBNP. These are listed below Mountain Lion FeZis concoZor - R in checklist fashion with their current status Bobcat FeZis rufUS - U (C = common; U = uncommon; R = rare: E = prev­ Even-toed Ungulates - Order Artiodactyla iously present, but no longer occurs; P = pos­ Peccaries - Family Tayassuidae sibly occurs) in the park. Collared Peccary (Javelina) DiaotyZes Pouched Mammals - Order Marsupialia tajaau - U Opossum - Family Didelphidae Deer - Family Cervidae Virginia Opossum DideZphis Mule Deer OdocoiZeus hemionus - U virginiana - P Lagomorphs - Order Lagomorpha During 1975-1976, we sampled small rodents Hares and Rabbits - Family Leporidae at 18 different sites along the riparian corri­ Desert Cottontail SyZviZagus dor. Each site was trapped (using Sherman live auduboni i - C. traps) a total of 720 trap nights resulting in Black-tailed Jack Rabbit Lepus 12,960 trap nights for the entire river corridor. caZifoPniaus - U A total of 1,292 rodents representing two fami­ !s Rodents - Order Rodentia lies (Heteromyidae and Cricetidae) were captured Squirrels - Family Sciuridae as follows (number trapped in parentheses): Texas Antelope Squirrel AmmospermophiZus Family Heteromyidae: Perognathus peniaiZZatus interpres - R (896); Perognathus neZsoni (2); Perognathus Mexican Ground Squirrel SpermophiZus fZavus (5); Dipodomys merriami (65). Family )- mexiaanus - R Cricetidae: Peromysaus Zeucopus (162); Peromyscus Ll Spotted Ground Squirrel SpermophiZus eremiaus (19); Sigmodon hispidus (70); Neotoma 1S spiZosoma - U miaropus (73). Perognathus peniaiZZatus was Pocket Gophers - Family Geomyidae overwhelmingly the most abundant small rodent Tons Yellow-faced Pocket Gopher in the riparian habitats and, for this reason, )t Pappogeomys castanops - C the total density of heteromyid rodents was i.al Pocket Mice - Family Heteromyidae greater than that of cricetid rodents. The Silky Pocket Mouse Perognathus three other heteromyid rodents were relatively fZavus - U rare along the river, although D. merriami was Desert Pocket Mouse Perognathus common at a few sites. Densities of the four Jci­ peniciZZatus - C species of cricetid rodents were more similar the Nelson's Pocket Mouse Perognathus to one another than the densities of the heter­ neZsoni - R omyid species. Peromyscus Zeucopus was the Ord's Kangaroo Rat Dipodomys most common cricetid and P. eremicus the least ordii - E common; Sigmodon hispidus and Neotoma micropus Merriam's Kangaroo Rat Dipodomys occurred in about equal numbers. merriami - C Beaver - Family Castoridae Borell and Bryant (1942) also found Beaver Castor aanadensis - U Perognathus peniaiZZatus to be the most abundant 1e New World Rats and Mice - Family rodent in the riparian corridor. However, com­ rea Cricetidae paring our data with that of Borell and Bryant und Cactus Mouse Peromyscus eremiaus - U (1942) for three other species (Dipodomys :d White-footed Mouse Peromyscus ordii, Peromyscus Zeuaopus, and Sigmodon ult Zeuaopus - C hispidus) reveals that significant changes in Deer Mouse Peromysaus manicuZatus - R abundance and distribution have occurred in Hispid Cotton Rat SigmodOn hispidus - C these species over the past 30 years. These 213 differences correlate with major vegetative attenuatus, we spent eight days (from 4 April changes associated with the cessation of exten­ 1976 to 12 April 1976) trapping at Upper sive livestock grazing. Early accounts (Taylor Tornillo Creek Bridge and other places where et aZ. 1944; Sperry 1938) describe the vegeta­ this species had been previously collected. tion along the river as open and severely Initially, 70 traps were set on both the east over-grazed. However, since ranching activities and west side of Upper Tornillo Creek. Later, ceased at the inception of the park, plant den­ the number of traps was increased to 110 on sities seem to have increased greatly so that the west side and 160 on the east side. The at several places (e.g., Johnson Ranch) mes­ traps were set out in various soil and vege­ quite forests now occur where the river bottom tation types ranging from deep sand·-sparse was once open and sparsely vegetated. Exten­ burro-brush (HymenocZea monogyra) , to packed sive fields of grass also occur today at sites sand-mesquite and gravelly-creosote (Larrea (e.g., Smoky Creek and Coyote) which formerly divaricata) flats. A total of 18 D. ordii were cultivated and farmed. and 21 D. merriami were caught during the first two nights of trapping. Thirty traps Generally,