Coastal populations of natterjack toad more threatened than previously thought (2/13/2008)

natterjack Toad

The genetic study carried out on the threatened populations of the natterjack toad (Bufo calamita) in the Basque Country (Txingudi Bay in Gipuzkoa and Azkorri strand in Bizkaia) concluded that it is essential to effect a management plan involving measures regarding both the species and its habitat. For the population in the Basque province of Gipuzkoa the primordial objective would be to keep the breeding sites connected (the various nuclei) and in neighbouring Bizkaia the task would be to increase the size of the population and make the habitat suitable by means of enhancing the breeding sites. The genetic study carried out on the coastal populations indicate that they should be conserved with the resident toads from each of their respective sites.

The natterjack toad is extant in practically the whole of the Iberian Peninsula. Nevertheless, the scant nuclei on the Cantabrian coast have got serious problems as regards their conservation. Two have been located along the coast of the Basque Country - one in the salt marshes of Txingudi, Gipuzkoa and the other on the beach at Azkorri, Bizkaia. Despite the precarious situation of the latter, only the Txingudi site appears in the Basque Catalogue of Threatened Species, under the category of "Vulnerable" (when the Catalogue was drawn up, the population in Bizkaia was unknown.

With the goal to work towards their recovery, the Aranzadi Society's Herpetology Section undertook the monitoring of both populations (from 2004 in Gipuzkoa and from 2005 in Bizkaia), and has just completed their genetic and population study (their bone chronology analysis - essential when carrying out a correct Conservation Plan).

The results show that the populations of Gipuzkoa and Bizkaia belong to different lines (groups with mated individuals) and so have different origins. Two lines have been found on the Iberian Peninsula and, curiously, each of the two Basque coastal populations belong to one each of these. The population in Bizkaia belongs to the line widely extended throughout the Peninsula and that of Gipuzkoa, on the other hand, is found in the north of the peninsula. During the last glaciations (some 20,000 years ago) the Iberian Peninsula functioned as a refuge for many species, the natterjack toad amongst them and, when the glacial age came to an end, the species relocated to the north of Europe, giving rise to its current distribution. Thus, the colonisation of the species towards Gipuzkoa and Bizkaia took place through different routes and by individuals belonging to different lines.

It has been observed that these coastal populations are genetically different from the rest of the populations - they are unique within the distribution of the species. They are found geographically and genetically isolated and, given this isolation, there has been no genic flow (migration of individuals) and, thus, genetic variability of the population is low. Moreover, in the case of Bizkaia, given that the number of individuals within the population is small, the level of genetic impoverishment increases considerably and, therefore, the risk of disappearance also. It should be pointed out that the two coastal populations are significantly different from each other, and so should not be reinforced by introducing individuals from the neighbouring population as this may eliminate their genetic identity.

As regards the populations analysed in Alava, Navarre and the Rioja, they are not greatly different genetically although genetic diversity was found to be high within the populations themselves. The river Ebro and its basin have played an important role in this area and have functioned as a natural corridor, ensuring communication between populations that are geographically distant.

Genetic study

The aim of the study was to evaluate the degree of isolation existing between the coastal populations of the Basque Country and the populations in the rest of the Iberian Peninsula. The origin of the populations of Bizkaia and Gipuzkoa has been identified, i.e. where they come from and with which other populations of the species they relate evaluated (they are genetically more similar). To this end, comparisons (of mitochondrial DNA) were made amongst individuals from the Basque Country, Navarre, Rioja, Asturias, Catalonia, Castille-León, Castille-La Mancha, Extremadura and Andalusia. Moreover, the similarities between the coastal populations and the neighbouring populations in Alava, Navarre and Rioja through the study of their diversity and genetic structuring (nuclear DNA), were evaluated. This information enables determining the existence or otherwise of genic flow, i.e. of the existence of migrating individuals amongst the populations, and is highly useful data for planning the population boosting campaigns (introducing individuals).

Situation in Gipuzkoa

The Txingudi Bay population presents a high level of fragmentation, caused mainly by building in the area. Thus, for example, the old sub-population of the Hondarribia back strand has disappeared, as have others in the urban centres in both Irun and Hondarribia (Juncal y Puntal respectively). Nevertheless, and taking into account the high capacity of the species for dispersion and colonisation, new population nuclei could appear with time in lands adjoining the existing sites. A clear example of these are the Zubieta meadows (Hondarribia), a place where a new population nucleus has been detected. Current distribution, apart from this site, is as follows: Plaiaundi, Osinbiribil, Kostorbe Alde and Arbes in Irún and the salt marshes of Jaizubia in Hondarribia.

The Txingudi Bay population has not shown any clear tendency in the past four years. While some nuclei consolidate over time, others find themselves in a delicate situation. A clear example of this was the 2007 campaign, the most successful as regards reproduction of the species, with more than 200 egg-laying sites counted and each nucleus increasing their numbers. Nevertheless, the number of individuals collected in two (Jaizubia and Kostorbe-Alde) of the six existing nuclei has decreased.

Threats

The Txingudi Bay area is highly affected by human presence and the environment greatly altered. The announcement of new urban projects and infrastructure could seriously compromise the survival of the only population in Gipuzkoa of the natterjack toad, which is a fragmented one and the main threats to which are: