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.