Herpetological Conservation and Biology 6(3):455−464. Submitted: 14 October 2010; Accepted: 14 November 2011; Published: 31 December 2011. 455 POPULATION STATUS AND NATURAL HISTORY NOTES ON THE CRITICALLY ENDANGERED STREAM-DWELLING FROG CRAUGASTOR RANOIDES (CRAUGASTORIDAE) IN A COSTA RICAN TROPICAL DRY FOREST HÉCTOR ZUMBADO-ULATE 1,2 , FEDERICO BOLAÑOS 1 , BEATRIZ WILLINK 1 , FERNANDO SOLEY-GUARDIA 3 1 Escuela de Biología, Universidad de Costa Rica. San Pedro, San José, Costa Rica, e-mail: hugozu1@yahoo.com 2 Asociación para la Conservación y el Estudio de la Biodiversidad, San Pedro, Costa Rica 3 Department of Biological Sciences, Macquarie University, Sydney, New South Wales 210, Australia Abstract.The Craugastor rugulosus species series has been one of the most affected clades by the decline of amphibian populations in Mesoamerica. These stream-dwelling frogs are threatened at all altitudinal ranges throughout their distribution. Craugastor ranoides is categorized as Critically Endangered on the IUCN Red List due to the disappearance of populations in the highlands and lowlands of Costa Rica. Currently the species is only found on the Santa Elena Peninsula. Additional ecological and natural history studies are necessary to formulate conservation plans for this species, which should include captive breeding programs and continuous monitoring of wild populations. We conducted a study of density, habitat use, and morphometrics of C. ranoides in three streams on the Santa Elena Peninsula Guanacaste, Costa Rica, during two consecutive dry seasons. The density of adult frogs and the probabilities of detection were similar during both dry seasons but we found differences in both parameters between streams. Counts of juveniles and subadults differed between seasons and between streams. Stream sector (50-m length) occupancy was approximately 80% during both dry seasons. We found most frogs motionless on boulders, but juveniles also frequented leaf litter. Sexual dimorphism was found in snout-vent length, mass, and tympanum diameter in subadults and adults. This study establishes a baseline for further monitoring of wild populations. Additional research and monitoring are necessary to detect possible changes in abundance and potential decline of these populations, which might be the only ones remaining in Costa Rica. Key Words.amphibian declines; Craugastor ranoides; Craugastor rugulosus group; endangered species; tropical dry forest INTRODUCTION Approximately 33% of the populations of existing species of amphibians have declined and are currently under threat, and many other species have become extinct (Stuart et al. 2004). Numerous cases of population declines and extinctions have occurred in undisturbed sites and protected pristine habitats worldwide (Crump et al. 1992; Fellers and Drost 1993; Lips 1998). Such declines have been associated with various factors, including disease and global climate change (Young et al. 2001). Fortunately, in the last few years some supposedly extinct species reappeared in several regions of the world (e.g., Retallick et al. 2004; La Marca et al. 2005) including Costa Rica, where populations of the following presumably extinct species have been reported: Isthmohyla angustilineata (Hylidae; Nishida 2006), Atelopus varius (Bufonidae; Rainmaker Conservation Project. 2007. Harlequin Toad. Available from http:// www.rainmakercostarica.org [Accessed 10 May 2011]), Incilius holdridgei (Bufonidae; Abarca et al. 2010), Duellmanohyla uranochroa and Isthmohyla rivularis (Hylidae), Lithobates vibicarius (Ranidae), Silverstoneia nubicola (Dendrobatidae; Bolaños 2009) and Craugastor fleischmanni (Craugastoridae; Ryan, M.J., F. Bolaños, and G. Chaves. 2011. Museums help prioritize conservation goals. Available from http:// www.sciencemag.org/content/329/5997/1272/reply#sci_ el_13658 [Accessed 10 May 2011]). The dramatic decline of amphibian populations, along with the rediscovery of species presumed to be extinct, emphasizes the necessity to prioritize field monitoring of declining species to gain a better understanding of their population dynamics (Abarca et al. 2010). In Central America, most declines have occurred at intermediate elevations (Bolaños and Ehmcke 1996; Lips 1998; Pounds et al. 1997, 1999; Puschendorf et al. 2006a), including the extinction in Costa Rica of the Golden Toad Incilius periglenes (Pounds and Crump 1994) and Craugastor escoces (IUCN, Conservation International, and NatureServe. 2009. Red List of Threatened Species. Version 2011.1. Available from http://www. iucnredlist.org [Accessed 14 Aug 2011]). At intermediate elevations, the stream-dwelling populations seem to be the most affected (Bolaños and Ehmcke 1996) and there are numerous reports of declines in the Craugastor rugulosus species group (Campbell 1998; McCranie and Wilson 2002; Lips et al. 2006; Puschendorf et al. 2006b; Bolaños et al. Copyright © 2011. Héctor Zumbado-Ulate. All Rights Reserved.