The arrival and establishment of the American bullfrog in Europe, and its recent appearance in the Ebro Delta, have set off all the conservation alarms because It is one of the 100 most harmful invasive species according to the IUCN. This is no anecdote: it is a large, voracious and highly adaptable amphibian that, when reproducing outside its native area, triggers a cascade of ecological impacts that affect fauna, flora and ecosystem processes.
In Spain, after detecting compatible tadpoles and confirming their genetic identity, governments and conservation organizations have activated emergency control measures. At the same time, it is recalled that the bullfrog, included in the Spanish Catalogue of Invasive Alien Species and on the European list, cannot be marketed or released, and that their presence can lead to serious biodiversity problems and, in certain circumstances, public health problems.
What is the bullfrog and why is it a threat?
Native to North America, with a distribution that extends from parts of Mexico to Canada, the bullfrog (Lithobates catesbeianus) It has expanded to Western Europe and multiple regions in South America and AsiaIn fact, it is already present on four continents and in more than 40 countries, a clear indicator of its enormous colonization capacity when it finds favorable conditions.
Its size stands out among amphibians: different sources mention individuals that exceed 20 cm and weigh around 600 grams, and there are also references to specimens that reach up to 460 mm and nearly 1 kilogram; even tadpoles can measure around 17 cm. In addition, it has very visible circular eardrums on both sides of the head and a green or grayish-brown mantle with dark spots; its deep vocalization, described as a deep baritone (jug-o-rum), It resembles the mooing of a cow and gives rise to its common name.
In its preferred habitat, it inhabits still or slow-flowing freshwater: ponds, lagoons, marshes, and swamps. Outside its native range, it shows an affinity for altered or impoverished environments, where competition and predator pressure are lower. Added to this flexibility is the ability to travel long distances, exhibit resistance to certain diseases, and, as an adult, It has virtually no effective predators, which increases its invasive success.
The diet is overwhelmingly opportunistic. This amphibian swallows small vertebrates and invertebrates: snakes, fish, small birds, rodents, other frogs, tadpoles, bats, worms, insects, and crustaceans. It hunts by stalking, taking advantage of the night to ambush any prey it can fit with its powerful hind legs and large mouth. Males are territorial and aggressive in defending their space, while females, it has been reported, can be slightly larger; overall, their predatory behavior and competitiveness explain much of its impact.
Documented ecological effects range from habitat modifications to trophic alterations. Changes in the hydrological regime, nutrient and mineral dynamics, light availability, and even parameters such as salinity and pH have been described. An effect on vegetation structure and the composition and relative abundance of native species has also been observed. as well as deviations from normal succession patterns plants that alter the ecological trajectory of the invaded systems.
- Impacts on physical and chemical processes of water: nutrients, light, pH and salinity may be altered.
- Modifications of community structure and vegetation: changes in abundance and species replacements.
- Direct pressure from predation and competition on amphibians and other native vertebrates, including native tadpoles.
- Health risk due to being a carrier of pathogens such as fungi, bacteria and viruses, with population-scale effects.
A critical point is its role as a reservoir for pathogens. It can harbor and transmit the fungus. Batrachochytrium dendrobatidis, identified worldwide in the decline of multiple amphibian species. This frog can also carry various bacteria and viruses, so when introduced into systems where the local fauna has not co-evolved with these agents, the risk of outbreaks and mass deaths increases in already vulnerable indigenous populations.
Beyond biodiversity, a potential risk to humans has been noted when consuming specimens from contaminated waters. In these scenarios, the bioaccumulation of harmful substances can have implications for food safety, an aspect that underscores the importance of The problem goes beyond the strictly ecological sphere when this species proliferates in degraded environments.
The situation in Spain: Ebro Delta, entry routes and response
In the Ebro Delta, during biological studies using a green filter in the left half of the delta, several tadpoles with compatible traits were detected in mid-June, which were later genetically and morphologically confirmed as bullfrogs. This marked the first time this species has been reproducing in the wild in Spain, a qualitative leap that, according to experts, increases the urgency of acting before the problem gets out of control.
Reporting the detection to the Ministry for Ecological Transition enabled the activation of the early warning system for invasive alien species and coordinated support for the Catalan government, which is responsible for management. From the very beginning, sampling and capture have taken place: one campaign managed to collect 406 specimens (all tadpoles at various stages, including some in metamorphosis), and another official update reported that around 600 tadpoles had been removed. At least four adults were located through monitoring. clear signs of an incipient population in the enclave.
Among the most immediate measures adopted was the closure of the water inflow and outflow routes of the green filter to confine the core. At the same time, work is underway to build an effective perimeter to prevent the escape of individuals, and once the enclosure is secured, captures will be intensified and hydrological methods will be evaluated to prevent their spread. These measures are complemented by a study of the hydrological functioning of the area and a continuous monitoring program. key to sizing the focus and assessing the effectiveness of control.
- Adult control through barriers and active capture, prioritizing the elimination of breeders.
- Monitoring in the affected area and its periphery to detect leaks or satellite nuclei, with periodic sampling.
- Monitoring of hydrological parameters of the system and adjustments in water management, including confinement.
- Evaluation of alternatives for the elimination of larvae within the green filter, within an eradication plan.
Technicians agree that the detection has occurred at an early and highly localized stage, which allows for eradication if action is taken decisively and quickly. SEO/BirdLife has asked the authorities to immediately implement the necessary set of measures, insisting that Every week counts to prevent the bullfrog from dispersing. to nearby habitats and gain ground.
This is not the first time the species has been observed in the country, but it is the first with proven reproduction. An individual was documented in 1999 in the Collserola Natural Park and another specimen in 2012 in the Ebro Delta itself; however, those findings were isolated cases. The difference now is substantial due to the presence of confirmed tadpoles and singing adults. It involves an active life cycle in the environment, which multiplies the risk of establishment.
How did it arrive? The most common routes of entry are captive breeding for consumption (ranch farming) and the pet trade. Between 1987 and 1990, farms were established in parts of Spain such as Cáceres, Madrid, and León, and although the inclusion of the species in the Spanish Catalogue of Invasive Alien Species in 2013 halted commercialization, illegal trade has not completely disappeared. Added to this are irresponsible releases or involuntary escapes, which the cited literature links as frequent causes of introduction. It would not be unheard of for individuals to release specimens when they became unmanageable due to size..
The regulatory framework is clear. The bullfrog is listed in Royal Decree 630/2013 (Spanish Catalogue of IAS) and in Regulation (EU) 1143/2014 on the prevention and management of invasive alien species, and is also among the IUCN's 100 worst invasive species. The public debate has pointed out that legal changes that facilitate releases for fishing or hunting purposes can generate dangerous incentives if not rigorously managed, a point on which various voices have called for reflection due to the potential for generate imbalances that are difficult to reverse.
The administrations are emphasizing coordination and applied science: genetic and morphological sampling, hydrological measures to encapsulate the outbreak, selective captures, and perimeter closures. SEO/BirdLife, for its part, has released technical reports and recommendations, while public bodies have disseminated reference documentation. For those who need further information, there are resources such as the Generalitat's control and eradication report and the ministerial technical sheet: management document y species sheet, that help to unify intervention criteria.
In addition to the technical and legal aspects, social responsibility is also required. Any sightings should be reported to environmental authorities; pets and tadpoles should never be released into the wild; and, in sensitive aquatic environments, extreme caution is advised to avoid inadvertently transporting organisms between bodies of water. These guidelines, as basic as they may seem, make a difference in systems as fragile as wetlands.
The available evidence paints a picture of a species with enormous ecological plasticity, a boundless appetite, and the ability to alter key processes in aquatic environments. Early detection in the Ebro Delta and the responses already underway offer a window of opportunity to curb it, but that window will not remain open indefinitely: If the bullfrog disperses and consolidates metapopulations, control costs and ecological damage will skyrocket.Therefore, acting quickly, adjusting water management, maintaining vigilance, strengthening the compliance framework, and maintaining control pressure are the most sensible levers today to protect local biodiversity and the health of aquatic ecosystems.