David R. Synatzske

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...

Demography of a collared peccary population in South Texas

Population dynamics of large herbivores in semiarid environments are regulated by rainfall. Reductions in reproduction and juvenile survival are the main mechanisms of regulation. We examined population characteristics of a harvested collared peccary (Tayassu tajacu) population in South Texas to model population responses to harvest. We sampled a peccary population from 1974 to 1993, intensively collecting data from 1989 through 1993. Mean (±SE) ovulation rate (2.13 ± 0.03, n = 181) and litter size (1.89 ± 0.03, n = 151) of adults did not vary by age class (P = 0.15) or year (P = 0.94), although litter sizes of yearlings (10-21 months old) (1.71 ± 0.19, n = 7) tended (P = 0.07) to be smaller than age-class 3 adults (5-7 yr old). Minimum age at predicted primiparity was 20 months. Pregnancy rate was positively associated (χ 2 = 6.20, P = 0.013) with rainfall in the previous year. Estimated fecundity (F young/F/yr) for yearlings and adults was 0.41 and 1.0, respectively. The sex ratio of harvested animals (53.3% M, n = 1,125, P = 0.027) was skewed toward males. Lack of age-specific variation (P = 0.23) in postnatal ratios suggested that harvest was not sex specific. Age structure varied (P = 0.08) among years, with fewer old animals in 1989-93 harvests. Male and female annual survival estimates were 0.90 ± 0.17 and 0.87 ± 0.05, respectively, from 1977 to 1983 telemetry data for unharvested herds, and did not differ (P = 0.88). From life table analyses of harvested herds from 1989 to 1993, male and female annual survival estimates were 0.65 ± 0.03 and 0.73 ± 0.04, respectively, and did not differ (P = 0.13). Year, age, and reproductive status affected (P < 0.05) condition indices, namely kidney fat, total mass, and eviscerated mass. Population modeling provided an estimated rate of juvenile (≤9 months) survival of 0.32 to maintain a stationary population under the 1993 harvest rate of 20%. We propose that juvenile survival is compensatory to adult mortality attributable to hunting. Relationships among juvenile survival, density, and forage resources need field investigation to explore the effect of density-dependent survival on dynamics of peccary populations

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.

Mark-Recapture Analysis of Population Parameters in a Texas Tortoise (Gopherus berlandieri) Population in Southern Texas

Techniques to assess population changes in reptiles across large landscapes are a conservation and management need. We studied a population of Texas tortoises (Gopherus berlandieri) on a large study area (6150 ha) of contiguous thornscrub vegetation in southern Texas from 1990 to 1999. We examined cohort and temporal variation in capture probabilities; and estimated survival, population size, and A (finite pop- ulation growth rate) for a population of Texas tortoises. We captured 2128 tortoises a total of 3132 times during the study period. The distribution of the frequency of captures by sex varied during the active season and across years, but annual capture probabilities were similar between sexes. The ratio of juvenile to adults varied by a month-by-year interaction. Tortoises exhibited a temporary response to marking that was modeled in survival analysis. Capture probabilities of adult tortoises on an annual basis ranged from 0.12-0.38, and annual survival rate of adults was estimated to be 0.79 ? 0.05. We estimated the density of the adult population to be 0.26 tortoises/ha, a level of magnitude lower than previous work conducted on habitat islands. Lambda (population growth rate) was estimated to be 0.981 (95% confidence limits: 0.945- 1.019) from a Jolly-Seber model. Road-cruising was an effective large-scale method for population moni- toring of the Texas tortoise in our study area and may prove useful in thornscrub habitats that compose the majority of its geographic range.

Temporal Differences in Activity Patterns of Male and Female Texas Horned Lizards (Phrynosoma cornutum) in Southern Texas

Abstract The Texas Horned Lizard (Phrynosoma cornutum) is a species of special conservation concern in Texas and the southwestern United States.This study was designed to compare capture rates of adult female and male Texas Horned Lizards by time and month. Horned lizards were captured by road cruising at Chaparral Wildlife Management Area in southern Texas. We found a female bias in morning capture rates compared with evening capture rates. These sex-specific differences in timing of activity may indicate an important difference in physiological constraints on the sexes. Sex-biased movement patterns must be accounted for in population sampling and plans to conserve this imperiled species.

Long-term effects of aeration and fire on invasion of exotic grasses in mixed-brush plant communities.

Abstract Invasion of exotic grasses into grasslands dominated by native plants changes fire cycles and reduces biodiversity. Brush management practices that create soil disturbance, such as aeration, may potentially result in invasion of exotic grasses and replacement of native vegetation. We tested the hypothesis that a long-term effect of aeration and prescribed burning is an increase in exotic grasses. The study was conducted at the Chaparral Wildlife Management Area in the western south Texas plains where four treatments were evaluated: aeration, warm-season burn, aeration followed by a warm-season burn, and no treatment (control). The experimental design was a randomized complete block with four replicates. We estimated percentage canopy cover of exotic grasses, native grasses, forbs, litter, bare ground, and woody and succulent plants in 2007. There was a multivariate main effect among treatments for the dependent variables absolute canopy cover of exotic grasses, native grasses, forbs, litter, and bare ground (Wilkss Lambda F15,179.84  =  2.78, P  =  0.001). Variables that contributed to the significant overall effect included litter (F3,69  =  4.32, P  =  0.008) and native grasses (F3,69  =  6.11, P  =  0.001). The multivariate main effect of treatment was significant (Wilkss Lambda F9,180.25  =  2.04, P  =  0.038) for the relative canopy cover of herbaceous species. Relative cover of exotic grasses was 31% higher (P  =  0.024) in control than in the prescribed burn treatment. Native grasses relative cover was 30% higher (P  =  0.003) in prescribed burn than in the control treatment. We did not detect differences among treatments in the percentage of total woody and succulent plants canopy cover (P  =  0.083). Under the environmental conditions at the time of the study, aeration and/or prescribed burning do not increase exotic grasses.

Female White-Tailed Deer Body Condition and Diet After a Large Spring Wildfire

Abstract Frequency of large rangeland wildfires may increase in the southwestern United States and northeastern Mexico as a result of exotic grass invasion and reduced emphasis on livestock production, but effects of such fires on white-tailed deer (Odocoileus virginianus) are poorly documented. A large wildfire burned > 90% of the 6 151-ha Chaparral Wildlife Management Area in southern Texas during March 2008, creating an opportunity to study short-term effects of wildfire on white-tailed deer food habits, body condition, and pregnancy. We harvested 26 female deer between 7 April and 20 June 2008 and recorded dressed body weight, body condition, number of corpora lutea, and number and size of fetuses. We used rumen contents to quantify forage classes consumed. Deer ate prickly pear (Opuntia engelmannii) pads and emergent grasses during April and shifted to forbs and browse as vegetation communities recovered. Deer consumed mesquite (Prosopis glandulosa) beans and prickly pear fruit during mid-June. Body condition measures did not vary during the collection period, suggesting deer were able to acquire sufficient nutrients to meet requirements. Fetal development rate appeared normal. Precipitation (11.4 cm) during late April and May probably allowed vegetation to recover from the wildfire. White-tailed deer are resilient opportunists and were able to maintain body condition and pregnancy after a large-scale wildfire.