Donald C. Ruthven

EFFECTS OF INVASIVE EXOTIC GRASSES ON SOUTH TEXAS RANGELAND BREEDING BIRDS

Abstract Invasive exotic plants are a major threat to many species of wild birds. When these plants become established and widespread, the floristic composition of native plant communities becomes simplified, which can result in long-term and often irreversible habitat degradation for birds and other animals. Until recently, few studies have focused on the effect of invasive exotic grasses on breeding birds in southwestern rangelands. During the 2001 and 2002 breeding seasons (May-June), we compared the abundance and species richness of breeding birds, native flora, and arthropods on South Texas rangeland plots dominated by native grasses and plots dominated by two invasive exotic grasses, Lehmann lovegrass (Eragrostis lehmanniana) and buffelgrass (Cenchrus ciliaris). Native-grass cover was >400% greater on native-grass sites than on exotic-grass sites. Forb and grass species-richness were higher on native-grass sites. Shrub canopy cover, bare ground, and vegetation height measurements were similar on native-grass and exotic-grass sites. Overall bird abundance was 32% greater on native-grass sites than on exotic-grass sites. Lark Sparrows (Chondestes grammacus) were 73% more abundant on native-grass sites. Four other species—Black-throated Sparrow (Amphispiza bilineata), Northern Mockingbird (Mimus polyglottos), Northern Bobwhite (Colinus virginianus), and Cassin’s Sparrow (Aimophilla cassini)— were 26–70% more abundant on native-grass sites. The guild of birds that foraged on the ground under open brush canopies was almost twice as abundant on native-grass sites. Arthropod abundance was 60% greater on the native-grass site we sampled. Specifically, spiders, beetles, and ants were 42–83% more abundant on a native-grass site than on a buffelgrass site. Compared with rangelands dominated by native vegetation, areas dominated by Lehmann lovegrass and buffelgrass in South Texas appear to provide less suitable habitat for breeding birds, especially for bird species that forage on or near the ground. Efectos de Pastos Invasores Exóticos en las Aves que Nidifican en los Campos de Pastoreo del Sur de Texas

Microhabitat selection by Texas horned lizards in southern Texas

The Texas horned lizard (Phrynosoma cornutum) has declined throughout its range. Understanding habitat selection by the Texas horned lizard is an important factor in its conservation. We examined daily and seasonal habitat requirements of Texas horned lizards and determined whether habitat selection differed among land management treatments in southern Texas. We used 5 study sites, each with a different burning and grazing treatment. Adult lizards caught in the study sites were fitted with backpacks carrying radiotransmitters and relocated daily. Habitat characteristics at radio locations and random points 10 m from the lizard were assessed using 50- × 20-cm quadrats. Relocations were made during 3 time intervals (morning, afternoon, evening) and 2 seasons (active, inactive). Horned lizards used bare ground and herbaceous vegetation similar to their availability in the morning and evening for thermoregulation and foraging purposes, but avoided bare ground in the afternoon. In the afternoons, lizards selected woody vegetation and litter as thermal refuges and cover from predators. Lizards also appeared less dependent on herbaceous vegetation and more dependent on woody vegetation and litter in the inactive season compared to the active season as a result of increased temperatures. We did not detect differences in habitat selection among land management treatments. Habitat management for Texas horned lizards should focus on creating a mosaic of bare ground, herbaceous vegetation, and woody vegetation in close proximity.

VARIATION IN TORTOISE LIFE HISTORY: DEMOGRAPHY OF GOPHERUS BERLANDIERI

Turtles provide excellent models for studies of life history strategies, but terrestrial species are underrepresented in these analyses. We present a life table and demographic parameters of an inland population of Gopherus berlandieri to contribute to the study of evolution of turtle life histories. Data were gathered during a mark–recapture and radiotelemetry study in the Tamaulipan Biotic Province in southern Texas, USA. A total of 835 individuals were captured, measured, and their ages estimated. Females matured at 131 mm carapace length at an average of 5 yr of age (range: 4–8 yr). Clutch size, as determined by ultrasound, averaged 2.07 ± 0.15 eggs (n = 49). No nests were found, and we estimated clutch frequency with a quadratic model to be 1.34 clutches·female−1·year−1. Survival was estimated from age frequency regression and telemetry. Annual male survival (0.828, 0.834) differed (P < 0.05) from female survival (0.728, 0.774) for both techniques of estimation. Age-specific female survival ranged fr...

Effects of dietary energy and protein restriction on nutritional indices of female white-tailed deer

Several physical and physiological indices have been used to assess white-tailed deer (Odocoileus virginianus) nutritional condition and habitat quality. We tested whether indices reflected inferred nutritional circumstances under controlled conditions of dietary protein and energy at 2 time scales. Our study used adult female deer that were captured in southern Texas and placed in captivity. We randomly assigned deer (n = 51) in the long-term study to 4 diet groups representing a factorial combination of 2 levels each of crude protein (CP) and energy for 6 months. We assigned deer (n = 31) in the short-term study to 6 diets (a factorial combination of 3 levels of CP and 2 levels of energy) for 60 days. At the end of the experimental periods, we sacrificed deer and assessed their nutritional status from serum (n = 11), ruminal (n = 3), fecal (n = 4), fat (n = 7), and carcass (n = 7) indices. Protein-energy interaction effects (P < 0.05) were observed for serum (calcium [Ca], phosphorus [P]), ruminal (neutral detergent fiber [NDF]), and fecal (NDF, dry matter) indices in both experiments. Six-month trials also had protein-energy interactions (P < 0.05) for serum (urea nitrogen, creatinine), ruminal digestible energy (DE), and fecal nitrogen. Sixty-day trials had protein-energy interactions (P < 0.05) for serum alkaline phosphatase, kidney mass (KM), relative KM, back fat depth, and eviscerated body mass (EBM). Nutritional indices in all categories (e.g., fecal, ruminal, carcass) varied (P < 0.05) by dietary protein status, dietary energy status, or protein-energy interactions. Multiple indices are recommended to assess deer herd condition and habitat quality. However, because multiple indices may not be available because of limited availability and resources, we provided variables in each index category that discriminate among nutritional levels.

Impacts of buffelgrass (Pennisetum ciliare) on a forb community in south Texas.

Abstract Since the 1950s, many south Texas rangelands have been seeded with buffelgrass, a perennial C4 bunchgrass native to Africa that is believed to contribute to reductions in biodiversity. Forb species represent a critical habitat component throughout the breeding period for many wildlife species as seed (summer to fall), as green vegetative material (spring to summer), and as habitat for arthropods (spring to summer). Reductions in richness and diversity of crucial ecosystem components such as forbs and arthropods have large implications for grassland birds and other wildlife. We sampled annual and perennial forbs within 1-m2 quadrats on 15 study plots (1 ha; n  =  20 quadrats/plot) at Chaparral Wildlife Management Area, in LaSalle and Dimmit counties, Texas, during 2005 and 2006. Study plots were divided into five light-buffelgrass plots (0 to 5% buffelgrass canopy coverage), five moderate-buffelgrass plots (5 to 25% buffelgrass canopy coverage), and five heavy-buffelgrass plots (> 25% buffelgrass canopy coverage). Buffelgrass in study plots was composed of naturalized plants, and was not deliberately planted. During 2005 we observed that plots with > 25% buffelgrass had a 73% reduction in forb canopy of native species, a 64% reduction in native forb species richness, and a 77% reduction in native forb stem density compared to plots with 0 to 5% buffelgrass. These trends in native forb reduction (−79% native forb canopy, −65% forb species richness, −80% forb stem density) were nearly identical in 2006, even with greatly reduced rainfall. Simple linear regression revealed negative relationships between buffelgrass cover, total exotic grass cover (buffelgrass and Lehmann lovegrass), and total grass cover and the richness, coverage, and density of forbs/m2. Reductions in diversity may have larger implications regarding ecosystem function and available useable space and densities of desired bird species such as northern bobwhite. Nomenclature: Buffelgrass, Pennisetum ciliare (L.) Link; Lehmann lovegrass, Eragrostis lehmanniana Nees.

Habitat selection by the Texas tortoise in a managed thornscrub ecosystem

Brush encroachment on semiarid shrublands resulting from livestock grazing has created global concern. Southern Texas is dominated by Prosopis-Acacia mixed brush communities typical of the Tamaulipan Biotic Province, and the geographic range of the state-threatened Texas tortoise (Gopherus berlandieri) is nearly identical to the boundaries of this biotic province in Texas. In light of the perceived threat to Texas tortoises because of habitat change caused by brush encroachment, we monitored 36 Texas tortoises by radiotelemetry during 1994-1996 to assess habitat selection on a site containing grazed and ungrazed pastures. Tortoises did not exhibit habitat selection at the level of locations within home ranges. Differential habitat selection at the level of home ranges within study areas was not apparent for sex, but was evident for treatment (grazed or ungrazed). Analysis of pooled data indicated that tortoises exhibited broad-scale selection for home ranges within study areas. Selection was expressed as preferential avoidance of old-field and riparian habitats, which represented vegetational extremes of canopy cover. However, tortoises tolerated the broad continuum of other brush communities on the study site. Apparent treatment differences may be an artifact of our inability to adequately pair study areas given the scale of tortoise movement. Our data indicate that increases in the extent of woody canopy cover resulting from grazing-induced brush encroachment will not be detrimental to Texas tortoises. Furthermore, large-scale range improvement practices, such as root-plowing, create unsuitable habitats for this species.

Woody vegetation response to various burning regimes in South Texas

Responses of woody plant communities on native rangelands in the western South Texas Plains to fire are not clearly understood. Our objective was to compare woody plant cover, density, and diversity on burned and nontreated rangelands. Five rangeland sites that received 2 dormant-season burns, 5 rangeland sites that received a combination of 1 dormant-season and 1 growing-season burn, and 5 sites of nontreated rangeland were selected on the Chaparral Wildlife Management Area, Dimmit and La Salle Counties, Tex. Woody plant cover was estimated using the line intercept method, and stem density was estimated in 25-x 1.5-m plots. Species richness did not differ among treatments. Percent woody plant cover was reduced by 50 and 41 % on winter and winter-summer combination burned sites, respectively. Honey mesquite (Prosopis glandulosa Torr.), twisted acacia (Acacia schaffneri S. Wats.), Texas persimmon (Diospyros texana Scheele), lotebush [Ziziphus obtusifolia (Hook.) T. & G.], wolfberry (Lycium berlandieri Dunal), and tasajillo (Opuntia leptocaulis Cand.) canopy cover was greatest on nontreated sites. Woody plant density declined by 29 and 23% on winter and winter-summer combination burned sites, respectively. Density of guayacan (Guajacum angustifolium Engelm.), wolfberry, and tasajillo was less on all burning treatments. Percent cover of spiny hackberry (Celtis pallida Torr.) and density of Texas pricklypear (Opuntia engelmannii Salm-Reif.-Dyck) declined on winter burned sites. Inclusion of summer fire into the burning regime did not increase declines in woody plants. Fire created a post-fire environment which resulted in the decline of many woody plant species. It is unclear to what degree other environmental factors such as herbivory and competition between woody plants and among woody and herbaceous vegetation may have interacted with fire in producing woody plant declines. Fire may be a useful tool in managing woody vegetation on native south Texas rangelands, while maintaining woody plant diversity.

The Effects of Winter Burning and Grazing on Resources and Survival of Texas Horned Lizards in a Thornscrub Ecosystem

Abstract The ecological effects of land-use practices on reptiles, especially endangered or threatened species, are of conservation and scientific interest. We describe the effects of rotational livestock grazing and prescribed winter burning on resources and survival of the Texas horned lizard (Phrynosoma cornutum) during the summers of 1998 to 2001 in southern Texas, USA. We evaluated survival rates of Texas horned lizards (n  =  111) on 6 study sites encompassing 5 different burning and grazing treatments. We also measured indices of cover (i.e., vegetation) and food abundance (i.e., harvester ants [Pogonomyrmex rugosus]). We telemetered and relocated adult lizards daily. We divided the study into 2 seasons, spring (15 Apr–30 Jun) and summer (1 Jul–15 Aug), corresponding to the relative activity of horned lizards. Winter burning provided an increase in food resources and led to increased survival of Texas horned lizards in the second growing season after fire, but grazing-induced changes in vegetation cover reduced survival, likely by increasing lizard vulnerability. Fire and grazing reduced litter and increased bare ground and forb cover but did not affect woody vegetation. Ant activity was greater in burned sites and varied with grazing level, season, and year. Summer survival functions of horned lizards varied by burning treatment, with higher survival observed on burned sites in the second year after burning. Survival rates were ordered from highest in ungrazed sites to lowest in heavily grazed sites. We recognize the limitations of our work resulting from a lack of spatial replication of treatments. However, our mensurative study provides fertile ground for future hypothesis testing regarding the effects of land management on shrubland and grassland reptiles. We propose that future studies focus on the population consequences of variation in burn frequency, burn timing, and grazing intensity.

GENETIC DIVERSITY WITHIN THE SOUTHERN PLAINS WOODRAT (NEOTOMA MICROPUS) IN SOUTHERN TEXAS

Abstract Genetic diversity within a population of the southern plains woodrat was examined using DNA sequences (967 base pairs [bp]) obtained from the control or d-loop region of the mitochondrial genome. One hundred fourteen individuals from 10 collection sites were assigned to 42 haplotypes. Haplotype diversity values were moderate to high (0.974 overall and ranged from 0.524 to 0.964 across collecting sites), whereas nucleotide diversity values were low (0.008 overall and ranged from 0.001 to 0.010 across sites), indicating that this population possesses a high number of closely related haplotypes. Seventy-nine percent of the genetic variability was partitioned within groups that corresponded to the collecting sites. In addition, 13 samples from Texas, New Mexico, and Mexico were included as references for evaluating the evolutionary history of haplotypes. Nested clade analysis revealed that restricted gene flow with isolation by distance in conjunction with contiguous range expansion was responsible for the observed pattern of genetic diversity. A test of neutrality supported the diagnosis of restricted gene flow, but failed to support contiguous range expansion due solely to population growth. Examination of the spatial distribution of the haplotypes indicated that most haplotypes were restricted to a single collecting site; however, a small number of haplotypes were found at 2 or more sites. A phylogenetic analysis indicated that some haplotypes (28.6%) were restricted to the study area whereas the remaining haplotypes occupied a broader geographic region.

Effects of root plowing on white-tailed deer condition, population status, and diet

Determining the effects of plant community changes on animal populations is an important area of ecological research. We compared condition, population characteristics, and diet composition of white-tailed deer (Odocoileus virginianus) from 2 root-plowed (low brush diversity) and 2 untreated (high brush diversity) sites in the eastern Rio Grande Plains of Texas to test the hypothesis that root plowing, which decreases woody species diversity-, reduced deer habitat quality. We collected 8 adult female white-tailed deer from each site every 3 months for 6 sampling periods. We assessed condition from several physiological, reproductive, and digestive indices and determined food habits by microhistological analysis of rumen samples. We estimated population size from helicopter surveys