Case Study 4 – American bullfrog

Case Study 4: American bullfrog

(Lithobates catesbeianus)

Description

The American bullfrog is a large, pond-breeding amphibian native to North America and is considered one of the world’s most harmful invasive species. It was introduced to Belgium in the late 1990s. As a gape-limited predator, its diet is limited only by the size of its mouth; it voraciously consumes invertebrates, other frogs, turtles, birds, and even small mammals. It is highly athletic, capable of leaping up to 1.8 metres and swimming at speeds of 7.2 kilometres per hour.

Habitat

This species primarily breeds in permanent freshwater bodies such as wetlands, ponds, lakes, and slow-moving rivers, but can be found in all types of freshwater ecosystems. It thrives in human-disturbed environments and often takes up residence in backyard or agricultural ponds. In Flanders, Belgium, the species has spread across an area of approximately 360 km2, forming eight distinct metapopulations primarily within river valleys like the Grote Nete.

Impacts

Biodiversity Loss: Due to its size and breeding capacity, it outcompetes and displaces native amphibians and salamanders. We have empirically shown that bullfrogs noticeably impact native amphibian community structure. Moreover, native species with a matching phenology with bullfrogs were found to be particularly vulnerable to their impacts.
Disease Vector: It carries and transmits the lethal chytrid fungus, which has caused massive amphibian die-offs globally. We found that there is a strong correlation between the site-level presence and density of bullfrogs and that of chytrid fungi.

Prevention and Control Actions

Management of the American bullfrog is complex due to its large non-native distribution, ability to spread through interconnected river corridors and the presence of inaccessible private ponds. The GuardIAS project is tackling this through several specialized strategies:

Genetic Mapping: Researchers use species-specific eDNA (environmental DNA) and droplet digital PCR (ddPCR) to precisely map the distribution and abundance of the species. Beyond its main population in the Grote Nete River Valley, bullfrogs were found to spread from a satellite population to the Netherlands, which has been bullfrog-free for multiple years.
Site Prioritisation: By measuring eDNA concentrations, teams can identify specific reproductive ponds and sites accommodating bullfrogs at the highest densities to focus management where it is most effective.
Holistic Management: The project combines active removal—such as trapping with fyke nets—with the introduction of native predators to increase the natural resilience of the ecosystem. A large-scale biocontrol experiment involving 20 ponds has been running for three years. These results will be out soon. Moreover, we tested whether aquatic structural complexity provided by vegetation could buffer the impact of bullfrogs on native amphibians, but we found no evidence.
Success Evaluation: By comparing eDNA concentrations before and after interventions, the project can estimate the proportion of the population removed and identify factors that help or hinder control.

Useful links

Download the factsheet
Paper that shows bullfrog impacts in study area: https://doi.org/10.1002/eap.3017
Paper that shows eDNA-based management evaluation: https://doi.org/10.1002/edn3.301
GuardIAS paper that shows a correlation between bullfrog presence and density and that of chytrid fungus:
https://doi.org/10.1007/s10530-025-03648-8
GuardIAS paper that shows that aquatic structural habitat complexity did not mediate bullfrog’s impact on native amphibians: https://doi.org/10.1016/j.gecco.2025.e03962

Meet the case study team

Research Institute for Nature and Forest (INBO): Teun Everts and Rein Brys

Case study contacts:

Teun Everts, teun.everts@inbo.be
Rein Brys, rein.brys@inbo.be

Anastasija Zaiko

“This project is not just about managing invasive species today—it’s about developing a proactive approach that anticipates future ecological challenges, ensuring robust ecosystems and sustainable marine environments for generations to come”

Sergej Olenin

“For over 35 years, since I first began studying non-indigenous species, I have dreamed of tools that could reveal the ‘Big Picture’ of the transfer, spread, and impacts of invasive aquatic species. Thanks to advances in new technologies, the GuardIAS project now offers an outstanding opportunity to make that vision a reality.”

There are 7 Work Packages (WP) focusing on different stages of invasive alien species ‘invasion’ in the GuardIAS project. Each Work Package is a distinct project area, but all are interlinked through GuardIAS as a whole.