Research Article |
Corresponding author: Corinne Watts ( wattsc@landcareresearch.co.nz ) Academic editor: Corinna S. Bazelet
© 2017 Corinne Watts, Danny Thornburrow, Ian Stringer, Vanessa Cave.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Watts C, Thornburrow D, Stringer I, Cave V (2017) Population expansion by Cook Strait giant wētā, Deinacrida rugosa (Orthoptera: Anostostomatidae), following translocation to Matiu/Somes Island, New Zealand, and subsequent changes in abundance. Journal of Orthoptera Research 26(2): 171-180. https://doi.org/10.3897/jor.26.21712
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Wētā, large wingless anostostomatid orthopterans, have been the most frequently translocated insects in New Zealand. Until recently, such translocations were only monitored intermittently to confirm presence. We investigate the spread of Cook Strait giant wētā (Deinacrida rugosa Buller, 1871) after its release on Matiu/Somes Island, Wellington, New Zealand, in 1996. Adult wētā were surveyed from 2008 to 2016 using footprint tracking tunnels and/or searching with spotlights at night. The population underwent a reversal in distributional abundance after 2008. In 2008, they were abundant in the north and rare in the south but by 2013 and 2015 they were relatively less abundant in the north and common in the south. Why they diminished in the north remains unknown but possible causes are predation on juvenile wētā by nocturnal geckos (detected in the north and east but not in the south), by some habitat change (mostly reduction of some lawn), or by a combination of these together with removal of wētā from the north for translocation elsewhere. Further research is required to confirm which of these factors affect wētā abundance, if there are other causes, and if any further change in distributional abundance occurs.
conservation, footprint tracking tunnels, gecko, skink, threatened species
Translocation, the deliberate movement of living organisms from one area to another (
In New Zealand, the flightless and often large bodied anostostomatid Orthoptera colloquially known as wētā, evolved since the Cretaceous in the absence of terrestrial mammals except for bats. Some species of wētā undoubtedly disappeared during the widespread local extinctions that occurred after the arrival of kiore (Rattus exulans) with Polynesians ca. 1300 years ago and after other rodents and predatory mammals were introduced by Europeans (
Here, we investigate the spread of the Cook Strait giant wētā, Deinacrida rugosa Buller, 1871, after 62 individuals were released on Matiu/Somes Island, Wellington, New Zealand, in 1996 (
Arrangement of tracking tunnel transects (shown in white) along the footpaths on Matiu-Somes Island. Each circle indicates the location of a tracking tunnel. The dark hatched area indicates where Deinacrida rugosa were released in 1996. The light hatched area shows where 186 adult D. rugosa were taken for translocation in 2007 and 2008. Note that no wētā were removed from the North transect.
During the course of this research we also acquired incidental observations on lizard distributions and include the results because of the possibility that predation by lizards may have contributed to changes in distributional abundance of D. rugosa on Matiu/Somes Island. We acknowledge that our methods for surveying wētā may not have been ideal for surveying lizards so we consider these results are only indicative.
Three geckos (Woodworthia maculata (Gray, 1845), Mokopirirakau granulatus (Gray, 1845), Naultinus elegans Gray, 1842) and four skinks (Oligosoma aeneum (Girard, 1858), O. kokowai Melzer, Bell & Patterson, 2017, O. nigriplantare (Peters, 1874), O. polychroma (Patterson & Daugherty, 1990)) were present on Matiu/Somes. W. maculata, O. aeneum, O. nigriplantare and O. polychroma survived the habitat changes on the island whereas the other lizards were released there between 1999 and 2007 (
Matiu/Somes Island (24.9 ha) was completely cleared of native forest when it was an animal quarantine station. Extensive restoration began in 1981 and about 14 ha of the island are now covered in the early stages of regenerating coastal forest and scrub. Tree species include karaka (Corynocarpus laevigatus), mahoe (Melicytus ramiflorus), broadleaf (Griselinia littoralis), lemonwood (Pittosporum eugeniodes) and coastal tree daisy (Olearia solandri), and shrubs include taupata (Coprosma repens), tauhinu (Ozothamnus leptophyllus) and flax (Phormium tenax). Approximately 11 ha (44%) of this vegetation at the northern end of the island is >4 m tall. Pasture and grass still cover about 1.8 ha, mostly towards the centre of the island and alongside some pathways, while the remainder is seashore, road paving and buildings (
Data were obtained using footprint tracking tunnels and visual searches during visits to Matiu/Somes Island on 11–15 February 2013, 14–18 February 2015. On 3–4 February 2016, no tracking tunnels were set and only searching was carried out. Differences in research effort followed from constraints in funding and the availability of field assistants. The distribution of D. rugosa from 12–15 February 2008 (3 nights) was published previously by
Six transects, each consisting of a series of tracking tunnels (‘Black Trakka’: Gotcha TrapsTM, www.gotchatraps.co.nz) spaced 30 m apart, were set out on existing footpaths. Three transects were positioned near each other at the north end of the island and three were spaced elsewhere around the footpath that circumnavigated the island (Fig.
The footprints of anostostomatid wētā are readily recognisable (e.g.
Footprints of geckos and skinks are also clearly identifiable (
A visual search using spotlights was made once along each of the six transects each night for four consecutive nights in 2013 and 2015 starting approximately 1 hour after sunset from 2120 to 0130 hours. On 3 and 4 February 2016, a search was made each night of the six transects but tracking tunnels were not set. Each search was extended 30 m beyond the first and last tracking tunnel (i.e. five searches were 420 m long and one was 390 m long). The path and up to ca. 1 m on both sides, together with vegetation up to ca. 2 m high, were systematically and thoroughly searched by a group of three people without disturbing the vegetation. Two people side by side at the front searched the path and the ground and low vegetation on their side of the path and the third person followed searching taller trees and shrubs. To reduce potential search bias we followed procedures outlined in
Each time a wētā was found, its position was taken with a GPS (estimated accuracy usually <5 m) and the wētā was marked with small individually numbered labels. The GPS positions enabled us to count the number of wētā found within distances of 15 m along the transects from each tracking tunnel position. Information obtained from marking was not used in the present investigation but is published elsewhere (
Finally, we counted the number of geckos seen on the paths during the last search (night of 4 February 2016) after we noticed that they were subjectively more abundant on paths where few adult D. rugosa were found during a search on the night of 3 February 2016. Geckos were not counted during previous years because the weather was cooler and they were rarely seen on paths.
Temperature and humidity were recorded with an Escort iLog EI-HS-D-32-L data logger set just above ground level under dense shrubs. This continuously recorded data for the duration of each survey except 2016. A ventilated plastic cover shielded the data logger from dappled sunlight. Temperature and humidity recordings were averaged for each search period.
The potential spatial and temporal changes in adult D. rugosa distribution were investigated using two sample z tests for proportions by comparing the percent of tunnels tracked per transect pairwise between years 2008, 2013 and 2015. Furthermore, a maximum likelihood chi-square test for association was used to assess whether the relative percentages of tracked tunnels per transect had changed over time. A contingency table permutation test, with the chi-square statistic calculated by maximum likelihood and 4999 random permutes, was used to test whether the relative frequency of wētā observed along the six transects differed between 2008, 2013, 2015, and 2016. The contingency table permutation test was also used to compare just the 2013 and 2015 data.
The effect of median temperature and median vapour pressure deficit (VPD) during the search period on the total number of adult D. rugosa found per night by searching six transects in 2008, 2013 and 2015 was assessed using simple linear regression. The effects of median VPD and median temperature on the percent of tunnels tracked by adult D. rugosa per night in 2008, 2013 and 2015 were also assessed using simple linear regression.
The 2008, 2013 and 2015 tracking data were used to investigate the relationship between the presence of lizards and adult D. rugosa. Separate linear regressions were used to assess whether the percent of tunnels tracked by wētā per transect per year was related to the percent tracked by i) skink and ii) gecko. For gecko, but not for skink, there was statistical evidence that the regression lines should differ between years (p<0.05). An analogous analysis was used to assess the relationship between the presence of lizards and other wētā (not adult giant wētā). For both skink and gecko, as there was no statistical evidence that the regression lines should differ between years (p>0.05), a common line was fitted to data over all years.
For all linear regressions, the residual diagnostic plots provided no evidence of any departure from the assumption of independent normally distributed residuals with constant variance.
All analyses were conducted in Genstat 18 (
In both 2013 and 2015, adult D. rugosa were most frequently found along the paths and on mowed lawn at the southern end of the island (Southwest transect).The lowest numbers were found in the north (North transect) and along the eastern side of the island (Northeast and Southeast transects). This contrasted with the distribution in 2008 when most were found along transects in the north (Central, North and Northeast transects; Table
The distribution of tracking tunnels with adult D. rugosa footprints present followed the same overall pattern of adult wētā observed by searching: in 2013 and 2015 adults were most often detected using tracking tunnels along transects at the southern and south-eastern end of the island whereas fewer were detected along the North and Northeast transects. In 2008, in contrast, adult footprints were most often found in tracking tunnels in the northern half of the island (Table
Overall, more adult wētā were found by searching per night in 2013 than in 2008 even though the temperatures in 2013 were slightly lower than in 2008 (Tables
The shift in distribution is not as obvious when presence–absence of wētā is examined using data from each tracking tunnel position combined with wētā found by searching within 15 m of each tracking tunnel. When the results at each position were summed for all nights during each visit to the island (Fig.
Percentages of the total numbers of adult Deinacrida rugosa found along six tracking tunnel transects during searches at night each year. Searches were made on successive nights: three in 2008, four each in 2013 and 2015, and two in 2016. (Data for 2008 from
Transect | 2008 | 2013 | 2015 | 2016 |
---|---|---|---|---|
1 North | 20.3 | 2.2 | 7.5 | 9.6 |
2 West | 12.6 | 9.3 | 8.2 | 7.4 |
3 Southwest | 2.8 | 55.1 | 48.9 | 40.4 |
4 Southeast | 3.5 | 7.7 | 6.8 | 3.2 |
5 Northeast | 19.6 | 3.4 | 6.0 | 3.2 |
6 Central | 41.3 | 22.3 | 22.6 | 36.2 |
Total seen | 143 | 323 | 133 | 94 |
Average proportions (%) of all tracking tunnels set in each transect that contained footprints of adult Deinacrida rugosa wētā in February 2008, 2013 and 2015. All tracking cards were baited with peanut butter. Results are averaged over 3 nights in 2008 and 4 nights in 2013 and 2015 (Data for 2008 are from
Transect | 2008 | 2013 | 2015 |
---|---|---|---|
1 North | 50.0 | 13.9 | 10.4 |
2 West | 41.7 | 52.8 | 31.3 |
3 Southwest | 13.9 | 50.0 | 20.8 |
4 Southeast | 38.9 | 55.6 | 31.3 |
5 Northeast | 75.8 | 21.2 | 9.1 |
6 Central | 38.9 | 36.1 | 14.6 |
Overall | 42.7 | 40.5 | 19.7 |
Although meteorological conditions differed between nights and visits to the island (Table
Total numbers of adult Deinacrida rugosa found each night by searching along six tracking tunnel transects together with temperature and vapour pressure deficit (VPD) during the search periods. Data for 2008 are from
Date | Number Wētā | Sex ratio (M:F) | Median temperature (°C) (range) | Median VPD (kPa) (range) |
|
---|---|---|---|---|---|
2008 | 13-Feb | 34 | 0.21 | 18.3 (18.3–18.4) |
0.36 (0.34–0.37) |
14-Feb | 63 | 0.37 | 19.0 (18.8–19.1) |
0.27 (0.26–0.30) |
|
15-Feb | 45 | 0.22 | 19.4 (19.1–19.8) |
0.91 0.86–0.95) |
|
2013 | 11-Feb | 61 | 0.27 | 17.8 (17.6–18.2) |
0 (0) |
12-Feb | 73 | 0.40 | 17.5 (16.6–19.7) |
0.04 (0–0.11) |
|
13-Feb | 98 | 0.53 | 16.2 (15.5–16.8) |
0 (0) | |
14-Feb | 91 | 0.52 | 16.3 (14.9–19.3) |
0.26 (0–0.82) |
|
2015 | 14-Feb | 16 | 0.33 | 13.2 (13.0–13.2) |
1.06 (1.00–1.12) |
15-Feb | 28 | 0.33 | 13.0 (12.8–13.3) |
1.39 (1.35–1.43) |
|
16-Feb | 51 | 0.46 | 13.7 (12.8–14.5) |
1.44 (1.37–1.53) |
|
17-Feb | 35 | 0.59 | 15.0 (14.4–16.1) |
1.51 (1.39–1.64) |
|
2016 | 3-Feb | 45 | 0.41 | 22.7 (21.9–23.4) |
0.98 (0.87–1.08) |
4-Feb | 49 | 0.26 | 20.9 (19.2–22.6) |
0.84 (0.56–1.12) |
Gecko footprints were present on tracking tunnel cards mostly in the north of the island in 2008, 2013 and 2015 but some were also detected along the Southeast transect in 2015 (Fig.
Geckos were frequently observed on the northern transects during the night of 3 February 2016 whereas subjectively fewer were seen elsewhere. When counted during the night of the 4 February 2016, geckos were common along the North and Northeast transects where few wētā were found. Few geckos were found along the Central and southern transects where wētā were most frequently seen. Few geckos or wētā were observed along the West and Southeast transects (Table
Numbers of adult Deinacrida rugosa and geckos seen along the tracking tunnel transects on Matiu/Somes Island during the nights of 4 February 2016.
Transect | No. wētā | No. geckos |
---|---|---|
1 North | 5 | 20 |
2 West | 4 | 4 |
3 Southwest | 24 | 0 |
4 Southeast | 0 | 8 |
5 Northeast | 1 | 24 |
6 Central | 20 | 3 |
Overall | 54 | 59 |
Distribution of adult Deinacrida rugosa presence as evidenced by combining detection with tracking tunnels baited with peanut butter and finding them by searching at night. Searches extended 15 m from each tracking tunnel. Results are presence-absence derived from three searches over three nights in 2008, and four searches over four nights in both 2013 and 2015. Areas searched (tracking tunnel transects) are indicated as white lines. 2008 data from
The percentage of tunnels tracked by adult D. rugosa had a negative linear relationship with the percentage of tracking tunnels tracked by geckos (linear regression with separate lines, R2=0.71, p=0.005), but no relationship was detected between adult D. rugosa and skinks (linear regression with a common line for all years; R2=0.07, p=0.305). Conversely, whilst there was no evidence of a relationship between the percentages of tunnels tracked by ’other‘ wētā and those with gecko footprints (linear regression with a common line for all years; R2=0.08, p=0.265), those with footprints of ‘other’ wētā showed a negative linear relationship with tracking tunnels tracked by skinks (linear regression with a common line for all years; R2=0.41, p=0.004).
Our observation that the population of D. rugosa expanded towards the south of Matiu/Somes Island between 2008 and 2013 confirms the suggestion by
D. rugosa also underwent a population shift on Matiu/Somes Island while the population was expanding over the island between 2008 and 2013–2016, because their abundance subsequently declined in the north while it increased in the south. This was evidenced by the results from both searches at night and from tracking tunnels (
It is not clear why numbers of D. rugosa diminished in the north after 2008 but habitat change, (particularly a reduction in available lawn area), harvesting for translocation elsewhere, predation by geckos or a combination of these may have been responsible. Another possibility suggested by G.W. Gibbs (pers. comm. 2015) to account for the population shift by H. crassidens is that some unknown nutritional factor higher in concentration in the south may be responsible, although
Habitat change, particularly a reduction in lawn, was probably responsible for some reduction in abundance of D. rugosa in the north between 2008 and 2016. This follows because adults were most frequently found on lawn adjacent to shrubbery. We did not monitor habitat but did observe the following subjective changes. Lawn along about 40% of the North transect became reduced from a strip 1.5–2 m wide in 2008 to <0.2 m wide in 2016 by overgrowth of adjacent shrubs. The increasing height of scrub reduced the size and numbers of scattered clumps of grass along the Northeast and West transects while bushes were progressively planted after 2008 in retired pasture at the eastern end of the Southwest transect. The latter formed an almost continuous low canopy by 2016 leaving only a fringe of grass alongside the path. We would have expected that habitat change would have favoured an increase in the abundance of H. crassidens in the north because continued growth of trees would result in increasing numbers of holes suitable for roosting.
The negative relationship detected between the numbers of nocturnally active geckos both seen and tracked in tracking tunnels does not demonstrate that these predators reduce the numbers of adult D. rugosa but it does indicate that further research is required to confirm this. W. maculata is the most abundant gecko on Matiu/Somes Island. It is known to be a generalist insectivore/frugivore which consumes large numbers of small invertebrates (
We detected no relationship between skinks and adult D. rugosa although our data on skink presence is unreliable because we tried to reduce their access to tracking tunnels during the daytime as described above. The skinks on Matiu/Somes Island are also primarily active on the ground during the day so they were unlikely to encounter juvenile D. rugosa which are nocturnally active.
Tuatara are known to eat large wētā (e.g.
The removal of giant wētā for translocation between 2007 and 2010 is unlikely, by itself, to have contributed much to the reduction in the numbers of these insects in the north (Table
Distribution of geckos and skinks as detected using tracking tunnels on Matiu/Somes Island. Data are combined presence-absence of footprints on cards from tracking tunnels baited with peanut butter during three nights in 2008 and four nights in both 2013 and 2015. 2008 data from
Numbers of Deinacrida rugosa collected from Matiu/Somes Island for release in Zealandia (Karori Wildlife Sanctuary), Wellington and Cape Sanctuary, Hawkes Bay. NR = not recorded. Data provided by R. Empson, K. Nakagawa.
Date removed | No. Male | No. female | Total |
1 Feb 2007 | 25 | 75 | 100 |
Feb 2008 | 22 | 64 | 86 |
20 Jan 2010 | 12 | 30 | 42 |
30 Mar 2010 | NR | NR | 8 |
16 Mar 2013 | 15 | 26 | 41 |
Our results, together with those of
Further research is required to confirm if predation by nocturnal geckos can reduce the abundance of D. rugosa. If predation by geckos explains the negative relationship between these species then it might also account for the southward population shifts experienced by both this wētā and H. crassidens on Matiu/Somes Island. We also recommend that D. rugosa and the reptiles present on Matiu/Somes Island be appropriately monitored at intervals of perhaps 5- or 10-years to document how their relative numbers and distributions change in case predation by reptiles is an important factor in establishing populations of this wētā elsewhere and in their long-term survival.
This research was supported by CoRE funding for Crown Research Institutes from the New Zealand Ministry of Business, Innovation and Employment’s Science and Innovation Group, under CO9X0503 and Department of Conservation (DOC) investigation No. 4091. We thank Jo Greenwood and Emma Dunning (DOC Rangers, Matiu/Somes Island), and Wellington Tenths Trust for their support. Thanks to George Gibbs for providing unpublished results and reviewing the manuscript, to Raewyn Empson (Zealandia) and Kahori Nakagawa (Cape Sanctuary) for data on wētā taken from Matiu/Somes Island, to Robbie Price for preparing the maps and Chris Winks for the use of the data logger. We are grateful to Andrea McCormick, Andrea Hoffman, Leanne Leelo and Mati Armentolicus for help searching for wētā at night. We appreciate information on lizard diet provided by Rod Hitchmough, Debra Wotton, and Trent Bell. Anne Austin, Eric Edwards, Fiona Carswell, Jo Monks, John Innes, Laurent Tatin, Steve Trewick and an anonymous referee provided useful comments on the manuscript.