While fossil remains of toads (Bufo spp.) have been recovered from the Maltese Islands, the only living amphibian that has ever been reported from the islands is the Painted Frog, Discoglossus pictus. This species is highly adapted to live in the semi-arid Maltese environment, where it manages to thrive in spite of a dearth of the freshwater that it requires to complete its life-cycle.
While other species of Discoglossus occur in the Mediterranean, the Painted Frog has a very limited distribution; one subspecies (D. pictus pictus) occurs in Sicily and the Maltese Islands, and another subspecies (D. pictus auritus) occurs in Algeria and has been introduced to parts of southern France and NE Spain.
Against this background, it came as a great surprise, therefore, to discover a thriving population of a different species of frog in the freshwater pool at Ta' Sarraflu in Gozo. This discovery was made after one of us (AS) investigated strange calls that were being heard from the pool area. These peculiar calls were first heard some four years ago but were dismissed as those of a water bird, since they were similar to the sounds that a number of water birds make. However, more careful study of these calls showed them to be different from those of birds and similar to those that some frogs make.
Investigation of the water in the pool revealed the presence of the Painted Frog, but also of what appeared to be a larger version, however, these larger frogs proved very difficult to observe since they disappear very rapidly under water and into the dense reed beds in the pool at the slightest movement. It was only recently and after a lot of hard work that we managed to take photographs of this animal and to record its calls.
It was immediately obvious that these frogs were not Painted Frogs. They had all the characteristics of a completely different group of frogs belonging to the Ranidae family and known as the 'Water Frogs' (the Painted Frog belongs to the Discoglossidae family).
From their size, colour and morphology the species at Ta' Sarraflu seems to belong to the southern European/North African group of the genus Rana, however this is a very difficult group to identify and we are still investigating what the species is.
At least six similar-looking species of this group occur in southeast Europe, some of which hybridise where their ranges overlap. Identifying these frogs is rendered more complex as some species have been transported outside their range and have mixed with other species to give populations that are partly composed of hybrids and partly of the parent species. In many cases, certain identification is not possible from appearance alone, and molecular or call analyses are required.
What is certain however, is that the population of water frogs at Ta' Sarraflu is not native to the Maltese Islands but has been introduced, possibly sometime in the late Nineties. Given their larger size, different coloration and they very loud noises these animals make, especially in spring and summer when the males establish territories and court females, it is not likely that these animals would have been overlooked, especially given that many naturalists, including ourselves, visit L-Ghadira ta' Sarraflu regularly.
The presence of an alien species of Rana at Ta' Sarraflu is worrying. L-Ghadira ta' Sarraflu is a very important habitat in the local context since it is one of very few pools where natural freshwater accumulates and persists throughout the year, even during the hot summer months, when most other freshwater dries up.
For this reason, a significant number of plant and animal species that require a constant supply of freshwater throughout the year occur there and are overall very rare in the Maltese Islands, since their habitat is rare. This is why the pool at Ta' Sarraflu has been scheduled as a Level 1 Area of Ecological Importance and Site of Scientific Importance under the Development Planning Act of 1992 (Government Notice 288 of 1995), while the entire western coastal area of Gozo, including L-Ghadira ta' Sarraflu, has been declared a Candidate Special Area of Conservation of International Importance in the Flora, Fauna and Natural Habitats Protection Regulations, 2003 (LN 257 of 2003). These regulations transpose the requirements of the European Union's Habitats Directive to local legislation.
What the impact of the alien Rana on the biota and ecology of the pool has been, or will be, is not known, but where species of Rana have been introduced into environments where they did not occur before, the native amphibian fauna has generally suffered, since Rana eat the tadpoles and froglets of smaller species.
Adult native Painted Frog and alien Rana certainly co-exist at Ta' Sarraflu, but the interaction between the two, especially predation by Rana on larval and juvenile Discoglossus is possible. It could also be that while the population of Rana is confined to the pool because of its requirements for water, that of Discoglossus is replenished from the surrounding environment since the native frog is able to transverse large areas of arid ground. The impact of Rana on Discoglossus and on the other pool biota needs to be studied, especially given the status of L-Ghadira ta' Sarraflu as a protected area.
What is important is that the alien Rana is not spread to other habitats. We therefore make a strong appeal to persons who keep alien species as pets or as a hobby, not to dispose of their unwanted animals in the environment. After habitat destruction, the introduction of alien species is the largest threat faced by local biota. At Ta' Sarraflu alone, apart from the alien Rana, somebody has introduced a population of Mosquito Fish (Gambusia), that has devastated the insect fauna, and two terrapins, while in the past there were also goldfish there.
While it is easy to introduce alien species, it is very difficult to eradicate them. In the same way that we would not dream of 'contaminating' our cultural heritage by, for example, introducing aluminium fixtures in historic buildings, we should not 'contaminate' our natural heritage by introducing alien species.
The authors thank Esther Schembri, Jeffrey Sciberras and Jonathan Abela for help with fieldwork, and Dr Fred Kraus (Hawaii) and Professor Ulrich Sinsch (Germany) for taxonomic advice.
Arnold Sciberras is an amateur herpetologist and Patrick J. Schembri is co-author (with Alfred E. Baldacchino) of the book Amfibji, rettili, u mammiferi fil-gzejjer Maltin. [Sensiela 'Kullana Kulturali' No. 39] Pubblikazzjonijiet Indipendenza.