Decreasing Population Size of the Philippine Limestone Frog, Platymantis insulatus By 1A. C. Alcala, 1A. A. Bucol, 1E. L. Alcala and 2R. M. Brown
T
he endemic Philippine limestone frog, Platymantis insulatus (Brown and Alcala 1970), is considered Critically Endangered (1) and is known to be restricted to the Gigantes group of Islands, off northeast Panay Island, Province of Iloilo, the Philippines (2, 3). Its microhabitats are limestone caves, rock crevices and rock fissures. In the present paper, we report our observations on the population density of this frog prior to and during the course of the El Niño Southern Oscillation (ENSO) Event in 20092010.
(27.44±0.20 S.E.), while relative humidity as measured by a sling psychrometer ranged from 75-92% (87.44±1.08 S.E.). The inner cave ambient temperatures, ranging from 25-28 °C (26.5±0.21 S.E.), were slightly lower than the temperature readings at the cave entrances. Some of the caves were observed to be drying up in 2009-2010. None of the caves had running water inside during the whole observation period.
The population density The two main islands of the Philippine limestone frog Platymantis insulatus in life. Photo: Abner Bucol. mostly outside caves for Gigantes and the small as2004-2005 was 400sociated islets were explored for amphibians, reptiles, birds and 638 individuals per hectare (2). In the El Niño years 2009-2010, mammals by the authors in 2004, 2005, 2009 and 2010 (30 April, the population density estimate outside caves was 5.0-6.7 individu1 May, 6-8 September 2004; 3, 4, 5 March 2005; 13-19 December als per 100 m2 (S.E.= ±.48-0.67) or 50-67 individuals per hectare. 2009; and 13-20 January, 21-28 February, 14 April, 15 May 2010). However, the density estimate inside caves was 1.0-4.30 individuA total of 17 small caves (ca. 1.0 m-6.5 m wide and 2.0 m-35.0 m als per 100 m2 (S.E.= ±0.33-1.86) or 100-430 individuals per hectlong) on the two larger islands were visited. These caves have been are. Assuming that the density estimate for 2004-2005 is correct, mined for their guano deposits and the forest cover has been re- the population density outside the caves would appear to have subduced by the expanding human population, altering the microhabi- stantially decreased in a time span of five years. The occurrence of tats of the frog. the El Niño in 2004-2005 and the disturbance of the habitat by humans could explain the population decline. It appears that the The years 2009 and 2010 were El Niño Southern Oscillation density inside the caves was higher than that outside caves, which (ENSO) Events (4), characterized by reduced precipitation (5.2-33 probably was due to the more equable conditions (cooler and more mm) in the entire Western Visayas Region from December 2009 to humid) inside caves, although the effect of guano mining cannot be March 2010. discounted. One data set gathered by us was population density of the Limestone frog. Visual counts were made in 100 m2 quadrats during daytime. The counts made at night included the number of calling males multiplied by two as part of the estimate of the number of individuals in a quadrat. A total of 42 quadrats were established during the entire survey duration in both islands in 2009-2010. The team also collected data on ambient air temperatures and relative humidity in the microhabitats of the frog on 13, 14, 15, 18 and 19 December 2009 and on 13 January 2010. Air temperature readings in cave entrances using a mercury thermometer ranged from 26-28 °C 1 Silliman University-Angelo King Center for Research and Environmental Management (SUAKCREM), Dumaguete City, 2nd Floor, SU-Marine Laboratory Building, 6200 Dumaguete City, Philippines .2Natural History Museum & Biodiversity Institute, Department of Ecology and Evolutionary Biology, The University of Kansas, Lawrence, Kansas 66045-7561 USA.
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We also observed some aspects of the ecology of the frog, which was generally found in moist and cooler portions of the caves, particularly near the entrance and in moist rock crevices. Except for the Harpa Cave which showed no frog signs, despite having the lowest temperature and highest humidity readings, the rest of the wet caves were observed to be inhabited by the species. We think the species’ absence in this cave might be due to the frequent human disturbance resulting from diggings for supposed treasurehunting. Conversely, we generally did not observe the species in dry caves with high temperature readings (e.g., Elepante Cave) and in a heavily disturbed cave (Langub Cave). In addition, one of the authors (AB) visited three caves in the adjacent islet of Cabugao Daku in February 2010 and observed only three P. insulatus individuals occurring in a single dry cave, indicating a small population of the species on this islet due to the dry condition of the cave.