Landscape-Scale Eradication of Bullfrogs

Introduction

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American bullfrogs (Rana catesbeiana) are native to eastern North America but have established populations globally outside of their native range. The International Union for Conservation of Nature (IUCN) considers the American bullfrog one of the “100 world’s worst invasive species invasive species
An invasive species is any plant or animal that has spread or been introduced into a new area where they are, or could, cause harm to the environment, economy, or human, animal, or plant health. Their unwelcome presence can destroy ecosystems and cost millions of dollars.

Learn more about invasive species .” Bullfrogs are 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, which have high survival rates to tadpole and adult life stages. Bullfrogs display aggressive feeding behavior which makes them equipped to predate on and outcompete native species. 

Non-native bullfrogs have successfully established populations across the United States, including the Southwest. Bullfrogs were first introduced to Arizona in the 1950s as a game species. They have since been introduced into and colonized the limited aquatic systems found in southern Arizona, including perennial and intermittent slow-moving streams, ciénegas (marshes fed by springs), ponds, and stock tanks. 

Historically, aquatic systems in southern Arizona supported a diverse assemblage of native fishes, reptiles, and amphibians. These systems co-evolved with the region’s unique environmental conditions, including flashy high flows during monsoon rains, hot summer temperatures, and cold, sometimes snowy conditions during winter. Examples of native organisms found in these systems include Gila Chub (Gila intermedia) and Gila Topminnow (Poeciliopsis occidentalis), Mexican gartersnake (Thamnophis eques), and Chiricahua leopard frog (CLF; Lithobates (=Rana) chiricahuensis). Due to various levels of anthropogenic disturbance of watersheds such as habitat loss and degradation, water withdrawals, and the introduction of non-native plant and animal species, several of the species endemic to aquatic systems in southern Arizona are federally listed through the Endangered Species Act. Research and management attention in these systems has increased habitat restoration efforts for native species recovery, but the threat of bullfrogs remains. 

To restore native amphibian populations decimated by non-native bullfrogs, a collaborative team of researchers, managers, and technicians (Frog Team) has been conducting mechanical removal at a landscape-level across southern Arizona since the 1990s, when it was initiated by university and agency biologists.

Key Issues Addressed

The CLF was listed as Threatened under the Endangered Species Act in 2002. The non-native bullfrog’s tolerance and adaptability to extreme conditions enable it to outcompete and consume populations of native species like CLF in aquatic systems throughout southern Arizona. Bullfrogs have colonized habitats historically occupied by CLF, and bullfrogs quickly outcompete and predate on CLF where they co-occur.

Large-scale CLF declines began in the 1970s, with the introduction of Batrachochytrium dendrobatidis (Bd), a fungal pathogen that causes chytridiomycosis, a disease that is fatal for many species of amphibians across the globe. CLF are susceptible to chytridiomycosis, and their populations can experience localized die-offs when exposed. Bullfrogs threaten recovery because they not only consume and outcompete CLF, but they are also resistant to chytridiomycosis, and they carry it with them when they invade a body of water.

Successful native frog recovery requires complete eradication of bullfrogs through intensive effort, long-term monitoring, and sustained financial support. This is especially difficult to accomplish due to the bullfrog’s widespread distribution in southern Arizona and their ability to reproduce with high fecundity, feed aggressively, disperse over large distances, and recolonize habitats quickly. Many regions in southern Arizona are characterized by limited water availability, with few existing perennial streams. Changing climatic conditions such as weakened summer monsoons, drought, and rising temperatures has the potential to further threaten native CLF, especially where the species has had difficulty becoming re-established due to the presence of Bd and bullfrogs within their native range. 

Project Goals

• Document non-native bullfrog abundance and distribution throughout southern Arizona
• Establish systematic methods for bullfrog removal 
• Conduct the systematic eradication of bullfrogs from prioritized invaded sites
• Establish and maintain “buffer zones” to prevent bullfrog re-colonization and increase recovery opportunities for native CLF
• Expand range and increase abundance of CLF

Project Highlights

Puddle Jumping: Biologists have documented Chiricahua leopard frogs dispersing and recolonizing habitat during the summer monsoon season once bullfrogs have been removed.

• Decades of Team Effort: This is the largest active bullfrog control project in the western U.S. with over 20 years of collaborative effort by a diverse group of team members from state and federal agencies, universities, non-profit organizations, landowners, and local volunteers. Efforts began in the 1990s at San Bernardino National Wildlife Refuge. By the early 2000s, public concern grew due to expansion of bullfrogs populations into Sycamore Canyon, a stream system that supports a diverse community of native amphibians, birds, and jaguar. This shifted the program’s focus to the Buenos Aires National Wildlife Refuge and surrounding area.
• Landscape-Level Control: The Frog Team has removed bullfrogs from many large areas prioritized within a project area of 10,230 square miles (a total area 66 miles wide by 155 miles long). In response, several CLF metapopulations have re-established within three targeted Recovery Units. Present CLF metapopulations consist of Peña Blanca Lake (CLF recolonized naturally after bullfrog removal in 2009), Sycamore Canyon/Peck Canyon (recolonization after bullfrog removal in the early 2000s), Cobre Ridge (recolonization after bullfrog removal from 2017-2019), and Las Cienegas (CLF were augmented after bullfrog removal and habitat creation from 2011-2014, and current buffer zone monitoring with expansion towards Sonoita/Elgin in 2019). 
• Taking Bullfrog Inventory: Frog Team members first conduct a complete census of an area to determine bullfrog presence and identify breeding populations using the standardized Visual Encounter Survey (VES) method. Preliminary surveys are conducted at night, generally once a month, using binoculars, powerful flashlights, and listening for frog calls. Project members confirm the amphibian species identification by sound, visually, or if uncertain at first contact, through collecting them by hand when the frogs resurface. 
• Prioritization Criteria for Targeting Bullfrog Removal: A combination of GIS, aerial imagery, and ground truthing is used to prioritize bullfrog removal areas. Occupied sites (e.g. stock tanks, canyons, and springs) that provide CLF habitat are prioritized first. The team uses aerial imagery, particularly in May/June when ephemeral sites usually dry, to examine water permanency. If water bodies are permanent and hold bullfrogs, they are prioritized because as potential breeding sites. The team also uses imagery and topo layers to determine potential bullfrog dispersal corridors, to predict where bullfrogs move on the landscape. Removal of adults is prioritized to prevent further breeding, before removing juveniles which are removed to prevent dispersal to adjacent sites. 
• Establishing and Maintaining “Buffer Zones”: Following successful bullfrog removal, buffer zones are maintained to prevent invasion from remaining nearby bullfrog populations. Buffer zones are then monitored, particularly during monsoon season, when bullfrogs disperse and breed, to prevent bullfrog re-invasion.
• Non-Native Aquatics Treatment Techniques: Team members (which may include agency or university biologists, including students, or volunteers) use 30 foot wide bag seine nets to remove tadpoles. Rifles with .22 caliber copper/polymer or tin bullets (both lead-free) are used to shoot adult bullfrogs at night. Additionally, in Cienega Creek the Bureau of Land Management left specified cattle tanks seasonally dry to prevent bullfrogs from spreading to perennial stream sections. Bullfrog biomass from the removals was mostly buried on-site. Targeted ponds and stock tanks may be temporarily drained if removal of bullfrogs and tadpoles remains a particular challenge, due to size and complexity of a site.

Lessons Learned

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Early eradication efforts were unsuccessful in the San Bernardino National Wildlife Refuge, where a dense cluster of ponds for native Yaqui-basin native fish conservation, and water management challenges along the U.S.-Mexico border, made bullfrog eradication almost impossible to achieve. In addition, field crews at the time targeted only adult bullfrogs. Plentiful juveniles quickly matured to take the place of removed adults. Field crews could not keep up since bullfrogs quickly mature from late adolescence to an adult (the Frog Team once recorded this in 10 days). Working alone at night along the border can be unsafe, so biologists are often paired up and checked in with Border Patrol along border area project sites. In addition, another issue along the border is for lotic systems such as the Santa Cruz River that flow across the border, or areas with large numbers of stock tanks adjacent to the border, as a lack of bullfrog control in Mexico limits the ability to control recolonization. Applying the lessons learned from San Bernardino and adapting techniques to new systems led to the later success in both Buenos Aires National Wildlife Refuge, which contains mostly stock tanks and isolated springs, and Cienega Creek, the most ecologically-intact cienega complex in the southwestern United States. This included removing all size classes of bullfrogs from new project sites. Successful bullfrog control efforts also depend on close collaboration with local landowners, for example working with ranchers to remove bullfrogs from cattle tanks.

CLF recolonized sites quickly once bullfrogs were eradicated. Quality habitat for CLF can include larger ponds with dense surface layers of vegetation (such as pondweed for cover, feeding, and predator avoidance), and bank cover, such as spike rush. With suitable habitat cleared of bullfrogs, CLF are equipped to recolonize on their own if there are occupied sites in the 5-mile (8 km) vicinity, without the need for translocations or re-stocking efforts. They are also able to disperse over large distances. At Cienega Creek, biologists documented an average dispersal distance of 7.43 km, ranging from 1.77 km to 15.13 km. 

The three most effective tools for bullfrog removal at this scale are bag seine nets, cattle tank draining, and .22 caliber rifles. Successful bullfrog eradication resulted from systematic and methodical removal techniques, which also has limitations. For example, team members preferred using CCI Copper-22 brand bullets, as they found them to have the best accuracy and most humane kill due to their high velocity. However, higher velocities mean greater risk for bullets ricocheting off the water, so team members must know what they are doing and how to shoot safely. Air rifles were tested by team members, but were not as effective, and subsonic (quiet) ammo is lead-based. Traditional methods such as gigging were too time and effort intensive for removal of thousands of bullfrogs. Private landowners can also be deterred by shooting at night for weeks on end. If so, the Frog Team will adhere to requests from the landowner, if given access, which may include not shooting at night (hand capture only) or limiting the number of repeated visits. 

Significant ecological challenges remain for recovering CLF. Bullfrogs are still readily equipped to re-colonize sites if not actively monitored, as they can disperse up to 16 km. Limitations to CLF re-colonization also still persist, due to chytrid and predicted climate scenarios that may result in decreased water availability and chytrid disease that causes CLF mortality. The successes of CLF recovery have been primarily documented in lower elevations, which may be related to the species’ ability to persist better with chytrid in the warmer climate at lower elevations. 

Although CLF populations can recover quickly following bullfrog eradication if CLF are reintroduced or a remnant CLF population is within dispersal distance, efforts in southern Arizona require ongoing monitoring that must continue to ensure long-term native species recovery. Each study area is ecologically diverse, and therefore management plans are not a one-size works for all approach. Researchers from this team emphasize the importance of initial surveys to census the entire invaded area to identify bullfrog populations before submitting a project budget. The successes of this project may easily be reversed if this project does not acquire long-term funding. The CLF is a heavily managed species, and the successes documented here come from intense monitoring and bullfrog eradication. It is significantly more cost effective to prevent future bullfrogs from invading by maintaining current buffer zones than to have to repeat large-scale removal efforts.

In Arizona, bullfrog control efforts benefit from discrete, generally small bodies of water on public lands. The Frog Team has successfully removed bullfrogs from several complex lotic systems. However, these have been relatively small stream systems in southern Arizona. It may be much more difficult to implement bullfrog control in larger, interconnected riverine systems in other geographies. 

Next Steps

• Maintain Buffer Zones and Target New Areas: The Frog Team will continue bullfrog removal while maintaining and expanding buffer zones within Recovery Units. For example, the Frog Team is expanding the Las Cienegas buffer zone, which will further protect the Las Cienegas metapopulation as well as a smaller CLF population at the Appleton-Whittell Research Ranch and in the Canelo Hills, where the Coronado National Forest has recently received funding to address the bullfrog problem. 
• Multi-Species Approach: Where possible, integrate bullfrog removal with multi-species approaches for ecosystem recovery. For example, CLF recovery supports Gila Topminnnow (Poeciliopsis occidentalis) and Desert Pupfish (Cyprinodon macularius) recovery efforts in several sites, including Las Cienegas National Conservation Area and the Appleton-Whittell Research Ranch. Continue reintroduction of more native aquatic species where bullfrogs have been eradicated.
• Expand Chytrid and Ranavirus Research: AGFD and USFWS are collaborating with a group of USGS researchers, who are analyzing CLF translocation data to determine what factors are important in getting CLF populations to establish successfully following release into a system (e.g. time of year of releases, life stage of frogs, frequency of releases). They are also investigating habitat factors, such as presence of chytrid, ranavirus, bullfrogs, and crayfish, to determine if they are negatively correlated with CLF establishment. The Frog Team has used eDNA to determine the presence of these pathogens and invasive species. In addition, the Frog Team is currently working with a researcher at Northern Arizona University who is investigating seasonal dynamics in presence and infection of ranavirus (specifically Ambystoma tigrinum virus, or ATV) within the range of CLF in Arizona. ATV is present in some CLF populations, but researchers do not know if it infects the species, and if so, its effects on mortality rates. 

Resources

• June 23, 2020 CCAST Webinar
• Final Report (2019): Local Population Dynamics of the Chiricahua Leopard Frog (Rana chiricahuensis) a Federally Listed Frog within the Las Cienegas National Conservation Area in Arizona by David Hall
• Presentation: Chiricahua Leopard Frog Habitat Restoration through Bullfrog Removal by David Hall
• Final Report (2013): Restoring Leopard Frogs and Habitat in the Sky Island Grasslands. 
• Annual Report: Chiricahua Leopard Frog Recovery in Arizona. Arizona Game and Fish Department. Note: Please contact Audrey Owens for a copy of this report.
• IUCN List: 100 of the World's Worst Invasive Alien Species
• Nature Conservancy Article on the Impacts of Bullfrogs on Native Species
• Tucson Herpetological Society’s Page on American Bullfrogs
• Bradley. et al. (2002). “Chytridiomycosis in Native Arizona Frogs.” Journal of Wildlife Diseases 38(1): 206-212.
• Drake et al. (2017). “Graph Theory as an Invasive Species Management Tool: A Case Study in the Sonora Desert.” Landscape Ecology 32: 739–1752.
• Howell, P. et al. (2020). “Survival Estimates for the Invasive American Bullfrog.” Amphibia-Reptilia 1(aop): 1-6.
• Kahrs, D. (2006). “American Bullfrog Eradication in Sycamore Canyon, AZ, a Natural Open Aquatic System.” Sonoran Herpetologist 19(7): 74-77.
• Mims et al. (2020). “Threats to Aquatic Taxa in an Arid Landscape: Knowledge Gaps and Areas of Understanding for Amphibians of the American Southwest.” WIRES Water 7(4): e1449.
• Scheele et al. (2019). “Amphibian Fungal Panzootic Causes Catastrophic and Ongoing Loss of Biodiversity.” Science 363(6434): 1459-1463.
• Rosen, P., and Schwalbe, C. (1995). “Bullfrogs: Introduced Predators in Southwestern Wetlands.” US Department of the Interior, National Biological Service, Washington, DC. 1995: 452-4.
• Rosen et al. (1994). “Introduced Aquatic Inverts in the Chiricahua Region: Effects of Declining Native Ranid Frogs.” In: Biodiversity and management of the Madrean Archipelago: the sky islands of the southwestern United States and northwestern Mexico, 251-261.
• Rosen, P. and C. Schwalbe. (1997). “Using Managed Waters for Conservation of Threatened Frogs.” In Proceedings of Symposium on Environmental, Economic, and Legal Issues Related to Rangeland Water Developments (pp. 180-202).
• The Frog Team
• The American Museum of Natural History Southwestern Research Station
• University of Arizona School of Natural Resources and the Environment
• Coronado National Forest

Contacts

• David Hall, Senior Wildlife Biologist, School of Natural Resources and the Environment, University of Arizona: davidhall31@gmail.com
• Audrey Owens, Ranid Frogs Project Coordinator, Arizona Game and Fish Department: aowens@azgfd.gov
• Cat Crawford, Biologist, Arizona Ecological Services, U.S. Fish & Wildlife Service: cat_crawford@fws.gov
• Philip Rosen, Research Scientist, School of Natural Resources and the Environment, University of Arizona: pcrosen@email.arizona.edu
• Cecil Schwalbe, Ecologist Emeritus, School of Natural Resources and the Environment, University of Arizona
• Blake Hossack, Research Zoologist, USGS Northern Rocky Mountain Science Center, blake_hossack@usgs.gov

Case Study Lead Author

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

Suggested Citation

Koeberle, A., L. (2020). “Landscape-Scale Eradication of Bullfrogs for Native Aquatic Species Recovery in Southern Arizona.” CART. Retrieved from https://www.fws.gov/project/landscape-scale-eradication-bullfrogs.