As with many islands, Christmas Island in the Indian Ocean has suffered severe biodiversity loss. Its terrestrial lizard fauna comprised five native species, of which four were endemic. These were abundant until at least the late 1970s, but four species declined rapidly thereafter and were last reported in the wild between 2009 and 2013. In response to the decline, a captive breeding programme was established in August 2009. This attempt came too late for the Christmas Island forest skink Emoia nativitatis, whose last known individual died in captivity in 2014, and for the non-endemic coastal skink Emoia atrocostata. However, two captive populations are now established for Lister's gecko Lepidodactylus listeri and the blue-tailed skink Cryptoblepharus egeriae. The conservation future for these two species is challenging: reintroduction will not be possible until the main threats are identified and controlled.

Faced with increasing rates of biodiversity loss and modest conservation budgets, it is essential that natural resource managers allocate their financial resources in a cost-effective manner and provide transparent evidence for extra funding. We developed the 'Cost-Effective Resource Allocator', a Microsoft Excel-based decision support tool to assist natural resource managers and policy makers, to prioritize the set of management strategies that maximize the total number of years that a suite of species is expected to persist given a budget constraint. We describe this tool using a case study of four locally threatened species from the Australian Commonwealth National Park of Christmas Island in the Indian Ocean. These include: a native fern (Pneumatopteris truncata), the Christmas Island Red Crab (Gecarcoidea natalis), the Golden Bosun (Phaethon lepturus fulvus), and Abbott's Booby (Papasula abbotti). Under a hypothetical budget of 8,826,000 AUD over ten years, in which all species are considered equal, our tool recommends funding: fern propagation and planting, rat control, cat control, and Yellow Crazy Ant (Anoplolepis gracilipes) survey and control. We found that the cost-effectiveness rankings of these strategies were sensitive to the importance that assessors' assigned to different species. The 'Cost-Effective Resource Allocator' can accommodate input from up to eight assessors, and analyse a maximum of 50 management strategies for 30 species.

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.

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