Successful Bullfrog Eradication in Yosemite

Introduction

Image Details

Known for its dramatic landscape and towering granite walls, Yosemite National Park is also home to a unique assemblage of native reptiles, amphibians, and fishes. In Yosemite, native aquatic species include California red-legged frogs (Rana draytonii), Foothill yellow-legged frogs (Rana boylii), Western toads (Anaxyrus (Bufo) boreas), Sierra newts (Taricha sierra), Western pond turtles (Actinemys (Emys) marmorata), and Rainbow Trout (Oncorhynchus mykiss). Several of these species are state and federally protected and endemic to California. American bullfrogs (Lithobates (Rana) catesbeianus), native to eastern North America, were first introduced to the Yosemite Valley in the 1950s with the first bullfrog documented in 1955 at Ahwahnee Hotel’s reflection pond. Since then, bullfrogs have likely contributed to the decline of many of Yosemite Valley’s native aquatic species. 

National Park Service (NPS) biologists have conducted bullfrog control in the Yosemite Valley since the 1990s, at first as a side project by Yosemite’s former wildlife branch chief, Steve Thompson, through the early 2000s. However, Thomspon secured funding and enabled more-targeted removal efforts, and by 2005 two full-time restoration technicians were hired by NPS to focus on bullfrog removal. Bullfrog removal continued regularly through 2015, with successful eradication achieved in 2019.

Key Issues Addressed

Bullfrogs are highly successful invaders due to a combination of biological characteristics. They have high plasticity, or the ability to express multiple life history tactics (e.g. different growth rates, age at maturity, size classes, etc.) to tolerate different environmental conditions. In addition, female bullfrogs can produce upwards of 20,000 eggs in one egg mass. Bullfrogs can reach large sizes as adults, are dietary generalists, and display aggressive feeding behavior which makes them equipped to prey upon and outcompete native species. Similar to bullfrog effects in southern Arizona, native aquatic populations have declined in Yosemite, likely due to bullfrog colonization by the 1960s. Impacted native species include several frogs, the Western toad, and Western pond turtle. In systems with established bullfrog populations, eradication is one of the few options for managers to recover native aquatic species.

Whereas southern Arizona aquatic habitat is often water-limited, with perennial and intermittent slow-moving streams, ciénegas (marshes fed by springs), ponds, and stock tanks, the Merced River that flows through Yosemite Valley is perennial with off-channel ponds forming during the spring and summer. In addition, one artificial pond (a reflection pond) is present at the Ahwahnee Hotel. The elevation in Yosemite Valley ranges from about 2,950 to 4,200ft and covers approximately 1500ha. The sides of the Yosemite Valley are contained by steep cliffs, waterfalls, and cascades, which make bullfrog emigration to other upstream habitats highly unlikely due to steep grades and high flow conditions. Above these features, tributary elevations are also too high to support bullfrogs (>6,000ft). In this setting, bullfrogs, without human assistance, are likely to re-invade the Yosemite Valley from source populations located further downstream. 

Given its unique geography, Yosemite Valley offers NPS biologists the opportunity to achieve bullfrog eradication with less chance of recolonization. Year-round flows and access to suitable stream habitat further aids native aquatic species recovery efforts in Yosemite Valley. Project biologists suspect the Yosemite Valley bullfrog population established through introductions at the Ahwahnee Hotel, rather than colonization from downstream of Yosemite Valley. Moreover, downstream bullfrog populations are separated from the Yosemite Valley by a section of narrow canyon with difficult terrestrial environments for bullfrogs to traverse and higher gradient and flow conditions that would prevent bullfrog colonization from below in most years. As such, project biologists focused on the Yosemite Valley as emigration from other sites would be unlikely following bullfrog removal efforts. Extensive bullfrog removal and ongoing monitoring provides NPS biologists the opportunity to increase native reptile, amphibian, and fish recovery through bullfrog removal.

Project Goals

• Eradicate bullfrog populations from Yosemite Valley in Yosemite National Park
• Use environmental DNA (eDNA) sampling to ensure remaining bullfrogs are detected
• Re-establish native aquatic species after achieving bullfrog eradication, including the California red-legged frog

Project Highlights

The Last Bullfrogs: NPS biologists removed over 8,000 bullfrogs from 2005-2019, with the last egg mass removed in 2012, last tadpole in 2013, and the last adult recorded in 2019; achieving successful eradication in Yosemite Valley.

• Annual Surveys: Bullfrogs were removed annually from 2005 to 2015 with a one to two person crew from mid-May (onset of breeding) to late August with mechanical removal techniques. Crews surveyed all available bullfrog breeding habitats including slack water, ponds, and stagnant streams. Bullfrog breeding was detected at six total locations (five natural locations and one artificial pond), and in 15 flooded areas. In 2015, NPS incorporated eDNA sampling to enhance detection from traditional survey techniques. From 2005 to 2018, bullfrog eradication was targeted at both sites where breeding was detected and flooded areas. In total, bullfrogs were eradicated from 1,500 hectares of the Yosemite Valley.
• eDNA to Monitor Success: NPS biologists incorporated eDNA in 2015 to increase their bullfrog detections. From 2016 to 2019, biologists were able to detect bullfrogs with eDNA that they did not observe with traditional visual survey methods. Through eDNA surveys, biologists were able to detect if bullfrogs were still present in the valley following mechanical removal efforts.
• California Red-Legged Frog Reintroductions: NPS biologists began California red-legged frog re-introductions in 2016. Red-legged frogs were captive-reared at the San Francisco Zoo, with over 1,000 frogs reintroduced into Yosemite Valley between 2016 and 2019.
• Non-Native Aquatics Treatment Techniques: Visual encounter surveys (VES) were used to detect breeding bullfrog populations (e.g. egg masses and tadpoles). Bullfrog egg masses resemble “large gelatinous mats,” which are easily distinguishable from other native frog species in Yosemite. Breeding surveys were conducted during the day from mid May to June and egg masses and tadpoles were removed via dip nets, paint strainers, and five-gallon buckets. Crews also used backpack electrofishing and seine nets to remove tadpoles in targeted sites. After breeding, crews removed adult frogs at night using visual surveys and 200+ lumen flashlights throughout the summer. Bullfrogs were collected by hand, spear (trident pole spears), dip and seine nets, hook and line, and shooting via pellet rifles. Baited funnel traps were deployed, however did not capture bullfrogs. The artificial reflection pond was also drained in 2006 to remove tadpoles. During bullfrog removal in Yosemite Valley, crews euthanized all other adult and sub-adult bullfrogs using a buffered solution of MS-222 (tricaine methanesolfonate) at a concentration of 2–3 g/l solution at pH 7.0–7.5 and/or skull blunt force and pithing protocol. Yosemite is currently continuing bullfrog removal at backcountry sites. For humane euthanasia at backcountry sites, bullfrogs are rubbed topically with a 10+% Benzocaine solution (Orajel) and then dispatched using a blunt force or pithing protocol.

Lessons Learned

Image Details

This project in Yosemite Valley demonstrates that, in the context of this particular location, successful bullfrog removal increased opportunities for native aquatic species recovery. Red-legged frog reintroductions have been successful, with egg masses observed at multiple locations in 2019, evidence that translocated adults from the San Francisco Zoo are successfully reproducing. In addition, while anecdotal, park visitors and residents have observed that tree frogs are much louder at night and are seeing western toads that haven’t been observed in decades. Overall, native amphibian populations visually appear to be increasing, although further surveys will need to be conducted before making population-level assessments. In Yosemite National Park, bullfrog removal was very well received by visitors and the general public. Generally, through outreach and education, bullfrog removal was well-supported to restore native populations of species especially like California red-legged frogs.

eDNA has been used to survey amphibians and fishes in Yosemite. eDNA surveys implemented by project biologists were a viable tool for detecting remaining bullfrogs that were not found with other survey methods. After 2015, eDNA sampling enabled project biologists to confirm if bullfrogs were still present following removal efforts. Because bullfrogs move from year to year, eDNA was less helpful in targeting exact areas where bullfrogs would be found, but proved useful for general surveillance of bullfrogs. eDNA, used in conjunction, with traditional methods can therefore be an effective strategy for monitoring non-native species like bullfrogs. 

There is a concern that eDNA techniques may detect DNA from residual or non-viable sources (i.e. dead bullfrogs or previously removed bullfrogs). Studies have found that eDNA sinks in a water column and eDNA is more concentrated in sediments than surface water in freshwater systems. DNA also degrades rapidly under high UV and heat conditions. As a result, a positive detection of bullfrog DNA from samples collected from surface water would indicate the DNA was from a live/viable species. Additionally, multiple positive detections (as we observed in Yosemite National Park), increases the strength in evidence that a living bullfrog is present and that the DNA is not from non-viable sources. 

Successful bullfrog eradication stemmed from over a decade of removal effort. Throughout this project, there were years with very high effort and other years where a dedicated crew was not available (lower effort, opportunistic surveys). Accordingly, in this case in Yosemite Valley, an “all-out assault” style of heavy removal efforts may not be necessary in all years to make a significant reduction in bullfrogs, depending on the site’s unique ecological factors. 

Project biologists recommend: 1) Focus on breeding areas and eggmasses during the breeding season, and access sites with early breeding early in the season to target eggmasses with tightly meshed nets or paint strainers (normal nets did not work for eggmass removal as eggs fell through the mesh). The project team generally removed eggmasses and then left the eggs on the sides of the bank to desiccate. 2) Focus on breeding adults throughout the season and remove both male and female adults. Adults were easiest removed during night visual encounter surveys using high lumen (300+) headlamps or flashlights to stupefy bullfrogs for easy hand capture, netting, or gigging (team members used a rubber band operated Hawaiian sling shot). 3) Opportunistically remove sub-adults and tadpoles, while searching for adults, especially during the fall when they metamorphose. 4) Following successful removal, continue visual surveys and consider eDNA surveys for bullfrogs. Early detection of bullfrogs is key to prevent re-establishment of a bullfrog population.

Next Steps

• Ensure Annual Bullfrog Monitoring: Long-term monitoring using visual encounter and eDNA survey methods will ensure that bullfrogs have not recolonized the Yosemite Valley from source populations further downstream.
• Continue Monitoring California Red-Legged Frogs: Monitoring efforts using visual encounter and capture, mark, recapture techniques will help biologists develop population estimates and better understand factors that contribute to successful recruitment. Once enough frogs have established, biologists intend to use the Yosemite Valley populations as a donor population for future sites within Yosemite National Park such as Miguel Meadows.

Funding Partners

• All bullfrog removal funded by National Park Service
• All California red-legged frog reintroductions funded by Yosemite Conservancy

Resources

• Amphibians of Yosemite National Park
• Aquatic Restoration in Yosemite National Park
• The Nature Conservancy Blog: “Recovery: A Plague of Bullfrogs” by Ted Williams
• The Fresno Bee “Threatened California frog, made famous by Mark Twain, returns to Yosemite Valley”
• High Country News: “Red-legged frogs successfully reintroduced to Yosemite”
• San Francisco Chronicle “Red-legged frogs - once killed off in Yosemite - are back, with help from SF Zoo”
• Kamoroff et al. (2020). “Effective Removal of the American Bullfrog (Lithobates catesbeianus) on a Landscape Level: Long Term Monitoring and Removal Efforts in Yosemite Valley, Yosemite National Park.” Biological Invasions 22(2): 617-626.
• Kamoroff, C., and C.S. Goldberg. (2018). “An Issue of Life or Death: Using eDNA to Detect Viable Individuals in Wilderness Restoration.” Freshwater Science, 37(3), 685-696.
• Turner, C.R., Barnes, M.A., Xu, C.C., Jones, S.E., Jerde, C.L., and D.M. Lodge. (2014). “Particle Size Distribution and Optimal Capture of Aqueous Macrobial eDNA.” Methods in Ecology and Evolution, 5(7), 676-684.
• Yosemite National Park
• Goldberg Lab, Washington State University

Contacts

• Rob Grasso, Aquatic Ecologist, Yosemite National Park: Rob_Grasso@nps.gov
• Ninette Daniele, Wildlife Biologist, Yosemite National Park: Ninette_Daniele@nps.gov
• Colleen Kamoroff, Biologist, U.S. Fish and Wildlife Service: cdkamoroff@gmail.com

Case Study Lead Author

• Alex Koeberle, CART Research Specialist, University of Arizona, akoeberle@arizona.edu

Suggested Citation

Koeberle, A.,L. (2020) “Bullfrog Removal at a Landscape-Scale in Yosemite National Park.” CART. Retrieved from https://www.fws.gov/project/successful-bullfrog-eradication-yosemite.