The effects of translocation on the spatial ecology of tiger snakes (Notechis scutatus) in a suburban landscape

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1 CSIRO PUBLISHING Wildlife Research, 2005, 32, The effects of translocation on the spatial ecology of tiger snakes (Notechis scutatus) in a suburban landscape H. Butler A, B. Malone A and N. Clemann B,C A Department of Zoology, Latrobe University, Bundoora, Vic. 3086, Australia. B Arthur Rylah Institute for Environmental Research, Department of Sustainability and Environment, PO Box 137, Heidelberg, Vic. 3084, Australia. C Corresponding author. nick.clemann@dse.vic.gov.au Abstract. In many suburban parts of Australia the removal of snakes from private property by licenced snake catchers is employed to mitigate perceived risks to humans and their pets. The number of snakes translocated around greater Melbourne, Victoria, each year can be very high (at least many hundreds). However, the effects of translocation on the behaviour and welfare of individual snakes, and the impact on existing snake populations at release sites are unknown. We used radio-telemetry of resident and translocated snakes to investigate the consequences of translocation on the spatial ecology of tiger snakes (Notechis scutatus) in a suburban parkland near Melbourne. Fourteen snakes (two female and four male residents, and four female and four male translocated snakes) implanted with radio-transmitters were tracked between spring 2002 and autumn Translocated snakes exhibited home ranges ~6 times larger than those of residents, although each group maintained core ranges of similar size. Translocated snakes travelled longer distances and were often located in residential areas adjacent to the park, whereas resident snakes were never located outside of the park. Introduction The study of reptile populations has predominantly focused on natural or semi-natural habitats, and there is little information about the ecology of reptiles living in suburban environments. Cohabitation of humans with wildlife in suburban areas frequently results in conflict that ranges from nuisance animals in human dwellings (e.g. brushtail possums (Trichosurus vulpecula): Miller et al. 1999) to attacks on humans by wildlife (e.g. attacks by Australian magpies (Gymnorhina tibicen): Jones and Thomas 1999). Translocation of wildlife into more suitable areas has been widely used to mitigate these issues (Jones and Nealson 2003; Clemann et al. 2004). Snakes found on private properties are usually unwelcome to their human cotenants, and capture and translocation of these animals remains a common management practice (e.g. Fearn et al. 2001; Shine and Koenig 2001; Clemann et al. 2004). However, the success of translocation is questionable (Galligan and Dunson 1979; Reinert 1991; Dodd and Seigel 1991; Reinert and Rupert 1999; Fischer and Lindenmayer 2000; but see Burke 1991): only 19% of monitored reptile translocations have proven successful, and none of these successful cases involved snakes (Dodd and Seigel 1991). Attempts to translocate snakes into novel environments are often unsuccessful. For example, long-distance translocation (over several kilometres) of rattlesnakes (Crotalus spp.) from reserves in the United States disrupts the snakes CSIRO 2005 normal activity patterns, resulting in relentless movements, and it may take over 12 months for an individual to settle into normal patterns of behaviour (Sealy 1997; Nowak 1998; Reinert and Rupert 1999). The greater frequency and distance of movements made by translocated individuals exposes them to greater risk of mortality (especially by humans), and translocated snakes may suffer 3 5 times greater mortality than residents (Reinert and Rupert 1999; Plummer and Mills 2000). In Victoria, hundreds of venomous snakes (Elapidae) are translocated from residential and suburban locations into safer areas annually in order to mitigate the perceived danger they pose to humans and pets (Clemann et al. 2004). However, the lasting benefit to humans and the fate of translocated snakes are unknown. Previous research concerning the effects of translocation on snakes has been limited to crotalid and colubrid snakes, for both wildlife human conflict and conservation purposes. Given their contrasting ecologies (Shine 1991; Greer 1997), it is possible that elapids may respond differently to translocation. Terrestrial colubrid and crotalid snakes are often ambush predators, whereas most large elapids are searching foragers (Shine 1991). The home-range size of snakes will also impose limits on the minimum area of habitat that is suitable as a release site for translocated individuals. The aim of this study was to determine the effects of translocation on the spatial ecology of tiger snakes (Notechis /WR /05/020165

2 166 Wildlife Research H. Butler et al. scutatus) in a suburban environment. Specifically, we intended to quantify the effects of translocation on the behavioural patterns and spatial ecology of individuals. Tiger snakes were studied because they are the most frequently encountered snake in suburban areas around Melbourne, and are often translocated (Clemann et al. 2004). The following questions directed our research. Do translocated tiger snakes released at our study site display different behavioural and spatial patterns to tiger snakes resident at the site? Do these translocated snakes move over larger distances than residents? And, do translocated snakes maintain larger home ranges than residents? Methods Study species The tiger snake (Notechis scutatus) is a largely terrestrial, medium-sized elapid (reaching a length of ~1.5 m) with highly toxic venom, and is responsible for a large proportion of human envenomations in Australia (Sutherland 1992). Tiger snakes are primarily diurnal, but crepuscular and nocturnal activity has also been recorded (Shine 1979; Greer 1997). Study site Our study was conducted at Westerfolds Park, in the Yarra Valley Parklands, located ~16 km north-east of Melbourne, Victoria. This 123-ha park encompasses 11 different vegetation communities (Beardsell 1996), but is dominated by grassy woodland with a sparse overstorey of river redgum (Eucalyptus camaldulensis) adjacent to the Yarra River. The park is bounded by the Yarra River to the north and west, and by suburban roads and residential areas to the south and east. A small creek fed by suburban stormwater bisects the park. Westerfolds Park receives an estimated visitors annually (Beardsell 1997), and is currently used as a relocation site for snakes captured in nearby residential areas (S. Watharow, professional snake catcher, personal communication). Surgical procedure Fourteen snakes (6 females and 8 males) were surgically implanted with radio-transmitters. The transmitters (Holohil Systems Ltd, Woodlawn, Canada: SI-2T, mm, 8.8 g, with a whip aerial) were coated in a biologically inert butyl-rubber resin, and were fitted with thermistors to allow collection of body-temperature data. Transmitters were gassterilised prior to implantation. Snakes ranged in weight from 209 to 609 g, thus ensuring that the transmitter weighed no more than 4.5% of an individual s body mass. This followed the protocol developed for other elapid snakes by Webb and Shine (1997). Surgery occurred as soon as practicable after capture (1 15 days, mean = 7.5) in order to minimise the time snakes spent in captivity. Snakes were anaesthetised using 5% Isoflurane, and respiration during surgery was maintained via direct intubation. Anaesthesia was maintained via positive-pressure ventilation of 2% Isoflurane. The transmitter was implanted into the peritoneal cavity via an incision between the first and second rows of body scales. The surgical site was sutured using 4-0PDS monofilament absorbable sutures. Ketoprofen (2 mg kg 1 ) was administered intramuscularly as an analgesic following surgery. To ensure that there were no adverse effects from the surgery, snakes were maintained in captivity for 1 10 days (mean = 3.7) after surgery and were released only during fine weather. Experimental design Three months prior to fieldwork (in August, at the end of winter), all licenced wildlife controllers in the Melbourne region were requested not to release any snakes into Westerfolds Park for the duration of the study, thus ensuring that any snakes we captured in the park were true residents (i.e. occurring naturally in the park, or naturalised within the park prior to this study). We captured six resident snakes (two females and four males) and, after transmitter implantation, these were released at their point of capture. Eight translocated snakes (four females and four males) were obtained from licenced wildlife controllers who had captured the snakes within 5 km of Westerfolds Park. After transmitter implantation, these snakes were released into suitable habitat at randomly selected points within the park. Suitable release points contained trees and either fallen timber, dense ground-level vegetation or some other form of cover. These criteria reflect the way in which licenced wildlife controllers usually select release sites (Clemann et al. 2004). Snakes were tracked 2 5 times per week (with at least 24 h between successive tracking episodes) from October 2002 to March The location of each individual was recorded using a hand-held globalpositioning system unit (Garmin, e-trex Vista GPS). Locations of snakes determined from radio-telemetry were used to calculate spatial ranges using the software BIOTAS v (Ecological Software Solutions 2001). Three different estimates of each snake s range were calculated, but snakes for which fewer than 25 data points ( locations ) were available were excluded from the statistical analyses. One hundred per cent minimum convex polygons (MCPs) were used to estimate the total range, 95% harmonic means were constructed as an estimate of home-range size, and 50% harmonic means were calculated as an estimate of the core range (Ujvari and Korsos 2000). For clarity, we will refer to these estimates thus: MCPs represent the entire area utilised by an individual throughout the study, whereas 95% and 50% harmonic means are referred to as the home and core range, respectively. Calculation of the 95% harmonic mean home range excludes 5% of outlying locations, helping to conform to Burt s (1943) home-range definition by excluding what could be considered occasional exploratory movements outside the true home range. Such an approach is particularly relevant to snakes immediately following translocation, when they may make a single, unidirectional movement before establishing a home range sensu stricto (Reinert 1991). Where appropriate, we log-transformed data to meet assumptions of normality and equality of variances. Mean MCP, home-range and corerange sizes of resident and translocated snakes were compared using independent-samples t-tests. Movements were recorded as the straight-line distance between successive locations, although this would have been an underestimate of the total distance travelled (Webb and Shine 1997; Whitaker and Shine 2003). Mean distances per movement were log-transformed and compared on a monthly basis using a two-way ANOVA (factors: (1) resident v. translocated, and (2) month). Post hoc independent-samples t-tests were then used to determine months in which significant differences occurred, with appropriate Bonferroni corrections to the α level. The mean proportion of days on which snakes moved was arcsintransformed and compared on a monthly basis using a two-way ANOVA (factors: (1) resident v. translocated, and (2) month). In cases where no significant difference was detected between resident and translocated snakes, data for both groups were pooled to allow comparisons between males and females. Before using parametric tests, assumptions of normality and equality of variances were checked using the Kolmogorov Smirnov test and Levene s test for equality of variances. Analyses were conducted using SPSS v for Windows. Results The size and sex of individual snakes, and the data obtained for each individual are presented in Table 1. Three of the telemetered snakes died during or after the radio-tracking

3 Effects of translocation on tiger snakes Wildlife Research 167 Table 1. Sex, length, mass and radio-tracking details for each telemetered tiger snake SVL, snout vent length Individual Sex SVL Mass Initial Tracking No. of No. of Mean distance/ (mm) (g) release period (days) locations movements movement (m) Resident A F Oct Resident B M Oct Resident C M Nov Resident D M Nov Resident E F Nov Resident F M Dec Mean ± s.e ± ± ± ± ± ± Translocated K F Nov Translocated L F Dec Translocated M M Dec Translocated N F Dec Translocated O M Dec Translocated P F Jan Translocated Q M Jan Translocated R M Feb Mean ± s.e ± ± ± ± ± ± study. Two resident male snakes died during summer; one of these was killed by a predator, the other succumbed to unknown causes. One translocated female snake was found dead during the winter following the formal radio-tracking period, but the cause of death was not determined. For each of our three estimates of home-range size, translocated snakes occupied larger areas, but there was considerable variation between individuals in both groups (Table 2). Translocated snakes had home ranges ~6 times larger than those of residents (mean: 28.1 ± 9.07 v. 4.9 ± 2.03 ha) (Table 2), and this difference was statistically significant (t = 2.329, d.f. = 9, P = 0.04). However, the MCPs occupied by both groups were not significantly different (t = 1.324, d.f. = 9, P = 0.22), although the difference between these groups in the range of the MCP data was considerable (residents 12.9 v. translocated 62.8 ha). This suggests that although both resident and translocated snakes moved over total areas of similar size, the movements of residents tended to be more confined, with fewer large excursions outside of the home range. Translocated snakes also displayed greater variation in home-range size than residents (Table 2), suggesting that individual snakes responded differently to translocation. Translocated and resident tiger snakes maintained core ranges of similar size (t = 0.294, d.f. = 9, P = 0.78) (Table 2). The core range of individuals occupied % of the home range (mean = 22%) (Table 2). Two snakes (individuals D and O) had multiple disjunct core ranges, representing distinct areas of frequent use that the snakes moved between. Home ranges showed considerable overlap within and between both groups. In fact, the home ranges of two residents (E and F) were almost completely overlapping. Conversely, both resident and translocated snakes appeared to maintain exclusive, non-overlapping core ranges. From October to March, translocated snakes travelled more than twice as far as residents (mean: 140 v. 64 m, two-way ANOVA with month and group as the factors, F 1,273 = 9.605, P = 0.002). There was high variability in the mean distances moved by translocated versus resident snakes (Table 1), perhaps indicating different individual responses to translocation. There was no difference in the mean distance travelled by translocated and resident snakes between months, or in the mean total distance travelled each month, suggesting an absence of strong seasonal effects on the distance of movements. Translocated snakes travelled further than residents in all months, but there was a significant difference only in December (post hoc independent-samples t-test, t = 3.08, Table 2. Home-range sizes (ha) of tiger snakes at Westerfolds Park One hundred per cent minimum convex polygons estimate the total range used by each snake; 95% and 50% Harmonic Means are an estimate of the home and core ranges, respectively (Ujvari and Korsos 2000) Individual Minimum convex Home Core polygon range range Resident A Resident C Resident D Resident E Resident F Mean (± s.e.) 3.88 ± ± ± Translocated K Translocated L Translocated M Translocated N Translocated O Translocated P Mean (± s.e.) ± ± ± 0.453

4 168 Wildlife Research H. Butler et al. d.f. = 57, P = 0.003) (Fig. 1), coinciding with the period in which half of the translocated snakes were released at the site (Table 1). This suggests that these individuals undertook large initial movements immediately after release. There was no difference between the overall frequency of movement of resident and translocated snakes (t = 1.113, d.f. = 12, P = 0.287), with snakes in each group having moved, on average, every second day (mean for resident snakes = 0.57, s.e. = 0.053, n = 6, for translocated snakes = 0.50, s.e. = 0.034, n = 8). Whilst snakes generally moved regularly from October to March, this period was punctuated with long periods of inactivity (up to several weeks) for some individuals. After pooling data for both groups of snakes, no difference was found between the proportions of days on which males (mean = 0.53 ± 0.059) and females (0.49 ± 0.022) changed location (t = 1.113, d.f. = 12, P = 0.287). Discussion Criticism of the use of translocation to solve human wildlife conflict is increasing (Dodd and Seigel 1991; Reinert and Rupert 1999; Fischer and Lindenmayer 2000). The results of our study lend further support to this criticism. During our study translocated tiger snakes had larger home ranges than did resident snakes within Westerfolds Park, and half of the translocated snakes did not remain within the park s boundaries. Within 1 16 days (mean = 9) after release, these snakes left the park and entered adjacent private properties. One translocated snake returned to an adjacent property four times, and, at the property owner s request, it was recaptured and returned to the park each time it was located on this property. This snake had a very large range, incorporating both the park and adjacent residential areas. It was the only snake that we recaptured and returned to the park, although others, of their own volition, occasionally re-entered the park from adjacent private properties. Although previous studies have shown that translocated snakes can take more than 12 months to settle into normal patterns of behaviour (Sealy 1997; Nowak 1998; Reinert and Rupert 1999), the Distance per movement (m) * * Residents Translocated October November December January February March (n = 2) (n = 5) (n = 1) (n = 5) (n = 5) (n = 5) (n = 7) (n = 5) (n = 7) (n = 5) (n = 7) Fig. 1. Monthly mean distance (m) travelled by resident and translocated tiger snakes. Bars represent mean ± 1 s.e. An asterisk denotes significance at P < 0.01 after Bonferroni correction for multiple post hoc tests. translocated snakes in the present study appeared to establish a form of home range within ~6 months of release. The difference between the MCP range of resident and translocated tiger snakes was not statistically significant. However, MCPs give no indication of how intensively the individual uses different parts of the range, and they often include large areas that animals never visit, owing to the disproportionate effect of single outlying locations (Kenward 2000). Conversely, using the 95% harmonic mean as the estimate of home range allows infrequent exploratory excursions (i.e. the outermost 5% of locations, as an arbitrary limit) to be excluded from analyses and permits a more realistic representation of the size and shape of the home range (Ujvari and Korsos 2000; Kenward 2000). This approach was particularly appropriate for translocated tiger snakes, which sometimes made single, long-distance movements from their initial release site before apparently establishing a true home range. The mean MCP size of resident tiger snakes at Westerfolds Park (3.88 ha) was similar to that of the broadheaded snake (Hoplocephalus bungaroides) (Webb and Shine 1997), less than 70% of that recorded for eastern brown snakes (Pseudonaja textilis) (5.8 ha: Whitaker and Shine 2003), but considerably larger than that of the activity ranges for tiger snakes from the New England region of New South Wales (0.75 and 0.77 ha: Shine 1979). However, these latter data relate to only two adult male tiger snakes that were tracked for a maximum of six weeks. These home-range sizes are relatively small compared with most of those recorded for snakes from other families (Slip and Shine 1988; Durner and Gates 1993; Secor 1994; Timmerman 1995; Shine and Fitzgerald 1996; Johnson 2000; Mullin et al. 2000). For example, diamond pythons (Morelia spilota spilota) have home ranges larger than 120 ha (Slip and Shine 1988; Shine and Fitzgerald 1996), whereas some crotalids and colubrids have home ranges up to 84.3 ha (Ciofi and Chelazzi 1994; Secor 1994; Timmerman 1995; Johnson 2000). An exception to this trend is the small home ranges (95% polygons of four adult males = ha) recorded using kernel density estimators for male gopher snakes (Pituophis catenifer, a colubrid) by Rodríguez-Robles (2003). The few studies on displaced snakes have found that translocated snakes occupy significantly larger and more variable home ranges than residents (Nowak 1998; Reinert and Rupert 1999; Plummer and Mills 2000). Reinert and Rupert (1999) found that translocated timber rattlesnakes (Crotalus horridus) made large exploratory movements (resulting in large home ranges), but later returned to their point of release. There is also limited evidence for homing behaviour in western diamondback rattlesnakes (Crotalus atrox), with translocated individuals making long unidirectional movements towards their original home range (Nowak 1998). Translocated tiger snakes showed no evi-

5 Effects of translocation on tiger snakes Wildlife Research 169 dence of homing behaviour, nor did any return to the point where they were released. However, one translocated individual returned four times to the same site in a residential backyard adjacent to Westerfolds Park, even after being repeatedly relocated to new locations within the park several hundred metres away. This suggests that learning had occurred, and that the individual remembered the whereabouts of this property. Core ranges of tiger snakes in the present study were largely non-overlapping, and translocated snakes maintained core ranges nearly 40% larger than those of resident snakes (1.33 and 0.96 ha respectively). For both groups, these core ranges may represent a trade-off between the minimum area necessary to incorporate an appropriate diversity of basking sites, refuges and foraging areas, and the maximum area from which conspecifics can be effectively excluded, if indeed core ranges are defended. Maintaining exclusive core ranges suggests that the snakes were either exploiting a limited resource, or that densities of snakes were low enough that core ranges did not need to overlap. Given that snakes sheltered in many types of refuges (in or under logs, under bark litter, inside grass tussocks, amongst rocks and inside rabbit burrows), it is unlikely that refuge sites were limiting. Similarly, although it has been suggested that basking sites may be limited in densely vegetated habitats (Greer 1997), densely vegetated habitats were relatively rare in Westerfolds Park, and were seldom utilised. The function of the core range (v. the MCP and home range) is rather enigmatic, and we hypothesise that the core range functions as a secure refuge during the relatively long periods of inactivity evident for this species. Three of our telemetered tiger snakes moved between multiple core ranges (two snakes had three core ranges, one snake had two, ~300 m apart) within their larger home range. This behaviour was not specific to resident or translocated snakes, nor to either sex. This has not been reported for other snakes. The reasons for the use of multiple core ranges is unclear, but may allow an individual to exploit different resources (e.g. prey or shelter) between seasons. Translocated tiger snakes made longer movements than residents (140 v. 64 m, on average, respectively). This difference was particularly obvious in December when most snakes were initially released, but the trend was apparent in all months. Similar results were reported for timber rattlesnakes where the distance per movement was also greatest after translocation (Reinert and Rupert 1999). Translocated individuals of other snake species also move further, and over larger areas, than residents (Nowak 1998; Plummer and Mills 2000). Although translocated tiger snakes moved further than residents, they did not move more frequently, implying that translocation did not affect the environmental or internal cues that stimulate movement. One limitation of our study was its relatively brief duration, so our results must be interpreted with some caution. This study was conducted over a particularly dry period, and the size of the tiger snake s home ranges may have been influenced by the effect of drought, which may reduce prey abundance/activity and cause snakes to forage less in order to conserve energy (Shine and Lambeck 1990). Of the six swamps present in the study area, all but one was dry prior to the onset of summer, and remained dry throughout the study. An associated reduction in foraging effort may have resulted in an underestimation of home-range size relative to wetter years. Other large elapids that occur in southeastern Australia show similar trends during dry periods: eastern brown snakes reduce their frequency of activity during drought (Whitaker and Shine 2002), and red-bellied black snakes (Pseudechis porphyriacus) suffer loss of condition due to reduced foraging success (Shine 1987). Alternatively, the home ranges of resident tiger snakes at Westerfolds Park could have been underestimated because monitoring ceased prior to the onset of the mating period. Tiger snakes mate after parturition in autumn (Shine 1977), and males of other species often move over larger areas whilst searching for mates (Durner and Gates 1993; Secor 1994; Timmerman 1995; Duvall and Schuett 1997; Whitaker and Shine 2003). Presumably, the movements of snakes that we recorded were not related to mating activity, and were probably driven by thermoregulatory or foraging activities. Snakes tend to move in a predictable manner and require familiarity with their environment (Sealy 1997; Plummer and Mills 2000; Whitaker and Shine 2003). The difference in MCP between resident (3.88 ha) and translocated (21.98 ha) tiger snakes appears to be the result of translocated snakes making long, exploratory movements either to familiarise themselves with the new environment, or to attempt to locate their prior home range (Reinert and Rupert 1999). Given that home ranges of resident snakes overlapped considerably, and that encounters between individuals are probably rare, it seems unlikely that territorial disputes caused translocated snakes to make large movements. On the other hand, there was very little overlap between core ranges, and translocated snakes may have moved over larger areas in search of suitable vacant space to adopt as their core range. Translocation clearly affects the behaviour and spatial ecology of tiger snakes. Our results show that, at Westerfolds Park, translocated tiger snakes have much larger home ranges, and make larger movements than do resident snakes. Thus, translocated snakes frequently breached the management boundaries of their release site and entered residential properties. We speculate that the release of large elapid snakes in metropolitan reserves may be adding to the problem it was meant to solve. Rather than remaining in the parks, many translocated snakes are simply including properties adjacent to these parks into their new home ranges. Whether this also occurs with other species and other release sites requires investigation.

6 170 Wildlife Research H. Butler et al. Acknowledgments We thank Dr Nataly Rourke and the Veterinary Department at Melbourne Zoological Gardens for conducting all veterinary procedures. This study received funding assistance from Parks Victoria (thanks to Sally Troy and John Wright) and the Australian Geographic Society. Libby Jude and the other Parks Victoria staff at Westerfolds Park facilitated fieldwork. HB thanks staff at the Austin and Repatriation Medical Centre for prompt and effective treatment of his bitten finger, and also for support from Kerry Bell, and Cindy and Gavin Butler. We thank the snake catchers who provided animals: Simon Watharow, Angela Reid, Jon Birkett, Nigel s Animal Rescue, Raymond Hoser and Wildpro Pty Ltd. Johnno Webb and Richard Shine provided advice on snake telemetry. Jenny Nelson and two anonymous referees provided constructive comments on an earlier draft of this paper. 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