Study site and species
We collected data on seven Izu Islands, namely Hachijo-Kojima (33.13°N, 139.69°E), Miyake (34.08°N, 139.52°E), Mikura (33.88°N, 139.60°E), Kozu (34.22°N, 139.15°E), Oshima (34.74°N, 139.40°E), Niijima (34.37°N, 139.26°E) and Toshima (34.52°N, 139.28°E), ranging in size from approximately 6 to 9076 ha (Figure S1 in Supporting Information). These volcanic islands are located along an axis radiating southwards from the Izu peninsula of mainland Japan, and are generally subject to the same mild climate carried over by the Kuroshio current (Hasegawa 1994; Brandley et al. 2014), with average annual air temperatures and total precipitation ranging from 17.3 to 18.8 °C, and 1810 to 2958 mm, respectively, for the year 2018 (http://www.data.jma.go.jp/).
P. latiscutatus is the most abundant lizard species overall in this system and is found on all islands, where, with little interspecific competition, it is usually found in high abundance, especially on islands with low predation (Hasegawa 1994). Amongst its terrestrial predators, E. quadrivirgata is the most widespread and abundant, with Oshima being the only island where there are other native species preying on the lizard. These include snakes, such as Lycodon orientalis and Gloydius blomhoffii , as well as the Japanese weasel (Mustela itatsi ), the only mammalian carnivore native to any of the islands (Hasegawa 1994). Therefore, aside from Oshima, the only other native predators to the lizards on the islands are birds (Hasegawa 1990, Hasegawa et al. 1996), notably the grey-faced buzzard (Butastur indicus ) and the Izu island thrush (Turdus celaenops ). Given that E. quadrivirgata is present on five of our study islands and absent on two, Hachijo-Kojima and Miyake, the only potential predation pressure for lizards on the latter two islands at the time of data collection was from avian predators, whereas predation on Mikura, Kozu, Niijima and Toshima was mainly by E. quadrivirgata (Hasegawa 1994). The evolutionary history of the relationship between P. latiscutatusand E. quadrivirgata is outlined in Appendix 1 of the Supporting Information.