Lassen Volcanic National Park Cascades frog and eDNA inventory final report
Authors:   WSP USA Environment & Infrastructure Inc.
Published: September 2023

The Cascades frog (Rana cascadae) is one of two frog species known to occur historically in Lassen Volcanic National Park (LAVO) in Northern California. However, no Cascades frogs have been documented in LAVO for several decades, despite repeated surveys. Threats to the species include natural predators, introduction of fish into historically fishless habitats, diseases like fungal pathogens, airborne drift of pesticides from nearby agricultural areas, ultraviolet (UV-B) radiation, wildfires, and habitat loss from human development and climate change. Objectives of this project were to conduct surveys for Cascades frogs at LAVO using both visual encounter surveys (VES) and environmental DNA (eDNA) surveys at locations with potential for reintroduction, conduct VES and eDNA surveys for potential predators (rainbow trout, brook trout, brown trout, and leeches) to determine locations unsuitable for reintroduction, and to develop a suitability model to assess reintroduction potential within LAVO for Cascades frogs. Fifty locations around LAVO were sampled during this study. Suitability for Cascades frogs was ranked according to suitable vegetation availability, elevation, climatic factors, and perennial water availability. Projected climate data was used to derive future climate suitability for Cascades frogs within and around LAVO.

No Cascades frog egg masses, larvae, or adults were detected during VES or eDNA sampling within LAVO in 2021. Pacific tree frogs were observed at 18 sites, indicating suitable larval frog habitat at these sites. At 12 of these 18 sites, trout were not detected with either VES or eDNA sampling and leeches were only detected in two locations. The eDNA survey did not appear to be an effective way to detect leeches, but this method was effective for trout species and Cascades frogs.

Climate, vegetation, elevation, and perennial water availability were summarized and combined to identify suitable locations within and around LAVO. Climate data for known locations of Cascades frogs in California were used to define a range of suitable climate conditions for the species and then extrapolated across the Cascade frog?s range in California. Within LAVO, occurrence data were not available and, thus, known habitat preferences were combined with spatial data to establish the location and density of suitable vegetation communities and the availability of perennial water. The most suitable areas within the park were identified to occur along the outer boundary and eastern side of the Park. These locations had optimal climate conditions, suitable vegetation communities and available perennial water sources. Many of the areas with high suitability in LAVO were impacted in recent burns (2021), which will reduce their current overall suitability until the vegetation recovers. Projected climate scenarios suggest that, in general, areas surrounding LAVO will decline in temperature suitability while areas within LAVO will increase.

Using all available information, sites (based on sub-watersheds) were determined to be of high, medium, or low reintroduction potential. Sites of high reproduction potential included the Cluster Lakes area; Feather Lake and nearby pond; Horseshoe Lake, Snag Lake and Grassy Creek, and vernal pool near Snag Lake trail; and Willow Lake. All the sites with high reproduction potential will require active management to prepare them for Cascades frogs and to maintain them as suitable habitat into the future.