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We used three-dimensional micro-computed tomography to compare the skeletal features of two species of male tree frogs with similar morphologies: the globally prevalent Hyla japonica and critically endangered Hyla suweonensis. One of the most prominent differences between the two species was the angle of the alary processes of the premaxilla. In H. suweonensis, the alary processes were inclined posteriorly at an angle of 77.05°, whereas in H. japonica, they were deflected at an angle of 93.27° (a difference of more than 16°). This difference accounts for the more protruded shape of H. suweonensis compared with that of H. japonica. Sizes and proportions related to skeletal compartments (e.g. skull, forelimb, vertebrae, and hindlimb) also differed between the two species, which is consistent with the general exterior observation that compared with H. japonica, H. suweonensis has a more acuminate snout and a more slender body. The full skeletal configuration could provide classification keys for the two sympatric species that will be applicable to ecological field surveys and conservation activities.

Simple Summary The American Bullfrog (Lithobates catesbeianus), known as one of the most problematic amphibians in the world, was introduced to South Korea in the 1970s. Although it has spread very rapidly over the past 50 years, there is no focused strategy to manage the frogs. With the introduction of American Bullfrogs into nature, numerous native species (mostly amphibians) are rapidly decreasing due to predation, a transmission of infectious diseases, and crossbreeding. In particular, the critically endangered Suwon treefrog (Hyla suweonensis) is the most affected by Bullfrogs. In this study, we modeled what environment the American Bullfrog settled in and how it could spread in the future. It was confirmed that the distribution of the two species overlaps in many areas at present, and the distribution of the American Bullfrog may spread to the distribution area of the Suwon treefrog according to future climate change. Through the results of this study, we intend to suggest a management direction for the spread of invasive species and the protection of endangered species. Our result could contribute to many countries that have problems with the American Bullfrogs at present and in the future. Abstract The American Bullfrog (Lithobates catesbeianus) has been imported into South Korea in earnest for food since the 1970s and introduced into nature due to release and escape. Accordingly, the influx and spread of American Bullfrogs are expected to have a direct impact on native species, but few related studies have been conducted on this. We predicted changes in the potential distribution and future distribution based on climate change scenarios to analyze how those changes affect critically endangered Suwon treefrogs. Suwon treefrog sites (63.9%, 78/122) overlapped with the distribution of Bullfrogs. According to the prediction of the future distribution of Bullfrogs, the overlapping of American Bullfrogs and Suwon treefrog will remain similar to the current level in the Representative Concentration Pathway (RCP) 4.5 scenario. On the other hand, in the RCP 8.5 scenario, the number of overlapping sites will increase to 72.1% (88/122) due to the spreading of the American Bullfrogs. The results show that climate change directly affects the distribution expansion of the American Bullfrogs but also indirectly can lead to an increased threat to Suwon treefrogs. In conclusion, our results strongly suggest why climate change should be actively addressed in terms of the spread of invasive species and the protection of endangered species.

Niche differentiation enables ecologically similar species to coexist by lessening competition over food and/or shelters and may be critical for reproductive isolation between closely related species in close proximity. Because no extra traits need to evolve, spatial and temporal differentiation may readily take place to complement other isolating mechanisms. Two closely related treefrog species occur together in Korea: the endangered Hyla suweonensis and the widespread Hyla japonica. Advertisement calls are differentiated, but it is unclear whether call difference is sufficient for reproductive isolation. We tracked individuals of both species to study fine-scale differentiation in microhabitat use in the diel cycle of the breeding season using a harmonic direction finder. tracking male movement patterns of both species revealed spatial and temporal differentiation in microhabitat use for calling and resting during the breeding season. Males of both H. suweonensis and H. japonica occurred in all 5 microhabitats identified in this study: rice paddy, ground, buried, grass and bush. Both treefrog species showed general similarities in calling from rice paddies and resting in grass and bush. However, H. suweonensis moved into rice paddies and produced advertisement calls 3 h earlier than H. japonica. These differences likely minimize contact between the species and provide an additional isolating mechanism. In addition, the activity of H. suweonensis may be contributing to the decline of this species, as resting in grass would increase dangers from predatory birds and habitat disturbance.

Biodiversity is plummeting worldwide, and the major causes of such decline include habitat degradation and climate change. While cities do contribute to the negative impact to the environment, they can also serve as strategic centres for conservation programs. Sites qualifying as biogeographic islands within metropolitan Seoul were studied for the occurrence of two hylid species: the endangered Hyla suweonensis and the abundant H. japonica. This study demonstrates that neither habitat diversity nor surface area, but solely the occurrence of aggregated rice paddies is a requisite for H. suweonensis, hypothetically due to its strict breeding requirements. On the contrary, H. japonica occurrence was not affected by any of these factors, and all types of habitats studied were adequate for this species. The presence of an endangered species within the boundaries of one of the most populated metropolises suggests a strong natural resilience, which should be enhanced with appropriate actions. We emphasize that the management plans therein can, and should, be used as the first step in the conservation of H. suweonensis in metropolitan Seoul.

Amphibian populations have been declining worldwide over the last few decades due to causes like habitat modification, climate change, and emergent diseases. Hyla suweonensis, a treefrog species endemic to Korea, has disappeared from much of its historic distributional range. Past studies reported this species to breed exclusively in man-made rice paddies. This study reports the habitat characteristics of a seminatural site for H. suweonensis. Out of 100 sites where H. suweonensis was present, only one was not a rice paddy. This seminatural site was characterized by a network of swamps bordered with forests of willow trees. H. suweonensis individuals located on willow trees called from 0.2 to 1.50 m above ground at the edge of the forest near the swamps. The seminatural site has a large percentage of rice paddies with a very low percentage of urban areas, a feature consistent with findings of a previous study. H. suweonensis was found calling with two other amphibian species. Although extracted from a sin...

Hyla suweonensis is an endemic species and is designated as the only endangered species I among amphibians in 2012 by the Ministry of Environment, however studies about its habitat are lacking. This study was carried out to analyze habitat of H. suweonensis based on the spatial information using Max...

A comparison between localities currently occupied by a species and localities where the species recently underwent local extinction is a powerful way to understand the causes of population decline. Two closely related treefrog species occur in the Republic of Korea: Hyla japonica, which is distributed widely throughout northeastern Asia, and H. suweonensis, which occurs sympatrically on a narrow strip of the western coastal plains in Korea. Males of both species produce species-specific advertisement calls. To determine the spatiotemporal distributions of H. suweonensis and H. japonica during their breeding seasons, auditory monitoring was conducted in 70 localities throughout the historic range of H. suweonensis. A group of volunteers conducted the monitoring, supplemented with call recordings for validation. The endangered H. suweonensis was found in only 14 of 70 localities, designated as syntopic, whereas H. japonica occurred in all 70 localities. The calling activity of H. japonica was constantly vigorous throughout the study period, whereas the calling activity of H. suweonensis was generally weak. The analysis of binary logistic regression identified the percentage of rice-paddy field as the significant variable. The syntopic localities had much higher percentages of the rice-paddy field than the non-syntopic localities did. Localities where the rice-paddy fields are fragmented and isolated by commercial and residential complexes are not likely to harbor the H. suweonensis populations. The spatiotemporal distributions of the two treefrog species throughout the breeding seasons confirm that H. suweonensis is indeed rare and requires swift and decisive measures, including the establishment of protected areas.

Although DNA taxonomy is readily available, morphological keys are still valuable for quick and easy identification of species on site. In Korea, Hyla japonica is widespread throughout the country,...

Reproductive interaction between closely related taxa may leave a distinctive signature in which populations of interacting taxa are more dissimilar in sympatry than in allopatry. An ideal condition for such a pattern of reproductive character displacement (RCD) may occur when a population has limited gene flow and experiences strong selection pressure, exerted by an interacting taxon in areas of sympatry. In Korea, there are two closely related treefrog species: Hyla japonica, which is distributed widely throughout the country, and Hyla suweonensis, which occurs sympatrically in a narrow strip of western coastal plains. H. suweonensis is only found within the distribution of H. japonica. These two species have a similar single-note call structure. Here, we tested the possibility of RCD in H. japonica by examining geographic variation in advertisement calls. Although means of temporal and spectral characters were significantly different between the two species, sympatric populations of H. japonica and H. suweonensis overlapped in distributions of most characters. Furthermore, allopatric and sympatric H. japonica populations did not differ in all call characters. Weak genetic differentiation between sympatric and allopatric populations of H. japonica implied either substantial gene flow or recent genetic isolation. Possible explanations for no RCD in male advertisement calls of H. japonica include a difference in fine temporal characteristics between the two species, migration between sympatric and allopatric localities in H. japonica, RCD in female preferences in H. japonica, and weak selection pressure by H. suweonensis.

Interspecific competition occurs when one species using a resource limits the use of that resource by another species. A dominance relationship between the species competing over a resource may result in asymmetric competition. Here, we tested the hypothesis that two sympatric treefrog species, the endangered Hyla suweonensis and the abundant H. japonica, compete with each other over calling sites. We observed the locations of calling individuals of the two treefrog species in rice paddies and tested whether removing one species affected the calling locations of the other species. Individuals of the two species were spatially isolated within rice paddies, with H. japonica at the edges and H. suweonensis in the interior. Male H. suweonensis moved towards the edges of rice paddies when male H. japonica were removed from the area, whereas male H. japonica hardly moved when male H. suweonensis were removed. The results of both studies are consistent with asymmetric interspecific competition, in which the calling locations of H. suweonensis are affected by the calling activity of H. japonica. In addition, H. japonica were found “sitting” on the substrate during call production, whereas H. suweonensis were “holding” onto vegetation. The difference in calling posture may represent an adaptive response to asymmetric interspecific competition.

The nucleotide sequence of a 447 base pair fragment in the mitochondrial cytochrome b gene and the complete sequence of the mitochondriat 12S ribosomal RNA gene, 938 bp, were analyzed to infer inter‐ and intraspecific genetic relationships of Hyla japonica and H. suweonensis from Korea and H. japonica from Japan. In the mitochondrial cytochrome b gene, genetic differentiation among H. japonica populations were 9.62% and 15.66% between H. japonica and H. suweonensis. Based on the Tamura‐Nei distance, the level of sequence divergence ranged from 0.45% to 2,75% within Korean H. japonica, while 8.31%‐8.87% between Korean and Japanese H. japonica and 11.51 %‐12.46% between H. japonica and H. suweonensis. In the neighbor‐joining tree, Korean populations of H. japonica were clustered first at 2.22% and followed by Japanese H. japonica and H. suweonensis at 8.51% and 12.29%, respectively. In mitochondrial 12S rRNA gene, genetic differentiation between H. japonica and H. suweonensis was 7,17% (68 bp) including 7 g...

Thirteen kinds of interspecific and intraspecific crosses were performed with Eurasian treefrogs of the genus Hyla. Progeny of eight kinds of interspecific crosses (between females of H. japonica and males of H. hallowellii, H. chinensis, H. suweonensis, H. savignyi, H. meridionalis, H. arborea arborea and H. arborea sarda and between a female of H. suweonensis and males of H. japonica and H. hallowellii) metamorphosed into froglets. The results are consistent with other lines of evidence that indicate close phylogenetic relationship among these Eurasian species. At metamorphosis, all of the hybrids had either nearly normal testes or abnormal ovaries. Sex ratio differed significantly from a 1:1 ratio among progeny in five kinds of interspecific crosses; most of the control froglets had normal ovaries or testes in nearly equal proportion. Gonadal structures in hybrids after one year were also abnormal, suggesting that fertility of adults would be very low, if not absent. The presumed low hybrid fertility supports the specific status of H. japonica, which has often been treated as a subspecies of H. arborea. It also supports the specific status of H. suweonensis, which was distinguished originally on the basis of its characteristic vocalization. This study also suggests specific status of H. savignyi and some genetic divergence between H. japonica of Japan and Korea.