This species is known only from the headwaters of Arroyo Valcheta in Somuncurá Plateau, an isolated basaltic plateau in the Río Negro Province, Argentinean Patagonia. It has an altitudinal range of 500-800 m asl (Basso et al. 2012 and references therein in Vaira et al. 2012). Given exhaustive surveys conducted between 2013-2016 to determine its distribution, it is considered to be a genuinely restricted species (Velasco et al. unpubl. data). Although the current understanding of this species' distribution is better than in the past and extends its range by 160%, it is still very small (Velasco et al. unpubl. data, F. Kacoliris pers. comm. December 2015), with an estimated spatial occupation along the stream and its shores of 1.8 km2 and an estimated area of occupancy (AOO) and extent of occurrence (EOO) of 32 km2. It is considered to occur in one threat-defined location, based on the ubiquitous presence of introduced salmonids in the region that occur throughout almost the entire stream in which the species is found (F. Kacoliris and M. Velasco pers. comm. February 2016).
Habitat and Ecology
It is an almost entirely aquatic species, although it can surface and sit on rocks or vegetation within streams (F. Kacoliris pers. comm. December 2015). Within the headwaters it can be found in shallow, slow-moving water and densely vegetated sections (F. Kacoliris pers. comm. December 2015). These sections are mostly close to the permanent thermal springs that originate the main stream, although they also occur elsewhere in the stream (F. Kacoliris pers. comm. December 2015). It is not present in modified habitats and breeds via larval aquatic development.
Over the course of three surveys in 2014, a total of 40 sites were visited across the species' range. Subsequently, an additional 160 new sites were surveyed, totaling 200 sites covering the four branches of the Valcheta stream's headwaters, and additional exhaustive surveys were also conducted beyond these sites in 2013-2016 (F. Kacoliris and M. Velasco pers. comm. February 2016). Approximately 39% of the visited sites were occupied (F. Kacoliris pers. comm. March 2016) and, while one of the sites where the species was thought to be locally extirpated was found to have one frog, another site where it was formerly common produced no records. This could be due to the construction of a small dam in 2009, after which no individuals have been recorded at the site (Velasco et al. unpubl. data). The lack of subsequent records suggests a population decline and local extirpation at this site. Other sites had all stages of development (eggs, tadpoles, juveniles and adults) (Velasco et al. unpubl. data). Given the species' almost completely aquatic nature, very low dispersal ability, and the fact that it is restricted to only certain parts of the stream, its population is considered to be severely fragmented following IUCN definitions (F. Kacoliris pers. comm. December 2015).
The greatest threats are introduced salmonids (i.e. Oncorhynchus mykiss, Salvelinus fontinalis), and sheep and goat livestock, with the latter degrading the species’ habitat and polluting it by defecating in the water (Basso et al. 2012 in Vaira et al. 2012, Velasco et al. unpubl. data). The building of a dam appears to have coincided with the loss of a subpopulation (Velasco et al. unpubl. data). In addition, sites where the frog was not recorded were found to have exotic trees, whose roots reach the springs and stream shores (Velasco et al. unpubl. data). Finally, the chytrid fungus Batrachochytrium dendrobatidis has recently been found in this species (Arellano et al. unpubl. data).
The range of the species is within the Somuncurá Provincial Reserve. Prior to 2012, there was limited management of this Reserve (Basso et al. 2012 in Vaira et al. 2012). However, since 2013 the Wild Plateau Initiative has been working on both in-situ and ex-situ efforts aimed at protecting this species, and it has an agreement in place with the local protected area authorities (F. Kacoliris, M. Velasco pers. comm. February 2016).
Additional recommendations include adopting urgent and concrete conservation measures that encompass the regulated use of streams and protection of headwaters in the plateau (Basso et al. 2012 in Vaira et al. 2012). It is suggested that the existing management plan for the area be implemented (Basso et al. 2012 in Vaira et al. 2012), as well as the continuation of the activities of the Wild Plateau Initiative, inclusive of the development and implementation of a Species Action Plan (F. Kacoliris, M. Velasco pers. comm. February 2016). A conservation breeding centre has been established and is being implemented, starting with this species (Kacoliris and Williams 2014, F. Kacoliris, M. Velasco pers. comm. February 2016).
More information is needed on this species' response to specific threats and a Population Viability Analysis would be helpful for management purposes.
Red List Status
Critically Endangered (CR)
Listed as Critically Endangered because its extent of occurrence (EOO) is estimated to be 32 km2, it is known from one threat-defined location and its population is considered to be severely fragmented, there is continuing decline in the extent and quality of its aquatic habitat on the Somuncurá Plateau, Argentina, and one subpopulation seems to have disappeared due to the construction of a dam.
IUCN SSC Amphibian Specialist Group 2016. Pleurodema somuncurense. The IUCN Red List of Threatened Species 2016: e.T20372A85948443. http://dx.doi.org/10.2305/IUCN.UK.2016-1.RLTS.T20372A85948443.en .Downloaded on 14 December 2018