Long-term monitoring of island night lizards on San Nicolas Island

We describe the results of long-term population monitoring of the island night lizard Xantusia riversiana on San Nicolas Island, California, following the species' removal from the U.S. Endangered Species list in 2014. Monitoring activities were carried out from October 2014 through November 2023, but we also incorporate data from earlier work dating back to 1993. Because of habitat loss on the western part of the island, the island night lizard is almost entirely confined to the island's eastern half. The species' distribution remains largely the same since studies in the 1990s, though small increases in distribution were noted at the island's west end. Numbers at most long-term monitoring sites appeared to show a slight decrease associated with a multi-year drought, with numbers returning to previous levels by the end of the monitoring period. Models fit to counts of newly recruited lizards suggested a positive association between winter precipitation and abundance; however, we found a simpler model including constant abundance throughout the study to be more parsimonious. Two monitoring sites showed marked decreases in numbers, with one site declining to zero following infestation of the area by non-native Argentine ants Linepithema humile. The island's vegetation and natural communities continue to gradually recover from a century of overgrazing by sheep and the widespread introduction of aggressive non-native plants. Habitat restoration efforts by U.S. Navy natural resources staff on the island may lead to increasing population numbers and genetic connectivity, but persistent threats remain for the island night lizard on San Nicolas Island.