Monitoring of Amphibians at St. Marks National Wildlife Refuge in Northwest Florida

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Freshwater wetlands provide critical habitat for a diverse array of organisms, including many amphibians. Yet, under the threat of climate change, these habitats are among the most imperiled ecosystems on Earth.

Mole Salamander, Ambystoma talpoideum
Mole Salamander, Ambystoma talpoideum

The Science Issue and Relevance: Freshwater wetlands provide critical habitat for a diverse array of organisms, including many amphibians. Yet, under the threat of climate change, these habitats are among the most imperiled ecosystems on Earth. Evapotranspiration losses from wetlands will most likely increase under projected climate change, resulting in shortened hydroperiods and an increased frequency of reproductive failures in amphibians, especially those that breed in ponds that are seasonal and isolated. Insufficient rainfall and shortened hydroperiods have already been linked to catastrophic reproductive failure and amphibian declines at some sites in the southeastern U.S. For this region of the country, amphibians that breed during the winter months are especially vulnerable to the effects of changing climate. In the last three decades, the average annual temperature in the southeastern U.S. has risen approximately 1.1°C, with the greatest seasonal increase occurring in winter. During this same time precipitation has decreased, primarily in winter (-9.6%) and spring (-29.2%). Continuation of these trends could negatively impact pond-breeding amphibians that rely on winter and spring rains to fill seasonal wetlands, trigger breeding, and ensure reproductive success. Understanding how amphibians generally cope with environmental uncertainty, and climatic variation and hydrologic fluctuations in particular, will aid in predicting how these and other pond-breeding amphibians may respond to future climate change.

Methodology for Addressing the Issue: We are monitoring two winter-breeding amphibians (the Ornate Chorus Frog, Pseudacris ornata, and the Mole Salamander, Ambystoma talpoideum) at the St. Marks National Wildlife Refuge (SMNWR) in the panhandle of Florida. The objective of the SMNWR research is to evaluate changes in site occupancy in response to climate-induced variation in pond hydroperiod. For Mole Salamanders, we sample wetlands with traps to capture aquatic larvae and paedomorphic individuals. For the chorus frog, we monitor sites with automated recorders (ARUs) to detect calling males. We then use single-species, multi-season models to estimate occupancy, detection probability, colonization, and extinction rates. By incorporating information on drought indices (such as the Palmer Hydrological Drought Index) into our models, we can evaluate the effect that drought may be having on changes in estimates of these parameters over time.

Ornate Chorus Frog, Pseudacris ornata
Ornate Chorus Frog, Pseudacris ornata

Future Steps: Knowledge of the length of time a seasonal wetland holds water (hydroperiod) – along with the timing, pattern and frequency of filling and drying of that wetland – is crucial to evaluating its suitability for reproductive success and persistence of amphibians. Traditionally, amphibian ecologists have used rainfall as a proxy for more direct measures of wetland hydrology to evaluate breeding conditions and other aspects of amphibian ecology. To relate site occupancy of amphibians to hydrologic variation within and among wetlands, there is a need for an integrated approach that incorporates continuous hydrological monitoring (e.g., wetland water levels and the extent and duration of wetland flooded area) into studies of amphibian ecology. Collaboration is underway between USGS biologists and hydrologists to monitor key hydrological, water quality, and biological properties at freshwater wetland ponds and to evaluate the biotic responses of amphibians to abiotic (hydrologic) variables that are likely to be affected by climate change. An understanding of the current hydrological variability of wetland ponds at SMNWR will provide a "benchmark" against which researchers and natural resource managers can monitor wetlands for the impacts of future climate change.