The lost lizards of Christmas Island: A retrospective assessment of factors driving the collapse of a native reptile community

Abstract

Until recently, the reptile fauna of Christmas Island in the Indian Ocean comprised five endemic species (two skinks, two geckos, and one snake) and one native, non‐endemic skink. Four of these species were common and widespread until at least 1979, but by 2012 had disappeared from the wild. During the years of decline, little research was undertaken to examine why the species were disappearing. Here, we use a retrospective expert elicitation to rank potential factors that contributed to the loss of Christmas Island's reptiles and to assess the likelihood of re‐establishing populations of two species now listed as Extinct in the Wild. We additionally considered why one endemic lizard, the Christmas Island giant gecko (Cyrtodactylus sadleiri), and three introduced lizards remain common. Experts considered that the introduced common wolf snake (Lycodon capucinus) was the most likely cause of decline, as its temporal and spatial spread across the island closely matched patterns of lizard disappearances. An Asian co‐occurrence in recent evolutionary timeframes of the common wolf snake with the Christmas Island giant gecko and three introduced reptiles was the most marked point of difference between the extant and lost lizard species. The demise in less than 20 years of 80% of Christmas Island's native lizard assemblage highlights the vulnerability of island fauna to invading species.

Invasive snake causes massive reduction of all endemic herpetofauna on Gran Canaria

“…Additionally, native reptiles became one of the most frequent prey of B. irregularis in Guam once native birds had been extirpated [5,7], which altered skink and gecko assemblages [8]. Similarly, the invasive horseshoe snake, Hemorrhois hippocrepis, affects native reptiles in the Balearic Islands [9,10], the common wolf snake, Lycodon capucinus, is presumed to be primarily responsible for the demise of endemic reptiles on Christmas Island [11], and the Indian wolf snake, L. aulicus, feeds on the endemic herpetofauna of La Réunion [12], though impacts per se have not yet been demonstrated, only presumed. These cases indicate that invasive snakes can potentially impact insular herpetofauna around the globe, even though such impacts remain unnoticed in most cases (see [1,2]).…”

Arthropod Predation of Vertebrates Structures Trophic Dynamics in Island Ecosystems

“…The remainder of the Phillip Island centipede diet (52.1%; 95% CI: 33.2-69.2) consisted of invertebrates (crickets). While skinks were not directly observed being hunted or consumed by Phillip Island centipedes, similarly sized giant centipedes are known to prey on skinks in other systems (e.g., Emery et al 2020Emery et al , 2021. It is likely that Phillip Island centipedes prey on skinks belowground or under loose leaf litter; thus, skinks were included in the dietary mixing model.…”

Invasive ants reduce abundance of small rainforest skinks

“…If so, there would have to be counteracting mechanisms to explain the lack of elevated C. rubrigularis abundance in high yellow crazy ant sites. Some Christmas Island reptiles have reportedly contracted to parts of the island with the highest yellow crazy ant abundance (Smith et al 2012), but a more recent review of the data posits that yellow crazy ants may pose indirect, secondary, or synergistic threats to lizard populations (Emery et al 2021). Clearly there is much more work to be done to elucidate the potential indirect mechanisms through which yellow crazy ants may affect forest reptile abundance.…”

“…Evidence for effects of yellow crazy ants on herpetofauna is much more tenuous. The ants had been implicated in the population declines of four native skink species on Christmas Island (Smith et al 2012), but a recent retrospective expert elicitation concluded there was little spatial concordance between the lizard declines and the highest densities of yellow crazy ants (Emery et al 2021). Feare (1999) asserted that on Bird Island, in the Seychelles, the introduction of yellow crazy ants resulted in the disappearance of an endemic skink but provided no evidence.…”