Polar Biol (2012) 35:1187–1195 DOI 10.1007/s00300-012-1165-8 ORIGINAL PAPER Spatio-temporal patterns of introduced mice and invertebrates on Antipodes Island James C. Russell Received: 21 November 2011 / Revised: 10 February 2012 / Accepted: 13 February 2012 / Published online: 1 March 2012 Ó Springer-Verlag 2012 Abstract House mice (Mus musculus) are a widespread Introduction introduced species with major but often overlooked impacts on ecosystems, proportionally greater when they House mice (Mus musculus and synonyms) are the most are the only introduced mammal present. Studies con- widespread introduced vertebrate in the world (Boursot ducted on the ecology of mice on Antipodes Island, where et al. 1990), having dispersed commensally with humans they are the only introduced mammal, are presented and for thousands of years (Auffray et al. 1990). On islands, compared to previous work over the past four decades. introduced mice are one of a group of introduced mammal Mice live-trapped on grids were more abundant in dense species which can have important negative effects on coastal tussock (147 mice/ha) compared to inland plateau native biodiversity (Courchamp et al. 2003). Commonly, grasslands (59 mice/ha), with a significant effect of age, but introduced mice on islands coexist with other introduced not sex, on both capture probability and range size. Body- mammals such as cats (e.g. neighbouring Auckland Island; size of mice has not changed over four decades, providing Harper 2010), but they can also be the sole mammal, either no evidence of gigantism, which on other Southern Ocean through historical accident or following the eradication of islands has been speculated to increase the predation risk to other introduced mammals (MacKay et al. 2007). On birds. Over 2,405 invertebrates from fourteen Orders were Southern Ocean islands in particular, mice have had a identified from pitfall traps and litter samples across five major impact on insular communities. Widespread impacts sites. Differences in invertebrate communities and taxo- on invertebrate communities have been documented nomic units attributable to habitat and altitude were (Angel et al. 2009; St Clair 2011). More recently, their detected among sites in both pitfall and litter samples on potential to impact avian communities has been demon- Antipodes Island. Differences in invertebrate communities strated, particularly on Gough Island (Cuthbert and Hilton were detected from litter samples on a neighbouring 2004; Wanless et al. 2007) and possibly Marion Island mouse-free island, with significantly greater abundance of (Jones and Ryan 2010). Given these impacts, eradication of large Amphipods and Collembola, but fewer Spiders. mice is a sensible conservation goal; however, mouse These data on introduced mouse ecology and invertebrate eradication can be challenging (MacKay et al. 2007), distribution on Antipodes Island contribute to the body of including potential issues with bait-uptake (Wanless et al. knowledge on Southern Ocean islands. 2008; Cuthbert et al. 2011) and non-target poisoning (Wanless et al. 2009). Keywords Body-size Á Density Á House mouse Á Mice were first reported in the castaway depot on the main Invertebrates Á Southern Ocean Á Spatially explicit Antipodes Island in the early twentieth century (Chilton capture–recapture 1909, p 600). The introduced species of mice on Antipodes Island have been shown to have a unique haplotype not found elsewhere in New Zealand, with affinities to Spain (Searle & J. C. Russell ( ) et al. 2009). This genetic evidence suggests mice did not School of Biological Sciences, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand arrive during castaway depot annual provisioning from e-mail: [email protected] mainland New Zealand as previously believed, but more 123 1188 Polar Biol (2012) 35:1187–1195 likely with a foreign shipwreck such as the Spirit of the Dawn spp.) and ferns (see Godley 1989 for a complete descrip- or President Felix Faure (Taylor 2006; personal communi- tion). On the main island, six sites of differing vegetation cation 2011). Mice are now widespread across the entire and altitude were re-surveyed in January and February island, but remain absent from the northern pair of Bollons 2011, corresponding to previous locations selected by and Archway Islands and presumably other offshore stacks. Taylor, Moors and McIntosh (Fig. 1). Sites were relocated Mice could potentially be eradicated from Antipodes Island, to within 100 m from written descriptions and photo- but challenges include the isolation of the island, elevated graphs. At these sites, vegetation is most dense (2 m failure risk with mouse eradications, and non-target impacts height) around low-lying coastal areas such as Reef Point, from primary or secondary poisoning on two species Anchorage Bay and Hut Creek, in contrast to smaller, of island endemic parakeets (Cyanoramphus unicolor and widely spaced, cushion plants (0.5 m height) in the inland C. hochstetteri), subantarctic skuas (Catharacta lonnbergi) elevated plateau areas such as North Plains and Mt and gulls (Larus dominicus). Galloway. These vegetation communities have remained The Antipodes Islands are isolated and only visited unchanged over the duration of studies reported here. In occasionally by scientists, where the visits are of a short addition, a brief comparative visit (1 h), allowing only duration and are strictly regulated. Previous unpublished litter sampling, was made to the mouse-free northern pair research has been undertaken on the mice of Antipodes of Bollons and Archway Islands before departure in Feb- Island. During the University of Canterbury February– ruary, which are similarly characterised by dense low-lying March 1969 expedition Rowley Taylor trapped and made coastal tussock grassland (Poa and Carex spp.). The ter- observations on mice, noting in particular their association restrial arthropod fauna exhibits substantial endemism with with Carex spp. seeding (Taylor 1969; Warham and Jones general affinities to other Southern Ocean islands of New 1975; hereafter ‘Taylor’). During the Bountys, Antipodes Zealand, and is likely to have been impacted in both And Snares (BAAS) expedition of November 1978 Phil abundance and composition by mice (Marris 2000). Moors intensively studied the mice using live-capture grids Mouse population density was estimated using capture– (Moors unpubl.; hereafter ‘Moors’), and during the recapture on a 7 9 7 grid of 49 Longworth live-traps at Department of Conservation Taiko search expedition 10-m spacing for 7 nights at two intensively trapped sites November 1995, Angus McIntosh and John Marris studied (Reef Point and North Plains plateau). Traps were checked the impact of mice on the invertebrate fauna on Antipodes every morning, and at Reef Point all mice caught were ear- Island and compared it to the mouse-free northern pair of tagged for short-term individual identification. On the final Bollons and Archway Islands (Marris 2000; McIntosh night of trapping, all mice captured were euthanized for 2001; hereafter ‘McIntosh’). The objective of this study autopsy. As part of a simultaneous bait-marker study, all was to repeat the methods of Taylor, Moors and McIntosh mice on the North Plains plateau grid were euthanized on to provide a spatio-temporal comparison of mouse and first capture. During autopsy, all mice were weighed to the invertebrate community relative abundance and mouse nearest 0.5 g (50 g Pesola Scale) and measured for body morphology from a rarely visited Southern Ocean island. and total length to the nearest 1 mm (30 cm ruler). With An important purpose of this time series analysis was to additional autopsy data from Anchorage Bay (see later), test for an increase in rodent body-size, as has occurred on data were used to compare body-size, sex and adult/juve- other Southern Ocean islands recently, and for any ongoing nile ratios with 1978 autopsy summaries at three sites from change in invertebrate communities. I also conduct the first Moors (Reef Point, Anchorage Bay and North Plains pla- reliable density estimates of mice on Antipodes Island. teau) and 1969 data at one site from Taylor (Reef Point). Differences in body-size metrics (weight, head–body length and total length) were individually tested with Materials and methods analyses of variance for the model including fixed effects for site (Reef Point, North Plains and Anchorage Bay) and The Antipodes Islands group (2,097 ha; 49°410S; sex, a random effect for year of study (1969, 1978 or 2011), 178°480E) lie 872 km south-east of Bluff, New Zealand. and a study–site interaction. Only adults were used in this The climate is characterised by strong south-westerly analysis to prevent confounding from different adult/juve- winds with frequent cloud, fog and rain, and cool tem- nile ratios among seasons. peratures (2–13°C) with little variation (Taylor 2006). The Density, D, was estimated using maximum-likelihood island group is an important and diverse breeding site for spatially explicit capture–recapture (ML SECR) imple- seabirds and two species of native parakeets (Warham and mented in program R and package SECR (Borchers and Bell 1979). Vegetation on the main Antipodes Island Efford 2008). I assume a Poisson distribution of range (2,025 ha) is entirely composed of tussock grassland (Poa centres (i.e. random) with a half-normal curve detection and Carex spp.) interspersed with some shrubs (Coprosma function parameterised by g0 (the probability of detection 123 Polar Biol (2012) 35:1187–1195 1189 Fig. 1 Antipodes Islands (New Zealand inset). Asterisks are sampling sites when trap and range centre coincide) and r (the spatial approximated by the 95% circular probability density area scale of the detection function). Removals in the popula- of capture as tion (i.e. accidental deaths during trapping) are assigned A ¼ pð2:45rÞ2 known capture histories of 0 with probability equals 1 following death. A conditional likelihood is used to derive As part of a bait-marker study, all mice on the North density, incorporating individual covariates of age and sex, Plains plateau grid were euthanized on first capture; and models are compared using an AIC framework.