Population Status of Three Endangered Lizards on Shimojijima Island and Irabujima Island, Ryukyu Archipelago, Japan

Abstract

The introduced Japanese weasel, Mustela itatsi, has had a significant negative impact on indigenous terrestrial vertebrates across several islands in the Miyako Group. The present study aimed to assess the current population status of squamate reptiles by investigating the distributions and abundances of three endangered lizards, Plestiodon kishinouyei, Takydromus toyamai, and Emoia atrocostata atrocostata, on Shimojijima Island and Irabujima Island. Route censuses for these species were conducted in the periods from 30 July to 4 August and 8–13 October 2022. Additionally, opportunistic sightings of lizards outside of the designated censuses were recorded. Eight P. kishinouyei, 20 T. toyamai, and seven E. a. atrocostata were recorded along four out of 101 survey routes, six out of 76, and two out of 39, respectively, with one additional E. a. atrocostata sighting at a non-census site. Plestiodon kishinouyei and E. a. atrocostata exhibited highly restricted distribution: the former was exclusively found in the northwestern part of Shimojijima Island, whereas the latter was confined to the southwestern part, with no presence on Irabujima Island. Takydromus toyamai was observed on both Shimojijima Island and Irabujima Island. The frequency of occurrence was low for the two skink populations on Shimojijima Island and T. toyamai on Irabujima Island, suggesting a potential population decline. These findings provide baseline data on the population status of these three lizard species. Such data will be crucial for evaluating their prospects for recovery, particularly considering the ongoing extensive eradication project targeting the introduced Japanese weasel on these islands.