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Asymmetric Competition Over Calling Sites in Two Closely Related Treefrog Species

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Asymmetric Competition Over Calling Sites in Two Closely Related Treefrog Species

Amaël Borzée et al. Sci Rep.

Abstract

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.

Conflict of interest statement

The authors declare no competing financial interests.

Figures

Figure 1
Figure 1. Distributions of distance to bank for H. suweonensis (solid, n = 15) and H. japonica (open, n = 123) in rice paddies.
At night, when male treefrogs produced advertisement calls, they distributed themselves on the rice paddy banks or inside rice paddies. “Distance to bank” was the distance between the bank and the location of a calling treefrog. When a treefrog called on the bank, distance to bank was zero.
Figure 2
Figure 2
(a) Boxplots of the distance moved for the two treefrog species in the removal experiment. Calling locations of the focal species (either H. japonica or H. suweonensis) were noted in rice paddies before and after individuals of the non-focal species were removed. Direction of movement was measured as “towards the bank,” “no movement,” or “towards the centre of rice paddies”. (b) Observed counts (bars) of movement in the two treefrog species. Horizontal lines represent the expected counts based on the assumption that the two species did not differ in the direction of movement.
Figure 3
Figure 3. Map showing the range of Hyla suweonensis and the sampling sites used in this study.
The species range (dashed area) is limited in the east by the elevation of the Taebaek mountain range and to the west by the Yellow Sea. H. japonica is present on all landmasses visible on the map. This map was generated with ArcMap 9.3 (Environmental Systems Resource Institute, Redlands, California, USA; http://www.esri.com/).

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