Evaluation of Solar Ultraviolet Radiation as a Factor in Amphibian Decline in Montane Habitats

Duration:  February 1999 - September 2003

Amphibian populations throughout the world are in decline.  A variety of factors have been proposed as causes of this decline, including habitat degradation, predation, competition from non-native species, parasites, disease, climate change, environmental contamination; and solar ultraviolet radiation (UVR). 
   

UVR irradiance tends to increase with both altitude and latitude.  Many montane amphibians breed and develop through their early life-stages in shallow temporary pools at high altitudes, where they are subject to high levels of UVR.   In addition, there is considerable evidence that UVR irradiance is increasing over much of the Earth's surface, due to depletion of stratospheric ozone.  Although several studies have implicated UVR as a factor in amphibian decline, there are many variables that can affect UVR exposure and its consequences.  Few of these variables are understood well enough to permit reliable identification of habitats at risk. 

UVR consists of two wavebands:  UVB (ranging from wavelengths of 280 to 320 nanometers (nm)) and UVA (ranging from wavelengths of 320 to 400 nm).  The UVB waveband is more energetic and thus more harmful than the UVA waveband. 

The intensity of UVB irradiance reaching an aquatic organism largely depends upon:  (a) atmospheric ozone concentrations, and (b) water composition and clarity.  Increased UVR irradiance arising from depletion of the ozone layer poses a threat to aquatic flora and fauna, and may be especially critical to amphibians whose populations are declining even in relatively pristine habitats. 

Our project is investigating solar UVR as a potential factor in amphibian decline in four National Parks in the Western United States.  The objectives of our study are:

• To determine UVR  (both UVA and UVB) irradiance in the aquatic habitats of developing amphibians.
  
  
• To assess the sensitivity of indigenous amphibians to current levels of UVR irradiance.
  
  
• To determine the influence of abotic and biotic factors on the UVR sensitivity of these organisms.

Locations:

Approach

We will measure surface and water column UVR with a scanning spectroradiometer at field sites representing the different types of amphibian habitats in Rocky Mountain, Glacier, Olympic, and Sequoia/Kings Canyon National Parks.  These sites include habitats where amphibian declines have been observed. They also include low and high elevations where a gradient of UVR occurs with altitude.  Sites at different latitudes will enable us to evaluate the latitudinal gradient of UVR. 

Laboratory studies will assess the sensitivity of amphibians collected from our study sites to simulated solar ultraviolet radiation.  Test organisms will be exposed to a range of UVR intensities representing:  (a) intensities measured in natural habitats, and (b) reasonable estimates of future UVR levels.  Sensitivity will be evaluated as increased mortality, delayed development, deformities, lesions, and abnormal behavioral function.   This study will determine if present and potential UVR levels are harmful. 

Abiotic and biotic variables that influence UVR exposure and sensitivity will also be measured: 

• Abiotic variables include concentrations of dissolved organic carbon, amount of shading vegetation and other habitat structure that could block the penetration of UVR through the water column, and hazardous substances that potentially increase injury caused by UVR.
  
  
• Biotic variables can influence the magnitude of exposure, and thus the "dose" of UVR.  These biotic variables include the animal's spatial behavioral preferences, reproductive timing, pigmentation, and the presence of parasites and disease.

Statistical techniques will be used to determine how these variables affect UVR exposure and sensitivity in the test organisms.

Application of Results

This project will provide information about the sensitivity of amphibian species in each National Park, as well as habitat measurements that will provide a critical data set for risk assessment of the species involved, particularly species at risk.  Our results will verify the application of the EPA PRIME Net radiometric data to specific habitats within the parks.  This project will also identify UV-mitigating variables that may be assessed in landscape-scale models of habitat availability for amphibian species.

	Principal Contact:  Dr. Edward E. Little, USGS, Columbia Environmental Research Center