The slow decline of a Columbia Spotted Frog population in Yellowstone National Park: A cautionary tale from a developed zone within a large protected area

Abstract

Field research and surveys from the past century have provided a valuable resource for documenting amphibian population declines and investigating their possible causes. In Yellowstone National Park, research from the 1950s provided us with an extraordinary opportunity to investigate a Columbia Spotted Frog population’s decline over time in an area modified by increasing human occupation. Herpetologist F.B. Turner’s investigation (1953–1956) of a Columbia Spotted Frog (Rana luteiventris) population set the foundation for decades of study. In the 1990s, acting on Turner’s observation of an apparently sharp reduction in the number of frogs, we replicated his 1950s mark-recapture study with the goals of determining the extent of the decline and examining its possible causes. We continued annual monitoring through 2020. The population declined by nearly 80% between the mid-1950s and mid-1990s. The number of active breeding sites fell from 3 sites in 1950s, to 2 and then 1 in the 1990s, and none after 2007. The decline was accompanied by significant changes in the 28-hectare study area: a road constructed in the 1970s bisected the study area, headwater springs were developed, and facilities were built in and around the study area to support expanding human uses. Changes in spatial characteristics of the remnant frog population are thought to indicate the impacts of local habitat fragmentation, modification and loss. Here we focus on how the activity ranges and patterns of frog dispersal within the study area shrank to a zone upstream of the highway, prior to the eventual extirpation of the local breeding population. Multiple direct and indirect human-caused changes negatively influenced habitat that supported reproduction, survival, and recruitment. Climate change was not found to provide an adequate explanation of the observed changes. While a breeding population of Columbia Spotted Frogs has persisted next to the historical study area, it has experienced large fluctuations in egg mass production and repeated disease outbreaks in the last 15 years. This long-term case study provides a cautionary tale about shifting baselines and how land use may affect amphibian populations even in protected areas. We recommend extended monitoring for amphibians in developed zones of Yellowstone National Park, as visitation grows and conditions are further altered by infrastructure upgrades as well as by climate change.