This is a continuation of a 2017 CREOi-funded project. Around the world, amphibians are threatened by disease, habitat loss, climate change, and other ecological changes and disturbances. In western Washington, lowland river floodplains have the potential to support large populations of native amphibians, but floodplain ecosystems have also been substantially altered by agricultural land use, the introduction of invasive species such as American bullfrog (Lithobates catesbeianus) and reed canarygrass (Phalaris arundinacea), and other human impacts.
This study assessed the potential for floodplain forest restoration to recreate favorable environments for native amphibians, especially northern red-legged frogs (Rana aurora), which are known to require forested land around breeding sites. Twelve floodplain ponds, representing a gradient from disturbed reed canarygrass meadow, to recently-restored forest, to more mature forest, were surveyed for amphibians from January to July 2020. Water levels, water chemistry, and microhabitat temperature and humidity were measured in and around breeding ponds.
Visual encounter surveys of breeding ponds found that Pacific tree frogs and northwestern salamanders are abundant in some sites dominated by invasive reed canarygrass, whereas northern red-legged frogs were only found in forested sites or in partially-forested restoration sites. Length of hydroperiod in breeding ponds (permanent vs. seasonal flooding) appears to be the strongest driver of amphibian occupancy, with Northwestern salamanders and northern red-legged frogs present mostly in permanent ponds, and Pacific tree frog and long-toed salamander present mostly in seasonal ponds. Ground-level microclimate was found to be quite variable among sites, but reed canarygrass-dominated meadows were found to be warmer and less humid than nearby forest or forest-restoration sites. The presence of northern red-legged frogs in restoration sites suggests floodplain restoration may be effective in creating habitat for this species, but very low numbers of egg masses in all sites suggest that population viability in both naturally-existing and restored forest areas is tenuous. ($15,000)