Polar Biol DOI 10.1007/s00300-010-0865-1 ORIGINAL PAPER Statistical models for monitoring and predicting eVects of climate change and invasion on the free-living insects and a spider from sub-Antarctic Heard Island Kendi F. Davies · Brett A. Melbourne · JeVrey L. McClenahan · Ty TuV Received: 7 March 2010 / Revised: 1 July 2010 / Accepted: 7 July 2010 © Springer-Verlag 2010 Abstract Terrestrial ecosystems of sub-Antarctic islands past 50 years, mean annual temperature has risen by more are particularly sensitive to global and local human than 1.2°C and mean annual precipitation has declined on impacts, including climate change and species invasion. average by 25 mm annually (Smith 2002). However, changes Invertebrates form a central component of these ecosys- in climate are often more complex than simple increases in tems. We conducted a stratiWed survey of 60 sites on sub- temperature and declines in rainfall. On Marion Island, there Antarctic Heard Island and used Poisson regression models are now also more clear-sky evenings, which has led to an to describe the spatial distribution and abundance of Wve of increase in the number and intensity of freeze–thaw events. the ten free-living species captured. Acari and Collembola Such climate changes can have direct eVects on biological were not considered. Five species were not caught in traps communities. For example, experimentally reducing precipi- in suYcient numbers to model. The distributions of species tation and increasing temperature on Marion Island signiW- were described by altitude, vegetation type and aspect. The cantly altered the community structure of the micro resulting distribution models can be used to both monitor arthropods of a keystone plant species (McGeoch et al. 2006). and predict the eVects of climate change and species inva- Although continuous temperature records are not avail- sion on this unique and valuable ecosystem. able, temperatures are warming and climate change is alter- ing terrestrial environments on Heard Island (Allison and Keywords Sub-Antarctic · Invertebrates · Keage 1986; Thost and Allison 2006). The mean temperature Climate change · Invasion · Heard Island at Atlas Cove for 1948–1954 was 1.3°C and for 1997–2000 was 2.1°C. This suggests possible warming of 0.8°C during the 50-year period from 1949 (Thost and Allison 2006), and Introduction in addition, since the late 1940s, 9.8% of the island has been deglaciated (Ruddell 2006). We should expect that increases Terrestrial ecosystems of sub-Antarctic islands are particu- in temperature and associated changes in solar radiation and larly sensitive to global and local human impacts. In particu- precipitation will alter the spatial distributions of species as lar, climate change, species invasions and their interaction the spatial distribution of suitable habitat changes, for exam- threaten the invertebrate and plant communities of these ple, along temperature gradients associated with altitude, islands (Davies and Melbourne 1999; McGeoch et al. 2006; assuming that species and their vegetation habitats are able to Chown et al. 2008). In general, temperatures are rising, and migrate; otherwise, we should expect species to go extinct. rainfall is declining on sub-Antarctic islands (Chown et al. To illustrate, for the vegetation community on Marion Island, 2008). For example, sub-Antarctic Marion Island is both le Roux and McGeoch (2008) recently showed a rapid ups- warming and drying with global climate change. Over the lope expansion in species’ distributions in response to 1.2 degrees C of warming since 1966. Sub-Antarctic islands are particularly vulnerable to inva- V K. F. Davies (&) · B. A. Melbourne · J. L. McClenahan · T. Tu sive species, and invaders have had major impacts on both Department of Ecology & Evolutionary Biology, UCB 334, University of Colorado, Boulder, CO 80309, USA the native biota and ecosystem processes of many islands e-mail: [email protected] (Brothers and Copson 1988; CraVord and Chown 1990; 123 Polar Biol Chapuis et al. 1994; Frenot et al. 2005; Chown et al. 2008). in the Southern Ocean and experiences the severe weather In general, invasions have greater eVects on island than typical of the region: mean sea level temperatures of 5.2°C mainland communities because islands have fewer species (maximum), ʕ0.8°C (minimum), mean windspeed of per functional group, lack some functional groups entirely 9.1 ms¡1, mean rainfall 1,400 mm. Snow falls throughout and have more keystone species (D’Antonio and Dudley the year but mostly in winter and spring (Thost and Allison 1995). Therefore, invaders can Wll unrepresented functional 2006). roles and thus have a signiWcant impact on ecosystem pro- Heard Island is dominated by the volcano complex, Big cesses (Cushman 1995; Chown et al. 2008). Recent exam- Ben (the highest point, Mawson Peak, is 2,745 m above ples of invertebrate invasions on sub-Antarctic islands sea level). Approximately 10% of the island’s 368 km2 is include two terrestrial crustaceans introduced to Macquarie currently ice-free (Kiernan and McConnell 1999) and Island (van Klinken and Green 1992; Richardson and Jack- available for establishment of vegetation. Vegetation son 1995; Greenslade et al. 2008). On Marion Island, where composition and patterns have been described recently repeated surveys have occurred since the mid-1980s, Wve (Bergstrom et al. 2002; Scott and Bergstrom 2006). BrieXy, new invertebrate colonizations have been detected in the Heard Island has 12 vascular plant species. Vegetation last 20 years. Three of these species have established popu- communities with closed canopies occur only in coastal lations, and the other two are predicted to establish popula- areas and some deglaciated valleys. Above about 50 m, the tions. As climate changes, the number of invasions is vegetation canopy is open, and above about 200 m, com- predicted to increase (Kennedy 1995; Frenot et al. 2005). munities are comprised of cryptogams only (Scott and Two lines of evidence provide support for these predictions Bergstrom 2006). (Chown et al. 2008): strong relationships between energy The terrestrial invertebrates of Heard Island are fairly availability and alien insect richness (Chown et al. 1998, well known (Chown et al. 2006). Here, we focus on the 2005) and the present day restriction of invaders to low alti- free-living macrofauna, which consists of Wve Coleoptera, tudes (Gabriel et al. 2001; Hulle et al. 2003). four Diptera, one Lepidoptera and one Araeae (Chown Species distribution models provide a baseline for et al. 2006). Also known from previous collections are 10 detecting, monitoring and predicting eVects of human Collembola, one Thysanoptera, 19 Phthiraptera, two Sipho- impacts on biota and ecosystem processes (Guisan and noptera, 34 Acari and seven Crustacea (Chown et al. 2006). Thuiller 2005). Strategically designed surveys provide a way to empirically estimate the distribution of species by Survey and analyses selecting sites along gradients of biologically important variables. Important determinants of species’ distributions The survey was undertaken between January and March include temperature, water, nutrients, sunlight, and in the 2001. Heard Island is characterized by distinct vegetation case of terrestrial invertebrates, the vegetation community. types and a sharp altitudinal gradient. We therefore selected These all vary at a range of spatial scales (Melbourne et al. sites for sampling by designing a stratiWed survey that sam- 2007) and can be captured by a stratiWed survey that sam- pled along gradients of altitude and vegetation type. We ples aspect, altitude, slope and vegetation. Correlative sta- included 60 sites in the survey design, distributed across tistical models can then be constructed from variables Heard Island in Wve geographic areas or regions: Round related to species distributions. We produced statistical Hill, Scarlet Hill, Cape Lockyer, Long Beach and Atlas models relating the distribution of invertebrates on Heard Cove/Mt Drygowski (Fig. 1). This level of sampling was Island to major environmental variables and vegetation. achievable within the Wve-month period and was suYcient The models provide a tool to assess, monitor and predict to produce reliable statistical models of invertebrate distri- the eVects of climate change and invasive species. Previ- bution on Macquarie Island (Davies and Melbourne 1999). ously, distribution models were produced for the inverte- We included altitude as a continuous variable (measured brates of Macquarie Island (Davies and Melbourne, 1999). to § 5 m), but for the purpose of getting a reasonably bal- anced design and for selecting sites, we deWned altitude classes as 0–100 m, 100–200 m, 200–300 m, 300–400 m, Method 400–500 m, 500–600 m, and 600+ m. We included Wve vegetation categories, which spanned the range of vegeta- The Island tion types. Our vegetation classes were Poa cookii domi- nated (>75%), Pringlea antiscorbutica dominated (>75%), At 53°06ЈS 73°32ÃE, Heard Island is 1,650 km north of Azorella selago dominated (>75%), feldmark (<50% vege- Antarctica, 4,350 km southeast of South Africa, 520 km tation but with the existing vegetation consisting of at least southeast of Îles Kerguelen and 5,340 km southwest of 50% moss) and “patchy Azorella” (50–75% Azorella but Western Australia. Heard Island is south of the polar front <75% total vegetation). Our design crossed vegetation with 123 Polar Biol Fig. 1 Map of Heard Island showing survey regions (AC Atlas Cove, CL Cape Lockyer, LB Long Beach, RH Round Hill, SH Scarlet Hill) and sites (circles) altitude. However, since not all vegetation classes occurred Bothrometopus gracilipes (C. O. Waterhouse) (weevil), at all altitudes, the design was unbalanced and included 16 Notodiscus hookeri (Reeve 1854)(mollusc), Embryonopsis altitude-vegetation combinations. At most, we managed to halticella (moth), Calycopteryx mosleyi Eaton (Xy) and Wnd between 10 and 13 sites in each of Wve regions giving Amalopteryx maritime Eaton (Xy). us a total of 60 sites. We also measured aspect and slope at each site.