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Survival of Adult Mountain Stone Weta Hemideina Maori (Orthoptera: Anostostomatidae) Along an Altitude Gradient As Determined by Mark- Recapture

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Survival of Adult Mountain Stone Weta Hemideina Maori (Orthoptera: Anostostomatidae) Along an Altitude Gradient As Determined by Mark- Recapture JOYCE, JAMIESON, BARKER: WETA SURVIVAL AT DIFFERENT ALTITUDES 55 Survival of adult mountain stone weta Hemideina maori (Orthoptera: Anostostomatidae) along an altitude gradient as determined by mark- recapture Shelley J. Joyce1, Ian G. Jamieson1,* and Richard Barker2 1Department of Zoology, University of Otago, P.O. Box 56, Dunedin, New Zealand 2Department of Mathematics and Statistics, University of Otago, P.O. Box 56, Dunedin, New Zealand *Corresponding author (E-mail: [email protected]) ____________________________________________________________________________________________________________________________________ Abstract: The mountain stone weta Hemideina maori, a tree weta, is a cold-adapted New Zealand insect that shows increasing body size with increasing altitude and decreasing temperature. This study modelled the monthly survival probability of adult weta at three sites (high, medium and low altitude) in the Rock and Pillar Range, Otago. Survival was predicted to be lowest at the low elevation site where weta are at the lower limit of their current altitudinal range. A total of 504 adult weta were marked and released at all three sites between November 1999 and May 2002. Mark-recapture analysis showed that survival varied over time, being lowest during the summer months. Survival also differed between the sexes, with females having a higher probability of survival than males, but there was no difference in survival between altitudes. Our findings that body size varied significantly with elevation but that survival was similar between sites, suggest that body size at each altitude might be adapted to the local environment. It would be of interest in a longer term study to model survival of Hemideina maori as a function of variation in average summer temperatures, to test predictions about the effects of climate change on populations of alpine ectothermic animals. ____________________________________________________________________________________________________________________________________ Keywords: altitude; body size; mark recapture; survival; weta. Introduction with sex and body size and also as a function of environmental variables (Lebreton et al., 1992). The Variation in life histories can result from differences in mountain stone weta Hemideina maori (Orthoptera: genotypes or from environmentally-induced differences Anostostomatidae) is a New Zealand insect that shows among individuals of a given genotype (Roff, 1992). increasing body size in association with increasing This latter phenomenon, termed phenotypic plasticity, altitude and decreasing temperature in the Rock and is one way for an organism to overcome the problem of Pillar Range, Central Otago (Dowsett, 2000; Koning living in a heterogeneous environment (Mousseau et and Jamieson, 2001). However, little is known about al., 2000). In ectotherms, growth rate tends to be the life history traits of Hemideina maori, and in slower and maturation delayed by development at cool particular how variation in body size and environmental temperatures. As a result, adult ectotherms attain larger temperature influence survival probability. body sizes at high altitudes and latitudes (Atkinson, To estimate survival probability in the field under 1994; Partridge and Coyne, 1996; Atkinson and Sibly, natural conditions, it is necessary to follow individually 1997; Arnett and Gotelli, 1999). Because body size is marked animals through time (Lebreton et al., 1992). an important ecological parameter, there is a need to Hemideina maori is suited for mark-recapture studies understand both the effects of temperature on body because individuals are large and flightless, meaning size and how body size ultimately relates to survival they can be easily tagged and monitored. Also, their (Atkinson and Sibly, 1997). This is particularly relevant habit of taking refuge under rock slabs during the day given current ecological interest in climate change, makes recapturing weta relatively easy (Jamieson et and that 99% of the world’s animal species are al., 2000; Leisnham and Jamieson, 2002; Leisnham et ectothermic (Atkinson and Sibly, 1997). al., 2003). Hemideina maori lives in an alpine It has been shown both theoretically and environment, is freeze tolerant (Ramløv, 1992; Sinclair empirically that survival probability is a major et al., 1999), and occurs between 1000 and 1500 m determinant of life history evolution (Charlesworth, a.s.l. (King et al., 1996). Weta are nocturnal and most 1980; Roff, 1992). Adult survival probability can vary active during the summer months. In the Rock and New Zealand Journal of Ecology (2004) 28(1): 55-61 ©New Zealand Ecological Society 56 NEW ZEALAND JOURNAL OF ECOLOGY, VOL. 28, NO. 1, 2004 Pillar Range summer night-time temperatures are on average 4°C higher at 1100 m than at 1450 m (summit elevation) (I. Jamieson, unpubl. data), and adult weta found at lower altitudes have smaller adult body sizes than those found near the summit (Koning and Jamieson, 2001). Little is known about the underlying factors causing this variation in body size and whether there are any associated differences in survival. This study assesses whether monthly survival probability of adult weta varies significantly across high, medium and low altitudes in the Rock and Pillar Range. Given that Hemideina maori is a cold-adapted insect, we predicted its survival rate would be lowest at low altitudes, where they are living at the lower limit of their altitudinal range and where they frequently experience what we assumed to be sub-optimal conditions such as warm temperatures. Methods Study sites The Rock and Pillar Range is located in Central Otago, 53 km inland from the east coast of the South Island of New Zealand (45°28'S, 170°02'E). The range runs SW Figure 1. Location of the three study sites on the southern to NE for 23 km and is 1450 metres a.s.l. at its highest slope of the Rock and Pillar Range. Dashed lines indicate point. Environmental conditions are harsh, with four-wheel drive tracks. persistent strong winds and low air and soil temperatures (Bliss and Mark, 1974). Characteristic of the range are the schist outcrops (tors), which Hemideina maori inhabits. Tors can range in size from one rock a few Site A (upper) 1400 m metres wide to several rock columns up to 30 m in Vegetation at the summit consists of low-lying cushion diameter (Jamieson et al., 2000; Leisnham and plants and herbfields (Bliss and Mark, 1974), and is Jamieson, 2002). Weta are found primarily under rock leasehold farmland lightly grazed by sheep. The study slabs that have broken off tors and that can be lifted site had three large tors approximately 750 m apart, relatively easily (King et al., 1996; Jamieson et al., with 19, 38 and 46 rocks on and around each 2000). respectively that were large enough for an adult weta The three sites for this study were located along to take shelter under. the gently rising southern slope of the range, on an altitudinal gradient (Fig. 1). Site A was at 1400 m a.s.l. Site B (middle) 1250 m (Infomap 260 H43 804380), Site B was 6 km south Vegetation at Site B is similar to that at the summit, west of Site A at 1250 m a.s.l. (Infomap 260 H43 with herb-fields and cushion plants between the tors. 765213) and Site C was 7 km south west of site B at The area is also leasehold farmland lightly grazed by 1100 m a.s.l. (Infomap 260 H43 756132). Site C is also sheep. The study site had three large tors with 34, 38 at the lowest altitude that weta have been found in the and 39 rocks on and around each of them, and one Rock and Pillars (I. Jamieson, unpubl. data). Study small tor with 13 rocks. Tors were at least 300 m apart. tors at each site were chosen because of their relative isolation from surrounding tors. Leisnham and Jamieson Site C (lower) 1100 m (2002) marked all adult weta at Site B over a four year Vegetation at Site C differs from that of Sites A and B, period to examine dispersal patterns. They found no being predominantly tussock grassland with scattered marked weta outside the study area and dispersal rates shrubs; the land is also more heavily grazed by sheep between tors within the study area were relatively low and cattle. The common gecko Hoplodactylus (< 4% of marked adult weta, n = 480). Therefore, we maculatus, which can prey on juvenile but not adult assumed for this study that few, if any, tagged weta weta (Toynbee, 2002), is also present at this site but not moved outside the study areas during the course of the the other two sites. Site C had three small tors, two of research. JOYCE, JAMIESON, BARKER: WETA SURVIVAL AT DIFFERENT ALTITUDES 57 φ φ which had 13 rocks and were 100 m apart; the third tor { (group*time) p(group*time)}, where and p represent had three rocks and was 400 m from the other two. survival and recapture probability, respectively. This Weta are much less common in the vicinity of Site C model allows survival and recapture probabilities to be than Sites A and B, and as mentioned previously, are different between groups for each time interval, i.e., an on the edge of their lower altitudinal range (I. Jamieson, interaction between the two factors. For this study unpubl. data). there were six groups; three sites, with two sexes at each site. Data from 11 sampling sessions were used Data collection in the analysis and there were 10 time intervals Data were collected from December to May over three corresponding to the periods between each session. Weta survived the interval between each session with successive breeding seasons (1999/2000, 2000/2001 φ and 2001/2002). During the 1999/2000 season and the a probability of (ij) and had a probability of being re- 2001/2002 season, sampling was undertaken once captured at each session of p(ij), where i indexes time every six weeks at each site.
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