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Population Trend Inferred From Aural Surveys for Calling Anurans in Korea

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Population Trend Inferred From Aural Surveys for Calling Anurans in Korea

Amaël Borzée et al. PeerJ.

Abstract

Amphibian populations fluctuate naturally in size and range and large datasets are required to establish trends in species dynamics. To determine population trends for the endangered Suweon Treefrog (Dryophytes suweonensis), we conducted aural surveys in 2015, 2016, and 2017 at each of 122 sites where the species was known to occur in the Republic of Korea. Despite being based on individual counts, the focus of this study was to establish population trends rather than population size estimates, and we found both environmental and landscape variables to be significant factors. Encroachment was also a key factor that influenced both the decreasing number of calling individuals and the negative population dynamics, represented here by the difference in the number of calling individuals between years. Generally, most sites displayed minimal differences in the number of calling males between years, although there was a large fluctuation in the number of individuals at some sites. Finally, when adjusted for the overall population size difference between years, we found the population size to be decreasing between 2015 and 2017, with a significant decrease in the number of calling individuals at specific sites. High rate of encroachment was the principal explanatory factor behind these marked negative peaks in population dynamics.

Keywords: Aural survey; Dryophytes suweonensis; Encroachment; Hylid; Population trend; Republic of Korea.

Conflict of interest statement

The authors declare there are no competing interests.

Figures

Figure 1
Figure 1. Locations of survey sites with local and temporary extirpations defined over four years.
The species range is drawn from Borzée et al. (2017a). Map generated through ArcMap 10.5 (Environmental Systems Resource Institute, Redlands, California, USA), with Service Layer Credits & Sources to Esri, USGS, NOAA and GeoServicesMap Esri Korea.
Figure 2
Figure 2. Frequency of the number of calling D. suweonensis at the sites surveyed in 2015, 2016, and 2017.
The highest number of calling males at a single site was recorded in 2016 while the lowest was recorded in 2017.
Figure 3
Figure 3. Frequency histogram for the difference in number of calling males between 2015 and 2017.
Although negative difference reached −41 individuals and positive variation reached 94 individuals, 90.00% of the variation was within −20 and +20 individuals.
Figure 4
Figure 4. Maps showing differences in the number of calling D. suweonensis between 2015 and 2017.
The species range is drawn from Borzée et al. (2017a) and Borzée et al. (2017b). Maps showing variation in the number of calling males between (A) 2015 and 2016, (B) 2016 and 2017, and (C) 2015 and 2017. Maps D, E, and F followed the same time series as A, B, and C, respectively, although they were based on corrected differences in the number of calling individuals to reflect differences at sites, independently of variations in the whole population. Maps were based on Kriging interpolations and generated through ArcMap 10.5 (Environmental Systems Resource Institute, Redlands, California, USA), with Service Layer Credits to DIVA-GIS (for administrative boundaries). Red represents a decrease in the number of calling individuals while green indicates an increase. Note the unbalanced colour scale.

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Grant support

This project was supported by four Small Grants for Science and Conservation in 2014, 2015, 2016, and 2017 from The Biodiversity Foundation, a grant from the Rotary Club Lectoure-Fleurance and a grant from the National Geographic Society Asia (Young Explorer #17-15) to Amaël Borzée; and Research Grants by the National Research Foundation of Korea (#2017R1A2B2003579), the Rural Development Administration (PJ012285) and from the Korean Environmental Industry and Technology Institute (RE201709001) to Yikweon Jang. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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