Benjamin A. Pierce

Auditory Sampling of Frogs: Detection Efficiency in Relation to Survey Duration

Abstract Call surveys are used widely to assess distribution and abundance of anurans. The durations of these surveys often are based on convenience rather than on empirical analysis. Knowing how frog detection varies with survey duration is valuable for designing sampling schemes, yet few studies have examined the relationship between survey duration and detection efficiency. We conducted call surveys for frogs in central Texas to assess effects of survey duration on detection efficiency. We controlled analytically for temporal and environmental covariates that had the potential to confound our assessment of survey duration. Cumulative detection efficiency of all species was 94% for 15-min surveys and did not increase appreciably with longer durations up to 30 min. Detection efficiency for number of species was significantly higher for 15-min surveys than it was for 5-min surveys, and the variability of detection efficiency decreased with increasing survey duration. Detection efficiency for number of calling individuals of Acris crepitans and Rana sphenocephala did not differ among 5-, 10-, and 15-min surveys. Of the temporal and environmental covariates examined, only the year in which a survey was conducted was significantly associated with detection efficiency for number of species. None of the covariates was significantly related to detection efficiency for A. crepitans or R. sphenocephala. When sampling resources such as time and personnel are limited, knowledge about detection efficiencies is essential for allocating survey effort.

Geographic Variation in Acid Tolerance of Connecticut Wood Frogs

Acid waters are often toxic to amphibians, but the ability of these animals to survive in acidic conditions varies considerably within and among species. We examined geographic variation in the acid tolerance of wood frog (Rana sylvatica) embryos and larvae from eight ponds in Connecticut, which differed in their levels of acidity. Embryos produced by adult frogs from the different ponds varied significantly in the ability to hatch in acidic solutions. Hatching success of the embryos was not correlated with the acid levels in the ponds. Tadpole survival in low pH solutions also differed among the ponds, and acid tolerance of tadpoles was correlated with pond pH (tadpoles produced by adults from acidic ponds tended to be more acid tolerant). Results of genetic crosses among frogs from different ponds suggested that the geograhic differences in acid tolerances of embryos are not genetically determined, but population differences in the tolerance of tadpoles to acidity appear to have some genetic basis.

Genetic variation in tolerance of amphibians to low pH

0bservations of within-species variation in acid sensitivity have prompted speculation that genetic variation exists in the response of amphibians to low pH conditions. Study of embryonic acid tolerance in the wood frog, Rana sylvatica, suggests that variation in hatching success at low pH is largely influenced by maternal factors, which may or may not be genetic. There is limited evidence that direct genetic factors influence differences in acid tolerance of embryos. Data from other species are largely consistent with these findings. Among wood frog larvae, there was no evidence of a maternal influence on survival time at low pH, but direct genetic variation was present

Effects of short-term acidification on growth rates of tadpoles

The effect of a short pulse of acidity on the growth rates of three species of tadpoles was investigated. Tadpoles of Xenopus laevis, Bufo valliceps, and Bufo woodhousei were exposed to pH 4 and pH 7.2-7.6 solutions for three days. At the end of the exposure period, the wet and dry weights of the tadpoles exposed to the low pH solution were about half that of tadpoles exposed to neutral solution. However, seven days after transfer to neutral conditions, the wet weights of the two groups did not differ, and time to metamorphosis was not affected by the short exposure to low pH. No differences in wet weight were observed when the tadpoles were not fed during the exposure period.

Lethal and Sublethal Effects of Salinity on Three Common Texas Amphibians

Amphibian populations worldwide are experiencing declines and contaminants are hypothesized to be one of the leading contributors to these declines. In particular, the salinization of amphibian habitats is a global issue that has received relatively little attention. Of the recent studies that have emerged, the majority focus on single-species responses, and because these studies often vary in methodology, assessing tolerances across species can be challenging. Thus, one step toward a broader understanding of salt loading is to assess tolerances across multiple species using similar methodologies. Additionally, in nature, amphibians can face low, sublethal concentrations of increased salinity. Though not lethal, these low concentrations can have long-term impacts on amphibian traits such as mass or their responses to future stressors. To address these issues, we first compared relative salinity tolerances of three common frogs in central Texas (Incilius nebulifer, Gulf Coast Toad; Lithobates berlandieri, Rio Grande Leopard Frog; and Acris crepitans, Blanchards Cricket Frogs). Next, we explored the effect of sublethal salinities on the mass of I. nebulifer. Finally, we investigated the potential for sublethal salinities to increase tolerance of I. nebulifer to a later exposure of lethal salinity concentrations (acclimation). We found that across the three amphibian species, A. crepitans tadpoles were the most tolerant, followed by L. berlandieri tadpoles, and then I. nebulifer tadpoles. Sublethal concentrations of salinity did not affect mass of I. nebulifer tadpoles. Finally, exposure to sublethal levels of salinity did not increase tolerance of I. nebulifer tadpoles to a later exposure of higher salinity concentrations. Instead, tadpoles exposed to sublethal concentrations of salinity died at a faster rate when exposed later to a higher salinity concentration. With the increasing potential for salt loading to amphibian habitats due to anthropogenic influences and global climate change, understanding the consequences of salinity across multiple species as well as the effects of sublethal concentrations has important implications for amphibian conservation.

Interobserver Variation in Frog Call Surveys

Abstract Auditory surveys are being used increasingly to monitor amphibians and assess amphibian declines. Interobserver differences in detected number of species during amphibian call surveys have not been widely studied, yet previous studies have assumed these differences are minimal. We examined inter-observer variation in 269 frog call surveys conducted along 20 standardized routes in central Texas. Two trained observers, listening simultaneously 10 m apart, agreed on number of species calling in 79.4% of 5-min surveys and 78.6% of 30-min surveys. The level of observer agreement varied among species. Observers in the present study were more likely to disagree about the presence of a species when only one or a few frogs called from distant sites. Wind and road noise had no significant effects on interobserver variation. Presence of moonlight was negatively associated with interobserver agreement. Because sampling variation and biases arising from observer effects may lead to inappropriate inferences and misdirected conservation efforts, it is important to control for interobserver differences during the design and analysis phases of research.

The relationship between genome size and genetic variation.

A strong negative relationship is demonstrated between the amount of variation in structural genes and the amount of nuclear DNA in salamanders. Significant correlations are seen in a group of 14 species of salamanders from seven families and among 15 species from a single genus. The limited evidence available suggests that this relationship may exist in other organisms. In salamanders, interspecific differences in amount of DNA appear to be due primarily to differences in amounts of repetitive DNA. One possible explanation for this phenomenon is that as potential for diversity at regulatory genes increases, selection for variation in structural genes decreases, leading to a negative correlation between genome size and variation in structural genes.

The relationship between allozyme variation and life-history traits of the spotted chorus frog, Pseudacris clarkii

Captive raised tadpoles were used to investigate the relationship between multilocus heterozygosity at nine electrophoretically detected loci and several important life-history parameters in the spotted chorus frog, Pseudacris clarkii. No significant correlation was found between multilocus heterozygosity and weight early in the larval period, weight at forelimb emergence, or weight at tail resorption. When the effect of weight at metamorphosis was removed, a weak, inverse correlation was observed between length of the larval period and multilocus heterozygosity-tadpoles with more heterozygous loci metamorphosed sooner than tadpoles with few heterozygous loci. Heterozygosity at an alcohol dehydrogenase locus contributed most to this relationship. Significant differences in growth rates were observed for genotypes at a phosphoglucomutase locus, but homozygotes were heavier than heterozygotes.

Effects of disturbance, position of observer, and moonlight on efficiency of anuran call surveys

Anuran call surveys are being used widely to monitor amphibian populations and study amphibian declines. To help optimize call surveys, we studied several factors that potentially affect the efficiency of this method. We studied whether the approach of observers to the listening site potentially disturbs calling amphibians and whether disturbance is reduced after 5 min using 230 roadside call surveys along 23 routes. On each route, listening time for five of ten sites was 5 min; listening time for the other five surveys on a route was 10 min. We detected nine species in these surveys. We found no significant difference in the number of species heard in the first 5 min of a 10-min survey compared to the second 5 min, nor a significant difference in the number of species heard when comparing 5-min surveys and the second 5 min of 10-min surveys. These results suggest that there is no detectable disturbance upon approach, or if disturbance occurs it does not dissipate after 5 min. The effect of the observers listening orientation on number of species heard was examined in 110 roadside call surveys along 11 routes. Changing orientation during the call survey did not increase the number of species heard. Significantly greater numbers of species were detected during surveys conducted under low moonlight.

Assessment of Quantitative Enclosure Sampling of Larval Amphibians

Abstract Reports of the disappearance and decline of amphibians have stimulated interest in developing more accurate methods for sampling amphibian populations. Specific sampling methods are often suggested because of logistical concerns or simple tradition, but the validity of underlying assumptions for particular methods are rarely confirmed. This study examined potential problems and biases with quantitative enclosure sampling of larval amphibians. Results indicate that movement within a pond by an investigator does not reduce the number of captured tadpoles any more than does approaching a sampling station from the shore. Disturbance created in taking a sample does not appear to reduce number of tadpoles captured in subsequent samples taken 5 m or even 1 m away. Net type can be critical in how efficiently animals are removed from the enclosure, but the effect of net type on capture efficiency may vary with the amount of vegetation present. Investigators should assess the validity of assumptions for enclosure sampling before using this technique to draw inferences about populations of larval amphibians.