Search ARMI Database

An increase in ultraviolet-B (UV-B) radiation has been posited as one potential factor in some amphibian population declines. This hypothesis has received support from laboratory and field experiments showing that current levels of UV-B can cause embryo mortality in some species, but little research has addressed whether UV-B is influencing the distribution of amphibian populations. We compared patterns of amphibian presence to site-specific estimates of UV-B dose at 683 ponds and lakes in Glacier, Olympic, and Sequoia-Kings Canyon National Parks. All three parks are located in western North America, a region with a concentration of documented amphibian declines. Site-specific daily UV-B dose was estimated using modeled and field-collected data to incorporate effects of elevation, landscape, and water column dissolved organic carbon. Of the eight species we examined (Ambystoma gracile, Ambystoma macrodactylum, Bufo boreas, Pseudacris regilla, Rana cascadae, Rana leuteiventris, Rana muscosa, Taricha granulosa), two species (T. granulosa and A. macrodactylum) had quadratic relationships with UV-B that could have resulted from negative UV-B effects. Both species were most likely to occur at moderate UV-B levels. Ambystoma macrodactylum only showed this pattern in Glacier National Park. Occurrence of A. macrodactylum increased as UV-B increased in Olympic National Park despite UV-B levels similar to those recorded in Glacier. We also found marginal support for a negative association with UV-B for P. regilla in one of the two parks where it occurred. We did not find evidence of a negative UV-B effect for any other species. Much more work will be necessary to determine if UV-B, either alone or in concert with other factors, is causing widespread population losses in amphibians.

We swabbed 45 amphibians and of these, 4 were positive for B. dendrobatidis. We found B. dendrobatidis on all 3 Rana catesbeiana captured in a small farm pond near Turin. Rana catesbeiana is a North America species introduced to Italy more than 50 years ago (Lanza 1962). Out of 41 native amphibians tested, we only found one individual that was positive for B. dendrobatidis. The positive individual was 1 of 10 R. esculenta tested at 5 ponds in Study Area C. We did not find any frogs that were dead or that appeared to be sick.

Of 242 amphibians swabbed, 52 were positive for B. dendrobatidis (Table 1). We found B. dendrobatidis on individuals from 11 of 22 populations and in 3 of the 4 study areas (A, C and D). All but 2 of the positive results were for Bufo boreas adults or juveniles. The other two positives were for Rana aurora adults. No R. luteiventris or R. sylvatica were positive. We did not find any frogs that were dead or appeared to be sick. For populations with at least one positive animal and with N  5 adults and juveniles, the percent of adults and juveniles testing positive averaged 34% and ranged from 10 to 80%.

Batrachochytrium dendrobatidis is a fungal pathogen that is receiving attention around the world for its role in amphibian declines. Study of its occurrence patterns is hampered by false negatives: the failure to detect the pathogen when it is present. Occupancy models are a useful but currently underutilized tool for analyzing detection data when the probability of detecting a species is less than 1. We use occupancy models to evaluate hypotheses concerning the occurrence and prevalence of B. dendrobatidis and discuss how this application differs from a conventional occupancy approach. We found that the probability of detecting the pathogen, conditional on presence of the pathogen in the anuran population, was related to amphibian development stage, day of the year, elevation, and human activities. B. dendrobatidis was found throughout our study area but was only estimated to occur in 53.4% of 78 populations of native amphibians and 66.4% of 40 populations of non-native R. catesbeiana tested. We found little evidence to support any spatial hypotheses concerning the probability that the pathogen occurs in a population, but did find evidence of some taxonomic variation. We discuss the interpretation of occupancy model parameters, when, unlike a conventional occupancy application, the number of potential samples or observations is finite

We used a large forested preserve (Olympic National Park, USA) to examine the habitat associations of a unique and environmentally sensitive stream amphibian fauna: Ascaphus truei Stegneger, Rhyacotriton olympicus (Gaige), and Dicamptodon copei Nussbaum. We quantified the relative abundance of stream amphibians and compared them to physical, topographic, climatic, and landscape variables. All three species were associated with the southwest to northeast climate gradient; tending to be most abundant in the southwest. Although a habitat generalist relative to the other two species, Dicamptodon copei was absent from the northeastern portion of the park. Ascaphus truei and Rhyacotriton olympicus were both associated with coarse substrates and steep gradients. Unlike studies in harvested forests, all stream amphibians were common in waters with unconsolidated surface geology (e.g., marine sediments that erode easily). Studies of ecological preserves can provide an important baseline for evaluating species associations with environmental gradients and can reveal patterns not evident in more disturbed landscapes.

In many permanent ponds throughout western North America, the introduction of a variety of exotic fish and bullfrogs (Rana catesbeiana) correlates with declines in native amphibians. Direct effects by exotics are suspected to be responsible for the rarity of some native amphibians and are one hypothesis to explain the prevalence of amphibian declines in western North America. However, the prediction that the permanent ponds occupied by exotics would be suitable for native amphibians if exotics were absent, has not been tested. I used a series of enclosure experiments to test whether survival of northern red-legged frog (Rana aurora aurora) and Pacific treefrog (Hyla regilla) larvae is equal in permanent and temporary ponds in the Puget Lowlands, Washington, USA. I also examined the direct effects of bullfrog larvae and sunfish. Survival of both species of native anuran larvae was generally lower in permanent ponds. Only one permanent pond out of six was an exception to this pattern and exhibited increased larval survival rates in the absence of direct effects by exotics. The presence of fish in enclosures reduced survival to near zero for both native species. An effect of bullfrog larvae on Pacific treefrog larval survival was not detected, but effects on red-legged frog larvae were mixed. A hypothesis that food limitation is responsible for the low survival of native larvae in some permanent ponds was not supported. My results confirm that direct negative effects by exotic vertebrates on native anurans occur, but suggest they may not be important to broad distribution patterns. Instead, habitat gradients or indirect effects of exotics appear to play major roles. I found support for the role of permanence as a structuring agent for pond communities in the Puget lowlands, but neither permanence nor exotic vertebrates fully explained the observed variability in larval anuran survival.