Pond-breeding amphibian assemblages of the pumice plain at Mount St. Helens : thirty-three years post-eruption

Abstract

Amphibian responses to intense volcanic disturbance have not been widely studied. Yet given the global distribution of volcanoes, their frequency of eruptions, and associated areal extent of disturbance, it is important to understand how amphibians are affected by volcanism. The 1980 eruption of Mount St. Helens in Washington State (USA) provides an outstanding opportunity to assess amphibian response to a range of volcanic disturbance types. Reported here is a case study conducted on the Pumice Plain, an area intensely disturbed by a number of volcanic forces that destroyed all pre-eruption life and set the stage for evaluating amphibian community assembly in the context of primary succession. Amphibians were monitored at 25 study sites that included four habitats (pond, lake, wallow, and seep) during summer 2013. Two survey methods were employed (aquatic funnel trap and visual encounter) at each survey site to obtain information on species presence and reproduction. Biophysical habitat characteristics were measured including study site area, elevation, water depth, sediment type, percent cover of vegetation, dissolved oxygen, conductivity, pH, and temperature. Results showed that five amphibian species were present on the Pumice Plain: Rana cascadae, Rana aurora, Pseudacris regilla, Anaxyrus boreas, and Ambystoma gracile. R. cascadae was encountered at 76% of the survey sites, while P. regilla was encountered at 72% of the sites surveyed. A. boreas and A. gracile were found only in the pond and lake habitats, occupying 20% of all sites surveyed. R. aurora is the least frequently encountered species on the Pumice Plain, and present at only 12% of all survey sites. A. boreas and P. regilla are the most abundant species on the Pumice Plain, with the majority of the captures being at the pond habitat. In regards to richness, the ponds contain the most species, with all five species present. Thirty-three years post-eruption, the Pumice Plain supports a diverse assemblage of pond-breeding amphibians. These pond-breeders demonstrated resiliency to intense volcanic disturbance. R. cascadae and P. regilla appear to be the most resilient of the species studied based on their broad distribution across the study sites.