Declining amphibian populations: the problem of separating human impacts from natural fluctuations

Effects of spray-irrigated wastewater effluent on temporary pond-breeding amphibians

Comparison was made of abiotic conditions, amphibian reproductive outputs, and the survival of embryonic and larval amphibians in wastewater effluent-irrigated and natural temporary ponds in an area in Centre County, Pennsylvania that has been spray-irrigated with secondarily treated, chlorinated wastewater effluent from The Pennsylvania State University for approximately 14 years. Three species of temporary pond-breeding amphibians were studied: wood frogs (Rana sylvatica LeConte), Jefferson salamanders (Ambystoma jeffersonianum Green), and spotted salamanders (A. maculatum Gravenhorst). Comparisons of physico-chemical parameters in 10 wastewater-irrigated and 10 natural temporary ponds over 19 weeks in 1997 indicated that wastewater -irrigated ponds had significantly higher median conductance, pH, ¿Na, ¿K, ¿Ca, ¿Mg, and ¿N-NO(3) and lower ¿dissolved oxygen. Many of the wastewater-irrigated ponds supported large mats of duckweed (Lemna spp.) that completely blanketed the pond's surface by mid-May. There were significantly fewer egg masses of all three species in wastewater-irrigated ponds than in natural ponds in both 1997 and 1998. In situ egg hatching success and larval survival (over a 6-day period) of all species was lower in wastewater-irrigated ponds than in natural ponds. Cumulatively, these studies suggest that wastewater effluent irrigation may impact amphibian populations by reducing the survival of amphibian eggs and larvae.

Quantitative evidence for global amphibian population declines

Although there is growing concern that amphibian populations are declining globally, much of the supporting evidence is either anecdotal or derived from short-term studies at small geographical scales. This raises questions not only about the difficulty of detecting temporal trends in populations which are notoriously variable, but also about the validity of inferring global trends from local or regional studies. Here we use data from 936 populations to assess large-scale temporal and spatial variations in amphibian population trends. On a global scale, our results indicate relatively rapid declines from the late 1950s/early 1960s to the late 1960s, followed by a reduced rate of decline to the present. Amphibian population trends during the 1960s were negative in western Europe (including the United Kingdom) and North America, but only the latter populations showed declines from the 1970s to the late 1990s. These results suggest that while large-scale trends show considerable geographical and temporal variability, amphibian populations are in fact declining--and that this decline has been happening for several decades.

Phase III interlaboratory study of FETAX. Part 3. FETAX validation using 12 compounds with and without an exogenous metabolic activation system

FETAX (Frog Embryo Teratogenesis Assay-Xenopus) is a 96-h whole-embryo developmental toxicity screening assay that can be used in ecotoxicology and in detecting mammalian developmental toxicants when an in vitro metabolic activation system is employed. A standardized American Society for Testing and Materials (ASTM) guide for the conduct of FETAX has been published, along with a companion atlas that helps in embryo staging and in identifying malformations. As part of the ASTM process, an interlaboratory validation study was undertaken to evaluate the repeatability and reliability of FETAX and to evaluate the potential teratogenic hazard of 12 compounds. Three different laboratories participated in the study. All three participating laboratories had extensive experience with the assay. FETAX intralaboratory and interlaboratory variability, as judged by coefficients of variation, were very low. Potential teratogenic hazard was evaluated using two major criteria from FETAX experiments employing metabolic activation systems (MAS). These were the teratogenic index TI (TI = 96-h lc(50)/96-h ec(50) (malformation)) and the minimum concentration that inhibits growth (MCIG). A compound was considered teratogenic by this criterion when the MCIG was significantly different from controls at concentrations below the 30% level of the MAS 96-h lc(50). Based on the results of this and other studies, a decision table was constructed in order to evaluate additional studies. Severity of malformations caused, especially near the MAS 96-h ec(50) (malformation), were also evaluated. Four compounds were non-teratogenic but two compounds were clearly teratogenic. The remaining six compounds were ranked as equivocal teratogens. The results were discussed in light of the difficulty of producing an adequate decision table. FETAX proved to yield repeatable and reliable data as long as care was taken during range-finding and technicians were adequately trained. The MAS was essential in using FETAX to predict developmental hazard in mammals, and still requires further development.

Amphibian declines: an immunological perspective

Many, but not all, amphibian populations have been declining on all six continents on which they live. Although habitat destruction, direct application of toxicants, and introduction of predators/competitors are obvious causes of amphibian declines, many amphibians are dying of infectious diseases in relatively pristine habitats on several continents. In this paper, we review the patterns of these disease outbreaks and the characteristics of amphibian immune systems. Hypotheses are presented to explain the apparent susceptibility of amphibians to these pathogens. Natural and man-made factors that can alter amphibian immune responses to pathogens are discussed. Additional research is needed on the biology of the specific pathogens, the pattern of immune responses they elicit, and the nature of environmental stressors that may increase susceptibility to infectious disease.

Intraspecific heterochrony and life history evolution: decoupling somatic and sexual development in a facultatively paedomorphic salamander

Morphological features such as size and shape are the most common focus in studies of heterochronic change. Frequently, these easily observed and measured features are treated as a major target of selection, potentially ignoring traits more closely related to fitness. We question the primacy of morphological data in studies of heterochrony, and instead suggest that principal sources of fitness, such as life history characteristics, are not only the chief targets of selection, but changes in them may necessitate changes in other (subordinate) elements of the organism. We use an experimental approach to investigate the timing of metamorphosis and maturation in a facultatively paedomorphic salamander, Ambystoma talpoideum. We determine that individuals possessing the well-known paedomorphic phenotype are peramorphic with regard to maturation, through the process of predisplacement (an earlier onset of maturation). Combining the well studied ecology of dimorphic A. talpoideum populations with theories of heterochronic mechanisms and life history evolution, we conclude that age at maturation is the principal target of selection and that morphological changes are secondary effects. Increased attention to the intimate connection between life history evolution and heterochrony is the most promising route to a better understanding of both.

Analysis of three amphibian populations with quarter-century long time-series

Amphibians are in decline in many parts of the world. Long tme-series of amphibian populations are necessary to distinguish declines from the often strong fluctuations observed in natural populations. Time-series may also help to understand the causes of these declines. We analysed 23-28-year long time-series of the frog Rana temporaria. Only one of the three studied populations showed a negative trend which was probably caused by the introduction of fish. Two populations appeared to be density regulated. Rainfall had no obvious effect on the population fluctuations. Whereas long-term studies of amphibian populations are valuable to document population declines, most are too short to reveal those factors that govern population dynamics or cause amphibian populations to decline.

Ground and surface water developmental toxicity at a municipal landfill: description and weather-related variation

Contaminated groundwater poses a significant health hazard and may also impact wildlife such as amphibians when it surfaces. Using FETAX (Frog Embryo Teratogenesis Assay-Xenopus), the developmental toxicity of ground and surface water samples near a closed municipal landfill at Norman, OK, were evaluated. The groundwater samples were taken from a network of wells in a shallow, unconfined aquifer downgradient from the landfill. Surface water samples were obtained from a pond and small stream adjacent to the landfill. Surface water samples from a reference site in similar habitat were also analyzed. Groundwater samples were highly toxic in the area near the landfill, indicating a plume of toxicants. Surface water samples from the landfill site demonstrated elevated developmental toxicity. This toxicity was temporally variable and was significantly correlated with weather conditions during the 3 days prior to sampling. Mortality was negatively correlated with cumulative rain and relative humidity. Mortality was positively correlated with solar radiation and net radiation. No significant correlations were observed between mortality and weather parameters for days 4-7 preceding sampling.

Bacterial diversity of a Carolina bay as determined by 16S rRNA gene analysis: confirmation of novel taxa

Carolina bays are naturally occurring shallow elliptical depressions largely fed by rain and shallow ground water. To identify members of the domain Bacteria which inhibit such an environment, we used PCR to construct a library of 16S rRNA genes (16S rDNAs) cloned from DNA extracted from the sediments of Rainbow bay, located on the Savannah River Site, near Aiken, S.C. Oligonucleotides complementary to conserved regions of 16S rDNA were used as primers for PCR, and gel-purified PCR products were cloned into vector pGEM-T. Partial sequencing of the cloned 16S rDNAs revealed an extensive amount of phylogenetic diversity within this system. Of the 35 clones sequenced, 32 were affiliated with five bacterial groups: 11 clustered with the Proteobacteria division (including members of the alpha, beta, and delta subdivisions), 8 clustered with the Acidobacterium subdivision of the Fibrobacter division (as categorized by the Ribosomal Database Project's taxonomic scheme, version 5.0), 7 clustered with the Verrucomicrobium subdivision of the Planctomyces division, 3 clustered with the gram-positive bacteria (Clostridium and relatives subdivision), and 3 clustered with the green nonsulfur bacteria. One sequence branched very deeply from the Bacteria and was found not to be associated with any of the major divisions when phylogenetic trees were constructed. Two clones did not consistently cluster with specific groups and may be chimeric sequences. None of the clones exhibited an exact match to any of the 16S rDNA sequences deposited in the databases, suggesting that most of the bacteria in Rainbow Bay are novel species. In particular, the clones related to the Acidobacterium subdivision and the Verrucomicrobium subdivision confirm the presence of novel taxa discovered previously in other molecular surveys of this type.

Uses of molecular phylogenies for conservation

Conservation requires the accurate targeting of resources together with information on population processes. Molecular phylogenies can potentially contribute in both areas. Interspecific molecular phylogenies can identify clades undergoing rapid diversification or extinction and can assess the contribution of individual lineages to overall genetic diversity. Whether and how this should translate to conservation priorities is open to debate. Within species, molecular phylogenies along with information on allele frequencies can be used to identify evolutionarily significant population units or areas. In relation to population processes, molecular phylogenies may provide a perspective on population growth and connectivity over evolutionary time. As threatened species typically have undergone rapid changes in population size an d/or migration rate, phylogeny-based estimates may be misleading about contemporary population processes. However, the comparison between historical and current estimates may identify significant changes to long-term trends, thus identifying population in need of urgent management. This information can potentially suggest strategies for managing declining or fragmented species, but this requires further theoretical and experimental study.

Developmental responses of amphibians to solar and artificial UVB sources: a comparative study

Many amphibian species, in widely scattered locations, currently show population declines and/or reductions in range, but other amphibian species show no such declines. There is no known single cause for these declines. Differential sensitivity to UVB radiation among species might be one contributing factor. We have focused on amphibian eggs, potentially the most UVB-sensitive stage, and compared their resistance to UVB components of sunlight with their levels of photolyase, typically the most important enzyme for repair of the major UV photoproducts in DNA, cyclobutane pyrimidine dimers. Photolyase varied 100-fold among eggs/oocytes of 10 species. Among three species-Hyla regilla, Rana cascadae, and Bufo boreas-for which resistance of eggs to solar UVB irradiance in their natural locations was measured, hatching success correlated strongly with photolyase. Two additional species, Rana aurora and Ambystoma gracile, now show similar correlations. Among the low-egg-photolyase species, R. cascadae and B. boreas are showing declines, and the status of A. gracile is not known. Of the two high-photolyase species, populations of H. regilla remain robust, but populations of R. aurora are showing declines. To determine whether levels of photolyase or other repair activities are affected by solar exposures during amphibian development, we have initiated an extended study of H. regilla and R. cascadae, and of Xenopus laevis, laboratory-reared specimens of which previously showed very low photolyase levels. Hyla regilla and R. cascadae tadpoles are being reared to maturity in laboratories supplemented with modest levels of UV light or light filtered to remove UVB wavelengths. Young X. laevis females are being reared indoors and outdoors. Initial observations reveal severe effects of both UVA and UVB light on H. regilla and R. cascadae tadpoles and metamorphs, including developmental abnormalities and high mortalities. Assays of photolyase levels in the skins of young animals roughly parallel previous egg/oocyte photolyase measurements for all three species.

Initial interlaboratory validation study of FETAX: phase I testing

An interlaboratory validation study was undertaken to evaluate the repeatability and reliability of the Frog Embryo Teratogenesis Assay-Xenopus (FETAX), which is a whole embryo developmental toxicity screening assay. A three-phase experimental program with seven participants was carried out. Phase I was a training and protocol evaluation phase where the identity of the three test materials was known. Hydroxyurea, isoniazid and 6-aminonicotinamide were tested in Phase I. Because the chemicals has been tested previously in FETAX, the same concentrations needed to establish the 96-h median lethal concentration (LC50) and the concentration inducing malformations in 50% of the surviving embryos (EC50) were used by all laboratories. The results of Phase I are presented in this report, and FETAX has proved to be as repeatable and reliable as many other bioassays. Some excess variation was observed in individual laboratories. Some of this variation may have been due to training difficulties. One change in protocol design necessitated by this study was the use of 6-aminonicotinamide as a reference toxicant. While 6-aminonicotinamide provided excellent concentration-response data in most laboratories, the protocol was written too strictly based on historical FETAX data. Phases II and III are currently in progress.

UV repair and resistance to solar UV-B in amphibian eggs: a link to population declines?

The populations of many amphibian species, in widely scattered habitats, appear to be in severe decline; other amphibians show no such declines. There is no known single cause for the declines, but their widespread distribution suggests involvement of global agents--increased UV-B radiation, for example. We addressed the hypothesis that differential sensitivity among species to UV radiation contributes to these population declines. We focused on species-specific differences in the abilities of eggs to repair UV radiation damage to DNA and differential hatching success of embryos exposed to solar radiation at natural oviposition sites. Quantitative comparisons of activities of a key UV-damage-specific repair enzyme, photolyase, among oocytes and eggs from 10 amphibian species were reproducibly characteristic for a given species but varied > 80-fold among the species. Levels of photolyase generally correlated with expected exposure of eggs to sunlight. Among the frog and toad species studied, the highest activity was shown by the Pacific treefrog (Hyla regilla), whose populations are not known to be in decline. The Western toad (Bufo boreas) and the Cascades frog (Rana cascadae), whose populations have declined markedly, showed significantly lower photolyase levels. In field experiments, the hatching success of embryos exposed to UV radiation was significantly greater in H. regilla than in R. cascadae and B. boreas. Moreover, in R. cascadae and B. boreas, hatching success was greater in regimes shielded from UV radiation compared with regimes that allowed UV radiation. These observations are thus consistent with the UV-sensitivity hypothesis.