900 I. M. Barata et al.


model (Supplementary Fig. 3). Extrapolating the estimated density across the whole fragment (1,284 ha) gives estimated male population sizes of 66,534 for A. vallani and 30,193 for A. helenae.


Discussion


Using presence and absence data obtained from multiple acoustic surveys,we provide the first estimates of population size for A. vallani and A. helenae, two Critically Endangered amphibian species from the central plateau of Madagascar. Both species are known to live in only a small number of forest fragments, one of which is outside Ambohitantely Special Reserve and has been subject to much deforestation since these frogs were discovered there (Vallan, 2000a). It is unknown whether these species are able to survive in the smaller forest fragments within the Reserve, given the differ- ent microhabitats and microclimates these smaller frag- ments probably have. For example, tree holes, which are an important reproduction site for A. vallani,may be absent in heavily logged forest fragments (K.E. Mullin, pers. obs., 2019, 2020). The reproduction of both species relies on high humidity and dew points, which could be lower in smaller forest fragments that are more affected by edge ef- fects. Although forest fragments of various sizes are import- ant to support a diverse amphibian community in Madagascar (Riemann et al., 2015), forest fragmentation in Ambohitantely Special Reserve decreases amphibian popu- lation sizes and increases species vulnerability by reducing the heterogeneity of the microhabitats available to frogs (Vallan, 2000b). Therefore, our estimates of population sizes refer to the single largest forest fragment in Ambohitantely Special Reserve, which is probably the lar- gest and most significant population refuge for these threa- tened frogs. Both species had moderate to high occupancy rates in


PLATE 1 The two Critically Endangered amphibian species studied: (a) Anilany helenae and (b) Anodonthyla vallani. Both species could potentially be misidentified as (c) Platypelis pollicaris but they can be differentiated visually and acoustically by an experienced observer (see spectrograms showing the differences in their calling patterns; Supplementary Fig. 1).


the estimated population size of male A. vallani was 855 (95%CI = 250–1,052), giving an estimated population den- sity of 52 individuals/ha. The estimated abundance of adult male A. helenae was four individuals per site (range = 1–8, SE = 2). The estimated population size of male A. helenae in the studied siteswas 388 (95%CI= 128–580), giving an es- timated density of 23 individuals/ha. For both species popu- lation numbers appear to be overestimations of the sam- pled distribution of the population based on the best-fitted


the studied forest fragment but we recorded relatively low density estimates compared to similar studies of leaf litter frogs: 470 individuals/ha of Mantella milotympanum in Madagascar (Vieites et al., 2005), 78–4,285 individuals/ha of four Sooglossus species in Seychelles (Gerlach, 2007) and 20–960 individuals/ha of multiple tree frog species in the Brazilian Atlantic Forest (Siqueira et al., 2009). Although we consider our estimates of population size at the surveyed sites to be robust, extrapolations to the whole fragment based on density do not account for heterogeneity in occupancy outside the surveyed area or variation in habi- tat suitability/availability across the fragment. Despite these uncertainties, such density estimations and extrapolations beyond surveyed sites provide valuable baselines for future monitoring and assessments of population trends. The acoustic surveys allowed us to collect a large dataset over a short period of time, facilitating the rapid assessment


Oryx, 2022, 56(6), 897–903 © The Author(s), 2022. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605321001034


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