Does UV-B Radiation Affect Embryos of Three High Elevation Amphibian Species in California?

Declines and extinctions of amphibians in well-protected habitats suggest that global atmospheric factors may be responsible. We tested effects of field exposures of ultraviolet radiation (UV-B) on embryo hatching success and time to hatching in three anurans that inhabit high elevation areas of the Sierra Nevada in California, USA. While few obvious environmental impacts have occurred in the high elevation area of the Sierra Nevada, two of the three most common anurans, the Yosemite Toad (Bufo canorus) and the Southern Mountain Yellow-legged Frog (Rana muscosa), have suffered severe population declines while the sympatric Pacific Treefrog (Pseudacris regilla) has remained relatively abundant. Previous studies have shown that hatching of P. regilla embryos at lower elevations are not affected negatively by UV-B radiation. We hypothesized that differences in UV sensitivity may help explain why P. regilla remain abundant while B. canorus and R. muscosa have declined sharply. We conducted field experiments at two remote sites above 3030 m elevation over two years. No effect of UV-B was found on hatching success or rate of development in embryos of B. canorus, P. regilla, or R. muscosa, except for a small, context-dependent increase in time to hatching in R. muscosa. We recommend that research efforts on these species in the Sierra Nevada concentrate on post-hatching effects of UV-B, or on other decline hypotheses.

Figure 1

The inset map shows the location of the two study areas in California, USA. The shaded region in California approximates the high elevation area (2,500–3,660 m) in the Sierra Nevada where the three species in this study co-occur. The Bufo canorus field experiments were conducted in and near Cloverleaf Lake (n = 2 water bodies), ∼60 km north of the second study area, Sixty Lake Basin (shown in the foreground). The water bodies shaded in gray and black are the study lakes for Rana muscosa (n = 4) and Pseudacris regilla (n = 2), respectively. Water samples analyzed for DOC and spectral analysis were collected from all six amphibian study lakes in Sixty Lake Basin and also from Lake 1 and Tulip Lake. Scale bar shown on bottom right is 0.5 km.

Figure 2

Mean percentage of embryos surviving to hatching ± 1 SE in Rana muscosa over two years in four water bodies (n = 25 egg masses; 18–70 embryos/experimental unit). Hatching success of each experimental unit is represented by an “X.”

Figure 3

Mean percentage of embryos surviving to hatching ± 1 SE in Bufo canorus (n = 9 egg masses; 30–52 embryos/experimental unit) and Pseudacris regilla (n = 59 egg masses; 13–31 embryos/experimental unit) in four breeding water bodies in 1999. Hatching success of each experimental unit is represented by an “X.”

Figure 4

Mean duration of development (fertilization to hatching) ± 1 SE in Rana muscosa (n = 1,777 hatched embryos), Bufo canorus (n = 1,119 hatched embryos), and Pseudacris regilla (n = 527 hatched embryos). Time to hatching of each experimental unit is represented by an “X”.

Figure 5

Irradiance (µW/cm²) measured in the field by the USEPA Ultraviolet Monitoring Program (UV-Net) at a Brewer station in Sequoia National Park near the field experiments and UV-B transmittance of the materials used in the field experiment. Field data are peak irradiance values for 13 June 1999. Values are shown for wavelengths ranging from 290 to 363 nm.

Figure 6

Percent transmittance of UV-B though 10 cm of water from samples collected in the Sixty Lake Basin in 1999 (study sites: Lake 12, Lake 30, Hyla 2, Hyla G; other sites: Lake 1, Tulip Lake). Lake and date are given for each sample. Lakes 12 and 30 were sampled three times; Lake 1, Hyla G, and Tulip Lake were sampled twice; and Hyla 2 was sampled once.

Contributor Notes

¹ Museum of Vertebrate Zoology and Department of Integrative Biology, University of California, Berkeley, Berkeley, California 94720. Present address: Department of Biology, Hensill Hall 754, 1600 Holloway Avenue, San Francisco State University, San Francisco, California 94132, e-mail: vancev@sfsu.edu. Send reprint requests to this address.

² Department of Integrative Biology, 3060 Valley Life Sciences Building, University of California, Berkeley, Berkeley, California 94720, e-mail: jmromansic@gmail.com

³ Department of Biology, Box 90338, Duke University, Durham, North Carolina 27708, e-mail: lauren.chan@duke.edu

⁴ Department of Ecology, Evolution and Marine Biology, Life Sciences Building, Room 4306, University of California, Santa Barbara, Santa Barbara, California 93106-6105, e-mail: tunstall@lifesci.ucsb.edu

Associate Editor: M. J. Lannoo.

Received: 27 May 2009

Accepted: 22 Mar 2010