Science for Saving Species Research Update Project 2.3.2 Options beyond captivity for two critically endangered Christmas Island reptiles
Conservation options for Christmas Island’s blue-tailed skinks
Project Overview Introduced predators
The blue-tailed skink Several introduced predators (Cryptoblepharus egeriae) threaten these lizard species, and and Lister’s gecko (Lepidodactylus were likely to have significantly listeri) are two endemic reptiles contributed to their extinction in to Christmas Island that are the wild. The wolf snake (Lycodon now presumed to be extinct in capucinus) and giant centipede the Wild. Both were once (Scolopendra subspinipes) are two common on the Island, however such species, and pose ongoing both declined rapidly from the threats to reintroduced wild 1980s, and by 2012 both had populations of skinks and geckos. vanished from the wild. The wolf snake was introduced Fortunately, in 2009 and early to Christmas Island in the 1980s 2010, Parks Australia, with the and is now found across the entire help of Perth Zoo, captured 66 island. On Christmas Island, the wolf blue-tailed skinks and 43 Lister’s snake is known to threaten native geckos to establish captive reptiles via predation. It has also breeding populations on been implicated in the extinction Christmas Island and at Taronga of the Christmas Island pipistrelle Zoo. Captive breeding has (Pipistrellus murrayi). circumvented extinction in Giant centipedes have been the short term, with both present since European settlement captive populations now over of the island in the late 1880s. 1000individuals. However, They predate on a range of native the facilities on Christmas Island species, including native reptiles Image: Renata De Jonge, Parks Australia have reached carrying capacity, like the blue-tailed skink and Blue-tailed skinks within a breeding and there is strong interest in Lister’s gecko. enclosure on Christmas Island. re-establishing wild populations of these two species.
Research aim The purpose of this project is to explore options outside of captivity for these two lizard species, in the form of reintroductions back onto Christmas Island and/or assisted colonisation to other islands. Using a range of desktop, experimental, and management techniques, we are investigating what mechanisms will increase the chances of permanently establishing the blue-tailed skink and Lister’s gecko in the wild. Progress to date
Population Viability harvesting scenarios. We will continue was making point observations Analysis to collect data over the next two from a number of locations on the years to reduce the uncertainty in outside of the exclosure. Lizards were The blue-tailed skink and Lister’s these models and develop additional monitored daily for approximately gecko are now confined entirely to models for trial reintroduction 4 weeks post release and then once captivity, so ensuring these captive populations on Christmas Island a week by Parks Australia staff. populations are viable now and or elsewhere. into the future whilst translocation Population decline and attempts are undertaken is critically First trial release failure to establish important. In April 2017, 139 blue-tailed skinks, One month after release (early May), Population viability analysis is a that were approximately one year old, we attempted to catch as many popular tool in conservation. It is a were released into an approximately lizards as possible to estimate the quantitative method for assessing a 0.25 hectare site (equivalent of population size and asses animal population and/or a species’ status 50m by 50m) at which introduced health. We were able to capture 37 and its likelihood of extinction. It predators were excluded by fencing. lizards of which 13 were already toe can also be used to develop and Before release, 57 blue-tailed skinks clipped. Unfortunately by the end of evaluate conservation management were weighed, measured and given June/early July population counts options, such as harvesting captive an individual toe-clip combination during monitoring had significantly breeding populations for translocation for identification, and to assist declined. Another mark-recapture programs. with capture-mark-recapture and session was undertaken four months Using eight years of captive breeding estimating population size over time. after release (early August). Only 7 data collected by Christmas This meant 42% of lizards were toe individuals were caught, all of which Island National Park staff, we have clipped at the time of release. had a unique toe clip combination. undertaken some preliminary analysis By the end of September no Lizards were monitored using two on the viability of the captive breeding individuals were seen (or detected methods. The first was using transects populations and have found they are with ink pads), and it was likely that set up within the site and the second resilient to a number of proposed none or very few were left in the site.
Image: Parks Australia Traditional captive breeding enclosures on Christmas Island have been highly successful in increasing the number of skinks in captivity but are a resource intensive strategy over the long-term. Researcher Jon-Paul Emery preparing habitat in a new soft-release exclosure. In the distant background a solid fence over 1 metre high excludes feral predators, while a lower barrier (middle of picture) is used to create individual enclosures for skink trial reintroductions. Photo: Will Copeland
The reasons and mechanisms Learnings from first trial The exclosure design and existing for the population decline and eradication methods have not been failure to establish remain unclear, The first trial established several effective against giant centipedes. however a few hypotheses include: pieces of evidence which will be Despite control attempts prior to and (1) unsuitable habitat, (2) predation, important in planning the second following skink reintroduction, giant and (3) stress from the release trial. In particular: centipedes remained within the soft of captive animals into the wild. Lizards have retained anti-predator release area in high densities. Additionally, we were unable to behaviour. This was indicated as A new giant centipede eradication locate deceased animals so numerous individuals were observed method is proving effective. investigation of causes of death actively avoid the Christmas Island Over the course of the first trial could not be undertaken. All three thrush (an endemic forest bird and nearly 400 giant centipedes were hypotheses will be investigated known predator). discovered and removed over through either experiments (see Food availability and foraging 6 months. Christmas Island National below) and/or explored more capability are adequate. The body Park staff have now implemented deeply in the next trial reintroduction. weights of all individuals that were a chemical eradication method captured during both mark-recapture which is working extremely well. Parks Australia staff behind one of the sessions were approximately the newer more natural captive breeding same or heavier than at release enclosures. Photo Renata De Jong indicating that food and foraging were not limiting factors. The exclosure design effectively excludes wolf snakes. No wolf snakes or signs of wolf snakes were ever found during surveys indicating that the fencing is effectively excluding them from the area.
Right: Inside a soft-release exclosure. Image: Jon-Paul Emery Image: Parks Australia
Future areas of research
Analysis of reptile Interactions between To address these questions, we extinctions on blue-tailed skinks, are planning to undertake a series of experiments to identify the Christmas Island red crabs, and giant relationships between blue-tailed We are currently undertaking a centipedes skinks, red crabs and giant centipedes. systematic assessment of the possible Several areas of further investigation The aim is to identify if red crabs causal factors that may have had were prompted by the outcome of could play a role in increasing the a role in Christmas Island reptile the initial reintroduction trial. One of likelihood of successful reintroduction extinctions. This will also be placed the most important questions is: for both lizard species. within the context of modern (post- what role, if any, did giant centipedes 1500) global island reptile extinctions, play in the failure of this trial? Second trial noting that island species comprise reintroduction onto Giant centipedes are voracious 25 of the 28 global extinctions of Christmas Island reptile species since 1700. predators and are known to prey on a wide variety of animals. However, In July 2018, Parks Australia are they are known to have been present planning to undertake a second on the island for over 100 years, so trial reintroduction onto Christmas native reptiles have persisted with Island into the introduced predator them over a long period. However, free exclosure. The team will the dynamics of many prey and adapt management in light of all predator populations on this island information learned from the first have been severely affected by trial, including increasing predator the marked decline of red crab management and increasing habitat numbers in many areas, due to the complexity and suitability of the establishment of supercolonies of site. Coupled with an adapted yellow crazy ants since the 1980s. monitoring regime, the team believes these management changes will So, a second research question is: significantly increase the likelihood what is the relationship between of establishing a self-sustaining Christmas Island red crabs (a population. keystone species), blue-tailed skinks and giant centipedes? Detailed monitoring of the second And could introducing red crabs trial along with concurrent research benefit the establishment of a will be used to refine management population of blue-tailed skinks? strategies and inform any future reintroductions or introductions.
A blue-tailed skink handled during monitoring. Photo Jon-Paul Emery
Further Information For more information contact University of Western Australia researcher Jon-Paul Emery - [email protected]
This project is supported through funding from the Australian Government’s National Environmental Science Program.
This project was also supported by The Holsworth Wildlife Research Endowment & The Ecological Society of Australia.