Interaction of Environmental Stressors and Batrachochytrium dendrobatidis (Bd) Pathogen Loads on Survival of Green Frogs (Lithobates clamitans)

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The U.S. Geological Survey Amphibian Research Monitoring Initiative (ARMI) is using a combination of swabbing, non-lethal tissue sampling, soil and water sampling, and collection of a variety of other environmental variables to determine the relationships between the prevalence and pathogen load of Bd infection and environmental stressors on green treefrog survival.

The Science Issue and Relevance: Batrachochytrium dendrobatidis (Bd) is a species of chytrid fungus and the causative agent of the infectious disease chytridiomycosis. In the last three decades, Bd has caused the catastrophic decline or extinction of more than 200 species of frogs. The fungus is prevalent in aquatic habitats in which many frogs breed, spreading via flagellated aquatic zoospores. The primary clinical sign of infection is thickening of the outermost layer of skin leading to a reduced capacity for osmotic regulation. The resulting ion disruption often leads to cardiac arrest and death. It is hypothesized that environmental stressors such as salinity, water temperature, pH, and bioavailable mercury (extent to which mercury is absorbed by the body) levels can exacerbate the effects of Bd. We are investigating how frog survival rates are impacted by the interaction of environmental stressors and Bd.

Green frog

Green frog (Photo by Brad Glorioso)

 

Methodology for Addressing the Issue: The U.S. Geological Survey Amphibian Research Monitoring Initiative (ARMI) began a nationwide study implementing capture-mark-recapture techniques in 2018. The study uses a combination of swabbing, non-lethal tissue sampling, soil and water sampling, and collection of a variety of other environmental variables to determine the relationships between the prevalence and pathogen load of Bd infection and environmental stressors. For three nights in a two-week span twice a year, we capture green frogs (Lithobates clamitans) at a small pond in Sherburne Wildlife Management Area in South-Central Louisiana. We note sex in adults, measure snout-vent length, swab the skin for the presence of Bd, and take toe clips for mercury analysis from all captured individuals. Lastly, we uniquely mark each individual by inserting a passive integrated transponder (PIT) tag into the body before release back into the pond. Recapturing PIT-tagged individuals allows us to estimate survival as a function of pathogen load and examine the effect of environmental stressors across time.

study pond

The study pond in Sherburne Wildlife Management Area in the Atchafalaya Basin, Louisiana (Photo by Brad M. Glorioso)

 

Future Steps: We will continue the current sampling protocol twice a year. Beginning in spring 2020, we will collect dragonfly and damselfly larvae at the study site to better understand the relationships among mercury levels in the environment, food items, and amphibians.