2003 Progress Report: Models and Mechanisms: Understanding Multiple Stressor Effects on an Amphibian Population

EPA Grant Number: R829086
Title: Models and Mechanisms: Understanding Multiple Stressor Effects on an Amphibian Population
Investigators: Palmer, Brent D. , Elskus, Adria , Sih, Andy , Shepherd, Brian , Crowley, Philip
Institution: University of Kentucky , University of California - Davis
Current Institution: University of Kentucky
EPA Project Officer: Packard, Benjamin H
Project Period: August 1, 2001 through July 31, 2004 (Extended to November 25, 2005)
Project Period Covered by this Report: August 1, 2002 through July 31, 2003
Project Amount: $522,832
RFA: Wildlife Risk Assessment (2001) RFA Text |  Recipients Lists
Research Category: Environmental Justice , Biology/Life Sciences , Ecological Indicators/Assessment/Restoration , Aquatic Ecosystems


The overall objective of this research project is to further our understanding of the effects of multiple environmental stressors, both natural and anthropogenic, on a wildlife population of the salamander Ambystoma barbouri. Our central hypothesis is that multiple stressors can interact to generate complex detrimental effects on populations, and these effects can be much more severe than those produced by any single stressor.The specific objectives of this research project are to: (1) build, implement, and evaluate a spatially explicit, individual-based population model; (2) conduct experiments to measure the effects of multiple stressors on parameters that enter into the model (e.g., survival, fecundity, growth, extinction rates); and (3) measure endocrine and physiological variables to investigate potential mechanisms underlying the effects of multiple stressors on the parameters that enter into the model. A. barbouri was selected because of increasing concern over the apparent global decline of amphibians and because amphibian life history characteristics make them particularly vulnerable to multiple stressors. To produce a realistic, situation-specific model, the combinatorial effects of multiple stressors likely to be encountered by this population will be used to model population demography (growth, survival, fecundity, extinction rates). These stressors include habitat ephemerality (desiccation), predator-prey interactions (chemoreception, locomotion, drift), and anthropogenic pollutants (atrazine, octylphenol, endosulfan, and carbaryl). In addition, physiological endpoints (hormone balance, enzyme induction) will be studied to elucidate mechanisms underlying amphibian response to multiple stressors. Parameters for the model will be derived from these and other empirical data; sensitivity analysis will identify the influence of each parameter, and the model will be validated against an existing data set.

Progress Summary:

Agricultural contaminants may be contributing to worldwide amphibian declines, but little is known about which agrichemicals pose the greatest threat to particular species. One reason for this is that tests of multiple contaminants under ecologically relevant conditions rarely are conducted concurrently. In this study, we examined the effects of 37-day exposure to the agrichemicals atrazine (4, 40, and 400 mg/L), carbaryl (0.5, 5, and 50 mg/L), endosulfan (0.1, 1, and 10 mg/L for 31 days and 0.1, 10, and 100 mg/L for the last 6 days), and octylphenol (5, 50, and 500 mg/L), and to a solvent control, on streamside salamanders (A. barbouri) in the presence and absence of food. We found that none of the agrichemicals significantly affected embryo survival, but that hatching was delayed by the highest concentration of octylphenol. In contrast to embryos, larval survival was reduced by the highest concentrations of carbaryl, endosulfan, and octylphenol. Growth rates were lower in the highest concentrations of endosulfan and octylphenol than in all other treatments, and the highest concentration of endosulfan caused respiratory distress. Significantly more carbaryl, endosulfan, and octylphenol tanks had larvae with limb deformities than did control tanks. Refuge use was independent of chemical exposure, but 10 mg/L of endosulfan and 500 mg/L of octylphenol decreased larval activity. Systematically tapping tanks caused a greater activity increase in larvae exposed to 400 mg/L of atrazine and 10 mg/L of endosulfan relative to solvent controls, suggesting underlying nervous system malfunction. Hunger stimulated a decrease in refuge use and an increase in activity, but this response was least pronounced in larvae exposed to the highest concentration of any of the four agrichemicals, possibly because these larvae were the most lethargic.

Under our ecologically relevant test conditions, octylphenol seemed to have the greatest detrimental effects on A. barbouri. Chronic exposure to octylphenol induced the greatest mortality, delay in hatching, growth reduction, and lethargy. Endosulfan also had deleterious effects, including increased mortality, reduced growth rates, respiratory distress, limb deformities, and altered behavior. Carbaryl caused significant larval mortality at the highest concentration and produced the greatest percent of malformed larvae, but did not significantly affect behavior relative to controls. Although atrazine did not induce significant mortality, it did seem to affect motor function. More studies are needed that concurrently examine the effect of multiple stressors and multiple agrichemicals on amphibians so that we can better identify and mitigate the effects of the agrichemicals that pose the greatest threat.

In addition, amphibian populations can be affected adversely by multiple biotic and abiotic stressors that together can contribute to their local and global decline. We focused on the combined effects of food limitation, drying conditions, and exposure to possibly the most abundant and widely used herbicide in the world, atrazine. We used a factorial design to evaluate the effects of exposure to four ecologically relevant doses of atrazine (approximate measured doses: 0, 4, 40, and 400 mg/L), two food levels (limited and unlimited food), and two hydroperiods (presence or absence of a dry down) on the survival, life history, and behavior of the streamside salamander A. barbouri from the embryo stage through metamorphosis. In general, food and atrazine levels did not interact statistically, and atrazine affected dependent variables in a standard, dose-dependent manner. Exposure to 400 mg/L of atrazine decreased embryo survival and increased time to hatching. Drying conditions and food limitation decreased larval survival, whereas 400 mg/L of atrazine only reduced larval survival in 1 of the 2 years tested, suggesting that the lethality of atrazine may be condition dependent. Sublethal effects included elevated activity and reduced shelter use associated with increasing concentrations of atrazine and food limitation. The larval period was lengthened by food limitation and shortened by 400 mg/L of atrazine. Drying conditions accelerated metamorphosis for larvae exposed to 0 and 4 mg/L of atrazine but did not affect the timing of metamorphosis for larvae exposed to 40 or 400 mg/L of atrazine. Food limitation, drying conditions, and 400 mg/L of atrazine reduced size at metamorphosis without affecting body condition (relationship between mass and length), although feeding rates did not differ significantly among atrazine concentrations at any time during development. This suggests that high atrazine levels may have increased larval energy expenditures. Because smaller size at metamorphosis can lower terrestrial survival and lifetime reproduction, resource limitations, drying conditions, and environmentally realistic concentrations of atrazine have the potential to contribute to amphibian declines in impacted systems.

Future Activities:

We will emphasize two thrust areas of the grant during Year 3 of the project. First, we will conduct analyses to elucidate the mechanisms of action for compounds, such as atrazine, on salamanders. We will focus on histological examination of reproductive organs and endocrine mechanisms, such as impacts on thyroid and reproductive hormones. This will further our understanding on how these agrichemicals may be eliciting their effects on development and reproduction.

Second, we will finalize a spatially explicit, individual-based population model. This model will help predict the effects of compounds such as atrazine on amphibian populations by utilizing the compounds’ effect on life history parameters to predict populational outcomes. Empirical data generated from our experiments will be utilized to fine tune the model’s predictive ability.

Journal Articles on this Report : 2 Displayed | Download in RIS Format

Other project views: All 25 publications 5 publications in selected types All 5 journal articles
Type Citation Project Document Sources
Journal Article Rohr JR, Elskus AA, Shepherd BS, Crowley PH, McCarthy TM, Niedzwiecki JH, Sager T, Sih A, Palmer BD. Lethal and sublethal effects of atrazine, carbaryl, endosulfan, and octylphenol on the streamside salamander (Ambystoma barbouri). Environmental Toxicology and Chemistry 2003;22(10):2385-2392. R829086 (2002)
R829086 (2003)
R829086 (Final)
  • Abstract: SETAC Journals Abstract
  • Journal Article Rohr JR, Elskus AA, Shepherd BS, Crowley PH, McCarthy TM, Niedzwiecki JH, Sager T, Sih A, Palmer BD. Multiple stressors and salamanders:effects of an herbicide, food limitation, and hydroperiod. Ecological Applications 2004;14(4):1028-1040. R829086 (2003)
    R829086 (Final)
  • Full-text: University of South Florida PDF
  • Abstract: ESA Journals Abstract
  • Supplemental Keywords:

    atrazine, octylphenol, carbaryl, endosulfan, salamander, larval, metamorphosis, development, behavior, water, exposure, risk assessment, effects, ecological effects, vulnerability, sensitive populations, dose response, animal, organism, population, stressor, cumulative effects, chemicals, toxics, terrestrial, aquatic, habitat, life history, conservation, environmental chemistry, biology, endocrinology, ecology, modeling, agriculture,, RFA, Health, Scientific Discipline, Air, Ecosystem Protection/Environmental Exposure & Risk, Aquatic Ecosystems & Estuarine Research, exploratory research environmental biology, wildlife, Ecosystem/Assessment/Indicators, climate change, Air Pollution Effects, Aquatic Ecosystem, Susceptibility/Sensitive Population/Genetic Susceptibility, Monitoring/Modeling, Ecological Effects - Environmental Exposure & Risk, Environmental Monitoring, genetic susceptability, Ecological Risk Assessment, Atmosphere, predicting risk, sensitive populations, anthropogenic stress, ecological exposure, demographic data, stressors, watersheds, endocrine disruption , amphibians, dose-response, endocrine disrupting chemical, exposure, amphibian, endocrine disruptors, modeling, multiple stressors, Wildlife Risk Assessment, animal models, salamander eggs, ecosystem stress, watershed assessment, ecological response, toxics, environmental hazard exposures, modeling ecosystems, amphibian population

    Progress and Final Reports:

    Original Abstract
  • 2002 Progress Report
  • 2004
  • 2005
  • Final Report