Biological invasions are one of the main causes of biodiversity loss worldwide, and they pose a particularly serious threat to amphibians and aquatic ecosystems. In Uruguay, one of the most emblematic cases is the American bullfrog (Aquarana catesbeiana), a species native to eastern North America that was introduced for aquaculture and is now established at several locations across the country. Its large size, high reproductive capacity, and extremely broad diet make it one of the most problematic invasive amphibians in the world.
The idea for the Bullfrog Project in Uruguay arose in the early 2000s, initially as a concern tied to conservation and to the risk posed by the bullfrog farms that had operated in the country since the late 1980s. At that time, knowledge about biological invasions was still very limited in Uruguay, and there were few regional precedents for how to address this kind of problem through research and environmental management. What we did know clearly was that the bullfrog represented a very significant potential threat to the conservation of native amphibians.
The start of the project was marked by a perception that proved correct over time: for those who understood how the frog farms worked and the difficulties of keeping the animals in captivity, the establishment of wild populations seemed inevitable. The vulnerability of the rearing systems, the ease of escape, and the lack of sustained controls all suggested that sooner or later the species would appear in natural environments. The detection of the first wild bullfrog foci quickly confirmed those suspicions. We went on to find five small populations, all close to where some of the farms had operated. Two of these populations failed to establish, and today only three persist, in the localities of Los Cerrillos (Canelones Department), San Carlos (Maldonado Department), and Aceguá (Cerro Largo Department).
From the outset, the project was conceived not only as a line of academic research but also as a long-term conservation and monitoring initiative. This meant reconstructing much of the history of the species' introduction in Uruguay, identifying former frog farms, traveling through rural localities, and building relationships with people who had taken part in those production ventures. Contrary to what one might assume, many of the former farmers showed a strong environmental sensibility and shared concerns about the risks the species posed. The aquaculture experience had in many cases been economically negative, and several producers actively collaborated, providing key information to locate sites and to understand the history of the invasion.
As the surveys began, another reality also became clear: much of the local community was unaware both of the invasion problem and of the enormous diversity of native amphibians present in the country. This led the project to incorporate outreach activities, talks, and communication aimed at schools, neighbors, and local governments. Over time, rural residents began to become essential allies in the monitoring.
One of the most interesting aspects was the role of the soundscape. Many people immediately recognized the deep, powerful call of the bullfrog, very different from that of native amphibians and foreign to the usual sound of Uruguayan wetlands. Its vocalization, comparable to the distant bellow of cattle, was unmistakable to those who knew these environments well. Even if they did not know the species, they knew that "there was a different frog." That perception made it possible to detect new foci, and showed that the people who know these environments well often notice ecological changes long before they are recorded by formal monitoring systems.
Over the years, Aceguá — in the northeast of the country, on the border with Brazil — became one of the project's main work sites. This region has very particular features within the Uruguayan landscape: a relatively well-preserved range of hills, with high biological diversity and a large number of small permanent water bodies, many of them tajamares — small livestock reservoir ponds — associated with extensive cattle-ranching systems. Unlike many studies carried out in lagoons, natural wetlands, or urban environments, much of the invasion in Uruguay takes place across networks of small ranch tajamares distributed throughout the rural landscape.
Although many of these water bodies were originally built to supply water for cattle, over time they develop a complex ecological structure, becoming surrounded by native vegetation and fauna until they function as true reservoirs of biodiversity. Aceguá lies in an area of hills, so its tajamares host a very diverse and distinctive amphibian assemblage, different from that of other regions of the country. The sampling nights in Aceguá are part of the project's identity: long shifts traveling among numerous tajamares, recording vocalizations, abundances, and changes in the aquatic communities and the anuran assemblage over time.
It was precisely in Aceguá that one of the project's most symbolic moments occurred. During the 2012 surveys, the team had selected invaded and uninvaded water bodies to begin new ecological comparisons. On arriving in the field, however, they began to find individuals and hear bullfrog calls at sites that had historically appeared free of the invasion. That moment produced a mix of surprise, concern, and frustration. For the first time it was clear that the invasion was no longer confined to a few isolated sites but was beginning to actively spread across the landscape. That year marked the shift from an apparently stable phase to a phase of active expansion. Over time, the monitoring made it possible to show that the invasion was advancing steadily, reaching an annual increase of close to 7.7% in the number of invaded water bodies.
Over these two decades, the project grew and diversified. With more than twenty years of continuous monitoring, it has become one of the longest-running studies of invasive amphibians in South America. What began as a survey of distribution ended up becoming a research platform on invasion ecology, emerging diseases, food webs, and amphibian conservation. Undergraduate and graduate theses, national and international collaborations, and multiple lines of work associated with the invasion were developed.
Among the main findings is evidence of negative impacts on native amphibians and aquatic communities. The studies carried out showed declines in the richness and abundance of native species, alterations in food webs, and important changes in the structure of invaded ecosystems. Recent models suggest that some systems could lose up to 40% of their amphibian species and around 50% of their abundances in barely a decade of invasion. The effects were not seen only in adult amphibians. Invaded tajamares contain fewer tadpoles of native species and show profound modifications in their aquatic communities. In addition, some fish species reach higher abundances, generating changes in trophic relationships and cascading effects on ecosystem functioning. The project also made it possible to study the bullfrog's role as a potential vector of emerging diseases, including the chytrid fungus Batrachochytrium dendrobatidis, which is associated with the global decline of amphibians. During the years of study, the project also contributed significantly to increasing knowledge of poorly known local amphibian species in Uruguay, such as Ololygon aromothyella and Dendropsophus minutus.
However, the Bullfrog Project in Uruguay did not produce only scientific papers. It also helped train new generations of researchers and conservationists. Many of us who are part of the team today began by taking part as undergraduate students and later developed our own theses and lines of research linked to the project. Over the years, university courses, outreach workshops, and training activities related to conservation, monitoring, and the management of invasive species were carried out.
One of the main lessons of the project was confirming that long-term studies are indeed possible in Uruguay, even in contexts with limited resources for research. The continuity of the work was made possible by integrating researchers, students, local communities, municipalities, and institutions that allowed the monitoring to be sustained year after year. The bond with the local population proved especially important. In many cases, residents not only collaborated by providing information but ended up becoming deeply involved in the work and in conservation.
That integration was reflected in a conversation that took place years ago with a mayor of Aceguá. During a discussion about the difficulties of getting environmental authorities to prioritize the problem, a reflection arose about the distance between the country's capital, Montevideo, and the interior. When someone from the team mentioned that we were also researchers "from Montevideo," the response was immediate: "No, you're from here." After so many years of work, monitoring, and coexistence with the local community, that phrase was taken as one of the most valuable forms of recognition the project could receive.
At the same time, these two decades also left a strong sense of frustration. The monitoring made it possible to detect the spread of the invasion early and to generate solid scientific information about its ecological impacts and possible management strategies. Yet turning that knowledge into concrete control actions proved much more difficult. Although the bullfrog was institutionally recognized as a priority species and some management plans moved forward, sustained and coordinated action on the ground has been scarce and limited.
This situation is not unique to Uruguay or to the bullfrog case. It reflects a frequent difficulty in conservation: the gap between the timescales of science and of local communities, and the response times of state structures. Even so, the project holds to the conviction that there are still opportunities to act and to limit the advance of the invasion, if greater coordination can be achieved among research, environmental management, and local participation. We also see great potential in developing tools through international collaboration among researchers.
After two decades following the spread of the bullfrog in Uruguay, the project continues to grow and adapt. Beyond the invasive species itself, the work has allowed us to better understand native amphibians, the landscapes of tajamares, and the complex relationship between science, conservation, and human communities. It has also left a clear lesson: monitoring sustained over time not only generates ecological knowledge but builds trust, lasting bonds, territorial memory, and the collective capacity to confront some of the most complex environmental challenges of our time.
Acknowledgments
We would like to recognize Vida Silvestre Uruguay, the Museo Nacional de Historia Natural (Uruguay), the Agencia Nacional de Investigación e Innovación (ANII), the Rufford Foundation, and Carolina Gómez from the Asociación de Cultura y Deporte de Aceguá for their support of this work. We also wish to thank the landowners, local residents, educators, and local authorities from the study areas whose collaboration and commitment have been essential to sustaining this long-term monitoring effort.