322 AvailableNew on-lineZealand at: Journal http://www.newzealandecology.org/nzje/ of Ecology, Vol. 38, No. 2, 2014 Impacts of invasive house mice on post-release survival of translocated lizards Grant Norbury1*, Michiel van den Munckhof1,4, Sophie Neitzel1,4, Andy Hutcheon2,5, James Reardon3 and Karin Ludwig2 1Landcare Research, PO Box 282, Alexandra 9340, New Zealand 2Department of Conservation PO Box 5244, Dunedin 9058, New Zealand 3Department of Conservation, PO Box 29, Te Anau 9640, New Zealand 4Present address: HAS Hogeschool University of Applied Sciences, PO Box 90108, 'sHertogenbosch, The Netherlands 5Current address: 197 McIntosh Rd, Brighton, Dunedin 9035, New Zealand *Corresponding author (Email: [email protected]) Published online: 9 April 2014 Abstract: Invasive house mice (Mus musculus) have detrimental effects on biodiversity, but their impacts can be difficult to detect and are often unquantified. We measured their effects on survival of a translocated population of an endangered lizard in New Zealand. Twelve captive-reared Otago skinks (Oligosoma otagense) were translocated to a 0.3-ha area of grassland/shrubland cleared of invasive mammals and surrounded by a mammal-resistant fence. Sixteen more skinks were released 2 years later but this was followed by an incursion of mice for c. 160 days. Peak mouse density was at least 63 per hectare, and they were seen attacking adult skinks (> 25 cm in length), which is previously undocumented for this lizard species. Using photo/re-sight methods and Program MARK, we estimated skink survival (phi) and detectability (p) in the presence of mice (second cohort: phi = 0.15 per annum, 95% Confidence Interval (CI) 0.01 – 0.48; p = 0.28, 0.20 – 0.38) and in their absence (first cohort: phi = 0.44 p.a., 95% CI 0.03 – 0.82; p = 0.29, 0.22 – 0.39). Survival of skinks from the first cohort during the mouse incursion was unaffected, presumably because they were already established and had access to familiar or more optimal refugia. Their survival over the entire 3 years of monitoring (0.83, 95% CI 0.60 – 0.93) compared favourably with published estimates for viable populations in the wild, protected from all invasive mammals. This suggests it may be feasible to re-establish captive-reared lizards in the wild, but mice should be considered a limiting factor, at least during the initial translocation phase. Keywords: Central Otago; invasive species; Oligosoma otagense; predation; reintroduction; Otago skink Introduction attributed to predation by invasive cats (Felis catus), weasels (Mustela nivalis), stoats (M. erminea) and ferrets (M. furo) Invasive house mice (Mus musculus) are distributed worldwide (Reardon et al. 2012), which are top predators in New Zealand and are increasingly recognised for their unwanted impacts on ecosystems. The effects of invasive house mice on Otago indigenous island species and ecosystems (e.g. Wilson et al. skinks, whether by predation and/or competition for food or 2007; St Clair 2011; Wanless et al. 2012). However, their refugia, are unknown. Our prediction was that skink survival impacts can be difficult to detect and are often unquantified would be lowered by predation and/or competition with mice. (St Clair 2011). Eradicating mice is expensive and problematic All skinks in this study were at least third-generation where reinvasion is likely, so having a clear understanding of captive-reared from an original founder population of 12 the potential ecological gains of mouse control is important. individuals taken from eastern Central Otago. The project Given their frequent incursions, mice are often the only therefore provided a further opportunity to assess whether mammalian pest remaining inside fenced sanctuaries in survival of the translocated captive-reared population would New Zealand (Innes et al. 2012). A number of New Zealand be lower than published survival rates of wild skinks (in studies have inferred that mice are harmful predators of small Reardon et al. 2012). indigenous lizards (Newman 1994; Lettink & Cree 2006; Hoare et al. 2007; Knox et al. 2012). Mice may also compete with lizards for food and shelter, or reduce basking opportunities. Materials and methods No study has measured the effects of mice on vital rates of lizard populations. Study site We assessed the impacts of mice on survival rates of a In 2009, a community conservation group, the Central Otago translocated population of an endangered skink species, the Ecological Trust, began a pilot study to test the feasibility Otago skink (Oligosoma otagense). Once widespread in Central of reintroducing captive-reared Otago skinks to an area the Otago (South Island, New Zealand), these skinks have declined species formerly occupied. In August 2009, we enclosed a dramatically over the past century and now occupy only 8–10% 0.3-ha area of grassland/shrubland habitat with a 1.9-m-high of their former range (Whitaker & Loh 1995). The species mammal-resistant fence (Pestproof Fences, Havelock North). is now classified as ‘nationally endangered’ (Hitchmough This release site was chosen because it contained high cover et al. 2013). The only extant populations are present near the of indigenous shrubs (e.g. Coprosma propinqua, Melicytis boundaries of their former range in the Macraes Flat and Lindis/ alpinus, Discaria toumatou, Muehlenbeckia complexa) Hawea districts (Whitaker & Loh 1995). Declines have been that provided suitable food and refuge for skinks. The site New Zealand Journal of Ecology (2014) 38(2): 322-327 © New Zealand Ecological Society. Norbury et al.: Invasive mice affect lizard survival 323 also contained numerous schist rock outcrops with deep cereal baits (Talon® Pellets rodenticide). There was no sign horizontal cracks that skinks use for refuge. The elevation of mice inside the fence 4 weeks after control began. of the site is 340 m (taken from Google Earth imagery), and average annual rainfall (in the town of Alexandra, 7 km Skink releases and monitoring away) is 363 mm (New Zealand National Climate Database Twelve adult skinks were taken from captivity in the North of the National Institute of Water and Atmospheric Research; Island, held in a Department of Conservation quarantine see http://cliflo.niwa.co.nz). Before releasing the skinks, to facility in Otago for 8 weeks for disease screening, and released ensure their adequate thermal protection from freezing, we inside the fence in November 2009 (Hare et al. 2012). In measured minimum temperatures in winter 2008 (a reasonably December 2011, another 16 adult captive-reared skinks were average winter), using temperature probes (DS1922L-F5# quarantined and released. Because Otago skinks are diurnal, Thermochron iButton) placed 1–3 m inside six rock crevices. spend much of their time sun-basking, and have unique body Minimum temperatures did not fall below 3°C. Given these markings, a non-invasive photo re-sight method can be used favourable characteristics, we believed the site could support to identify individuals and estimate survival rates (Reardon at least 60–80 skinks. et al. 2012). Skinks were monitored every 15 days on average (range, 1–94 days) on 75 occasions. Monitoring was less Mammal eradication and monitoring frequent during winter because skinks were less active then. To eradicate any mammals inside the enclosure before A monitoring session involved one person (same person on translocating the skinks (in November 2009) we used 20 60% of occasions) walking quietly through the area for 1–2 h kill-traps (spring-loaded metal-jawed traps: ‘DOC-250’ photographing the lateral surfaces (snout to foreleg) of skinks. (Department of Conservation 2014) and ‘Timms’ traps (www. Images were compared by eye with a photographic library of philproof.co.nz/gen_showproduct.php?cat=1)); baited with known individuals. Monitoring began 7–10 days after each fresh rabbit meat over a 3-month period. Twenty footprint- translocation and occurred only during good conditions for tracking tunnels (using ‘Black Trakka’ cards from Gotcha observing skinks (i.e. warm, sunny, little or no wind) from Traps, Warkworth) baited with peanut butter (Cunningham & November 2009 to November 2012. The first mouse was seen Moors 1996) were deployed to detect rodents. To ensure no in January 2012, 10 days after the release of the second skink mammals were present, traps and tunnels remained in place cohort (during the first follow-up visit to the site). for 6 months before skinks were released. Kill-traps were de- activated when skinks were released to avoid accidental capture Data analysis of skinks, but tunnels remained active for the duration of the Mouse density was estimated as the number removed by study to monitor continuously for the presence of rodents. trapping until no more were captured, divided by the area Tunnels were checked approximately every month and the enclosed by the fence. This calculation assumed that all of ink replenished. No mammals or their scats were detected or the mice present were captured, and that the population was observed during regular visits to the site, until January 2012, ‘closed’ during the 5-day trapping period. The estimate does when mice were seen inside the fence and tracking tunnels not account for mice that may have been poisoned later, and were marked with mouse prints (Table 1). Suspected entry therefore represents a minimum estimate. points through the fence were blocked 4 months later and the To estimate skink survival rates, we analysed the photo/ mice eradicated using 20 live-capture Elliott traps (Tasker re-sight data using the Cormack–Jolly–Seber model in Program & Dickman 2002) cleared daily for 5 days, followed by MARK (version 6.0; White & Burnham 1999). We assumed poisoning over 4 weeks using 16 bait stations elevated 30 cm no emigration (we have no knowledge of adult Otago skinks above ground by wooden posts to make them less accessible escaping through mammal-resistant fences), no differences to skinks.