AUTUMN HABITAT OF NORTHERN LEOPARD FROGS (LITHOBATES PIPIENS) IN WASHINGTON

Yesenia Koite

Amphibian populations are declining worldwide due to a multitude of threats, including habitat fragmentation, invasive species, and diseases. To minimize and mitigate these threats to sensitive species, wildlife managers conduct reintroduction programs that consist of translocations, captive breeding, and/or captive rearing. To help improve the success of these conservation efforts, wildlife managers rely on scientifically rigorous assessments of possible reintroduction sites, which requires knowledge of both aquatic and terrestrial habitats used by the target species. Additionally, information regarding threats, such as predation risk, is important for decision-making. One species in need of such information is the Northern Leopard Frogs (Lithobates pipiens, NLF), a species that has experienced marked declines across their northwestern range and is classified as state endangered in Washington.Here, we examined habitat use of NLF prior to their overwintering in the Desert Unit of the Columbia Basin Wildlife Area, southwest of Moses Lake, Washington, USA. We selected two areas to work within: Site A where ponds are periodically inundated by rising water levels in the neighboring Potholes Reservoir, and Site B where ponds were never inundated by the reservoir but rather experienced periodic recharge from groundwater. We used radio telemetry, visual encounter surveys, and borescope photography to characterize NLF habitat use during October – November, anticipating migration to their overwintering locations. We recorded various environmental covariates, including the occurrence of invasive American Bullfrogs (Lithobates catesbeianus, bullfrogs; a known predator of other frogs) and vegetation characteristics. We hypothesized that NLFs would use areas away from bullfrogs with higher vegetation coverage. We found the sex ratio of our captured frogs was male-biased (χ21 = 4.79, p = 0.029) and did not differ between sites (χ21 = 0.95, p = 0.329), and the size distribution likely did not include the smallest and largest frogs within the population. Tracked males traveled farther than females, but most individuals relocated with radiotelemetry remained within their initial capture pond, except for one male. We suspect these longer and more frequent movements by males in initial capture ponds may have contributed to higher chances of detecting males, resulting in our observed male-biased sex ratios. From the timing of our study and our results, it appears that these frogs selected temporary refugia by early October prior to the ponds refilling. We found no evidence of habitat selection in relation to vegetation cover, but we found evidence that NLFs had a higher probability of habitat use proximately to bullfrogs, contrary to our hypotheses. This relationship varied across size classes; the odds of using habitat close to bullfrogs decreased by about half for every 5 g increase of NLF body weight, with the probability dropping below 0.5 for frogs larger than 32.24g (p = 0.011). This relationship did not differ between the sexes (p = 0.658). We also observed NLFs underground in small earthen crevices (i.e., holes) along dry pond edges, where some co-occurred with invasive bullfrogs of similar size, indicating that crevices may function as refugia during the low-water season. Some of these crevices were occupied by large numbers of NLFs (n > 9) and bullfrogs (n > 4). This study highlights previously undocumented size-structured associations in the relationship between NLFs and invasive bullfrogs and potential dependency on crevices for NLFs during the pre-overwintering season for many frogs at this site. This contributes to the greater understanding of the habitat requirements of NLFs in their northwestern range during this time of year. The results of this study can help inform future NLF reintroduction and conservation strategies across the Pacific Northwest.

AUTUMN HABITAT OF NORTHERN LEOPARD FROGS (LITHOBATES PIPIENS) IN WASHINGTON

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