Integrating copper toxicity and climate change to understand extinction risk to two species of pond-breeding anurans

Long-term, landscape-level assessment of aquatic pesticide exposure to identify amphibian ontological traits affecting vulnerability

Amphibians worldwide are threatened by habitat loss, some of which is driven by a changing climate, as well as exposure to pesticides, among other causes. The timing and duration of the larval development phase vary between species, thereby influencing the relative impacts of stochastic hydroregime conditions as well as potential aquatic pesticide exposure. We describe the stages of breeding through metamorphosis for eight amphibian species, based on optimal hydroregime conditions, and use a model of pesticide fate and exposure representative of central Florida citrus groves to simulate hydrodynamics based on observed weather data over a 54-year period. Using the Pesticide in Water Calculator and Plant Assessment Tool, we estimated daily wetland depth and pyraclostrobin exposure, with label-recommended application quantities. Species' timing and duration of larval development determined the number of years of suitable hydroregime for breeding and the likelihood of exposure to peak aquatic concentrations of pyraclostrobin. Although the timing of pesticide application determined the number of surviving larvae, density-dependent constraints of wetland hydroregime also affected larval survival across species and seasons. Further defining categorical amphibian life history types and habitat requirements supports the development of screening-level assessments by incorporating environmental stochasticity at the appropriate temporal resolution. Subsequent refinement of these screening-level risk assessment strategies to include spatially explicit landscape data along with terrestrial exposure estimates would offer additional insights into species vulnerability to pesticide exposure throughout the life cycle. Computational simulation of ecologically relevant exposure scenarios, such as these, offers a more realistic interpretation of differential agrichemical risk among species based on their phenology and habits and provides a more situation-specific risk assessment perspective for threatened species. Integr Environ Assess Manag 2024;00:1-10. Published 2024. This article is a U.S. Government work and is in the public domain in the USA.

Amphibian Dispersal Traits Not Impacted by Triclopyr Exposure during the Juvenile Stage

Exposure to agrochemicals can have lethal and sublethal effects on amphibians. Most toxicology studies only examine exposure during the aquatic larval stage. Survival of the juvenile stage is the most important for population persistence and it is critical to understand the potential impacts of exposure during this life stage. We investigated how short-term exposure to triclopyr, an herbicide commonly used in forestry management, might impact several juvenile traits. To determine if juveniles perceived exposure as an environmental stressor, we measured their release of corticosterone. We also examined dispersal traits by measuring foraging and hopping behavior. We found no evidence that exposure negatively impacted these traits or was a stressor. Our results provide a preliminary assessment of the potential impact of triclopyr on juvenile amphibians, but we recommend additional research on the effects of agrochemicals on juvenile amphibians.

Projected Climate and Hydroregime Variability Constrain Ephemeral Wetland-Dependent Amphibian Populations in Simulations of Southern Toads

Amphibian populations are threatened globally by stressors, including diminishing availability of suitable wetland breeding sites, altered hydroregimes driven by changing weather patterns, and exposure to contaminants. Ecological risk assessment should encompass spatial and temporal scales that capture influential ecological processes and demographic responses. Following the PopGUIDE framework of population model development for risk assessment, we used matrix population models, in conjunction with existing hydroregime predictions, under a climate change scenario to evaluate the effects of environmental stochasticity and aquatic pesticide exposure on amphibians that are dependent on ephemeral wetlands. Using southern toads (Anaxyrus terrestris) as an example, we simulated population dynamics with breeding success dependent on hydroregime suitability. Years were defined as optimal, marginal, or insufficient for successful toad recruitment, based on the duration of their potential breeding season and rate of larval development to metamorphosis. We simulated both probabilistic and chronologically specific population projections, including variable annual fecundity, based on hydroregime suitability and reduced larval survival from carbaryl exposure. In our simulations, populations were more negatively impacted by prolonged drought, and consequently multiple sequential years of reproductive failure, than by aquatic pesticide exposure. These results highlight the necessity of reliable climate projections to accurately represent the effects of altered hydroregimes on amphibian populations. Risk assessment approaches could be improved with flexible modifications that allow inclusion of various extrinsic stressors and identification of demographic and ecological vulnerabilities when precise data are lacking.

Investigating potential toxic effects of pollutants on population growth rates and probability of extinction for a representative squamate

Chemical contamination has been suggested as an important contributing factor to reptile population declines, but direct links are rarely reported. Population modeling provides a quantitative method to understand the long-term effects of contaminants on population persistence. We created a matrix model for Sceloporus lizards and investigated hypothetical toxic effects by reducing survival and reproductive parameters by 0 to 100% in 10% increments. We report effects on population growth rate (λ) and elasticity values for each stage due to these reductions. We then incorporated stochasticity to the model to simulate the variation seen in demographic data and quantified extinction risk. The deterministic model yielded a λ of 1.07 suggesting stability in some wild Sceloporus populations. A yearly reduction of 20 to 30% in demographic parameters was needed to push λ to decline in both our deterministic and stochastic simulations. Surprisingly, our baseline stochastic simulations had a 30% extinction probability despite a stable deterministic model. We tested three adjustments to the stochastic model, (1) increased survival/fecundity parameters, (2) higher starting densities, and (3) a density-dependent juvenile survival function. The model with density-dependent juvenile growth had the lowest extinction risk. Ultimately, 20 or 30% mortality every year is likely unrealistic, but our results provide insight in linking toxicity to population effects. Ultimately, very little reduction in demographics is needed to cause declines in these populations. Our generalized models provide important tools for screening-level risk assessment of chemical contamination, especially for taxonomic groups that tend to receive less research interest.

Acute toxicity of copper to the larval stage of three species of ambystomatid salamanders

Copper (Cu) appears to be consistently more toxic to anuran species relative to other vertebrate taxa. There are limited Cu toxicity data for salamanders; of the few studies conducted on salamanders, most examined Cu effects on the embryonic, but not the larval, stage. We performed acute toxicity experiments, to quantify LC50s, on Harrison stage 46 larvae (free swimming hatchlings with egg yolk completely absorbed) of three ambystomatid salamander species. Each LC50 experiment used exposure concentrations of 10, 20, 30, 40, 50, and 60 µg/L with 10 replicates per concentration each containing one larva. We found very high toxicity for all species compared to previously published research on the embryo stage. Specifically, the 4-d LC50s for Ambystoma tigrinum and A. opacum were 35.3 and 18.73 µg/L, respectively. The same Cu concentrations caused similar toxicity to A. talpoideum (LC50 = 47.88 µg/L), but exposures required up to 48 d to elicit the same level of mortality. A time-to-event analysis indicated that time to mortality was significantly affected by Cu concentration. Additionally, for A. talpoideum, we observed that elevated levels of Cu decreased growth rate. Comparisons with previously reported Cu toxicity for embryos suggest that, as with fish, Cu may be more toxic to larval salamander stages than for embryos. Further, our data suggest that Cu is an important environmental contaminant that deserves increased scrutiny on the potential for population-level effects where contamination has occurred in wetlands and streams inhabited by salamanders.

Simulated developmental and reproductive impacts on amphibian populations and implications for assessing long-term effects

Fish endpoints measured in early life stage toxicity tests are often used as representative of larval amphibian sensitivity in Ecological Risk Assessment (ERA). This application potentially overlooks the impact of developmental delays on amphibian metamorphosis, and thereby reduced survival, in amphibian populations constrained by habitat availability. Likewise, the effects of reduced productivity or altered sexual development as a result of chemical exposure are not presented in terms of lower population fecundity in these surrogate tests. Translating endpoints measured in toxicity tests to those that are more representative of amphibian ecology and population dynamics provides a means of identifying how developmental effects result in long-term impacts. Here we compare effects of developmental delay on metamorphosis success in six anuran species and simulate population-level impacts of subsequent reductions in larval survival as well as potential reductions in fecundity as a result of developmental impacts. We use deterministic matrix models to compare realistic combinations of amphibian demographic rates and relative impacts of reduced growth on larval survival and subsequently on population growth. Developmental delays are less detrimental in species with longer and less synchronous larval periods. All six species were most sensitive to changes in first-year survival, and damping ratios were generally a good indicator of resilience to perturbations in both larval survival and fecundity. Further identification of species and population-level vulnerabilities can improve the evaluation of sublethal effects in relevant context for ERA.