The Herpetological Journal
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Volume 16, Number 2 April 2006 ISSN 0268-0130 THE HERPETOLOGICAL JOURNAL Published by the BRITISH HERPETOLOGICAL SOCIETY The Herpetological Journal is published quarterly by the British Herpetological Society and is issued1 free to members. Articles are listed in Current Awareness in Biological Sciences, Current Contents, Science Citation Index and Zoological Record. Applications to purchase copies and/or for details of membership should be made to the Hon. Secretary, British Herpetological Society, The Zoological Society of London, Regent's Park, London NWl 4RY, UK. Instructions to authors are printed inside_the back cover. All contributions should be addressed to the Scientific Editor (address below). Scientific Editor: Wolfgang Wi.ister, School of Biological Sciences, University of Wales, Bangor, Gwynedd, LL57 2UW, UK. E-mail: W.Wu [email protected] Associate Scientifi c Editors: J. W. Arntzen (Leiden), R. Brown (Liverpool) Managing Editor: Richard A. Griffi ths, The Durrell Institute of Conservation and Ecology, Marlowe Building, University of Kent, Canterbury, Kent, CT2 7NR, UK. E-mail: R.A.Griffi [email protected] Associate Managing Editors: M. Dos Santos, J. McKay, M. Lock Editorial Board: Donald Broadley (Zimbabwe) John Cooper (Trinidad and Tobago) John Davenport (Cork ) Andrew Gardner (Abu Dhabi) Tim Halliday (Milton Keynes) Michael Klemens (New York) Colin McCarthy (London) Andrew Milner (London) Richard Tinsley (Bristol) Copyright It is a fu ndamental condition that submitted manuscripts have not been published and will not be simultaneously submitted or published elsewhere. By submitting a manu script, the authors agree that the copyright for their article is transferred to the publisher ifand when the article is accepted for publication. The copyright covers the exclusive rights to reproduce and distribute the article, including reprints and photo graphic reproductions. Permission fo r any such activities must be sought in advance from the Editor. ADVERTISEMENTS The Herpetological Journal accepts advertisements subject to approval of contents by the Managing Editor, to whom enquiries should be addressed. Herpetological Journal website: http://biology.bangor.ac.uk/�bss 166/HJ FRONT COVER: Leiope/ma hamiltoni (P. J. Bishop) HERPETOLOGICAL JOURNAL, Vol. 16, pp. 97-106 (2006) A MODELLING APPROACH TO DETERMINE A TRANSLOCA TION SCENARIO FOR THE ENDANGERED NEW ZEALAND FROG LEIOPELMA HAMILTON/ M ANDY D . TOCHER 1, DAVID FLETCHER 2 AND PHILLIP J. BISHOP3 1 Science, Technology and Information Services, Science and Research Unit, Department of Conservation, Dunedin, New Zealand 2Department of Mathematics and Statistics, University of Otago, P. 0. Box Dunedin, New Zealand 56, 3Department of Zoology, University of Otago, P.O. Box Dunedin, New Zealand 56, A stage-structured population model was developed to predict which of nine hypothetical translocation scenarios was likely to produce the best outcome for the rare Hamilton's frog (Leiopelma hamiltoni McCulloch). Model outcome was measured in terms of population growth rate and probability of extinction. Only fe males were modelled. The model predicted that moving at least20 fe male adult frogs was the tiest strategy, and moving subadult frogs alone, or no frogs at all was the worst in terms of mean growth rate of both populations combined. When the new population was considered separately, introducing subadults alone was the worst strategy in terms of mean growth rate and extinction probability. Extinction of the donor population was most likely when 40 adult fe males were removed, and the extinction risk was reduced when only 20 were removed. We consider the most reasonable management strategy - confirmed by the modelling and supporting qualitative data- is the translocation of20 adult and 20 subadult female frogs (with the concurrent translocation of 40 males). This scenario provides a balance between risk of extinction in the donor population and probability of success in the translocated population. Key words: amphibian conservation, population viability analysis, relocation INTRODUCTION translocation of a specific cohort to a nearby predator fr ee island. Translocation to a nearby island with Hamilton's frogs (Leiopelma hamiltoni) are likely to similar habitat is a priority management action for L. have once been relatively widespread in New Zealand hamiltoni (Newman, 1996). Managers are faced with (Worthy, 1987), but are now present only on one small deciding how many frogs (and fromwhich age groups) island in the Cook Strait at the top of New Zealand's to remove from the only existing population in New South Island (Stephens Island: but see Bell et al., 1998; Zealand in order to attempt the establishment of an Holyoake et al., 2001). The estimated population size of other. In particular, a combination of subadult and L. hamiltoni on Stephens Island at the time of writing was adult frogs (n= l 0 in total) collected at random from the approximately 300 individuals, almost entirely living in a donor population (frog bank) each year for three years rock-tumble fragment of total area less than 300 m2 was being considered as a translocation strategy prior (Newman, 1990; Brown, 1994; Thomson, 1996; this pa to the modelling exercise carried out here (Mike A viss, per). Three adult frogs are known to reside in a second Department of Conservation, pers. comm. 2002). fragment on Stephens Island, less that 70 m from the main Three translocations have been previously carried rock tumble (Brown, 1994; Tocher & Brown, 2004). out with Leiopelma spp. In 1992, twelve L. hamiltoni Clearing of vegetation in the early 20th century de were transferred from the frog bank on Stephens Island stroyed most of the forest cover on Stephens Island, and to a man-made habitat 70 m away (Brown, 1994). Al generally improved conditions for the tuatara (Sphenodon though several frogs returned to their original site punctatus). L. hamiltoni is now confined to the rock-tum (Tocher & Brown, 2004), a new population fo unded ble fragment (frog bank) by high numbers of predatory by three frogs that remained at the release site, seems tuatara and severe weather conditions that are pro to be establishing. One hundred L. pakeka were nounced in the absence of forest. Recent data suggest the translocated to a forest remnant (Boat Bay) on Maud population could be increasing, and may be reaching car Island in 1984-85 (Bell et al., 2004). L. pakeka was rying capacity. Density has apparently quadrupled from subject to a second translocation in 1997 when 300 in the 58 per 100 m2 reported in the 1970s (Newman, 1990), dividuals were translocated to Motuara Island from to 220 per 100 m2 (this paper). Maud Island (Tocher and Pledger, unpublished data). Management options for L. hamiltoni include enhance Two main conservation lessons have emerged from ment of habitat on Stephens Island, captive breeding, and these translocations. Firstly, individual growth rates of Correspondence: M. D. Tocher, Science, Technology and L. pakeka were remarkably high following Information Services Science and Research Unit, Department translocations to both Boat Bay (Ben Bell, Victoria of Conservation, Private Bag 1930, Dunedin. University of Wellington, pers. comm. 2001) and E-mail: [email protected] 98 M. D. TOCHER ET AL. Motuara Island (unpublished data), presumably lower bank. As is usual for populations in which the sex ratio ing age to first reproduction. Secondly, there is evidence is close to I: 1, only females were modelled. Two that subadults are more likely to remain in the vicinity of populations were considered simultaneously: a donor the release site, following a translocation, than adults. population (population 'D'; frog bank) from which Adult L. hamiltoni have homed over 70 m from the re frogs were removed to create a new population (popula lease site to their original capture site (Tocher & Brown, tion 'T'). The structure of the model was the same forthe 2004). two populations, but the values forage at firstreprodu ction Intensive monitoring of L. hamiltoni on Stephens Is (AFR) were allowed to be lower in population T. land began in July 1997 to gather data on population We programmed the model in an Excel spreadsheet. demographics and in particular to determine the stability There were three stages in the model: juveniles, of the L. hamiltoni population over a six-year study. subadults and adults. Time spent as a juvenile before Data on L. hamiltoni juvenile survival rates in the wild becoming a subadult was estimated conservatively as 12 are sparse, and indicate high mortality between hatching months, based on capture-recapture data collected from and one year of age. Juveniles are thought to have spe the frog bank. We used a "pre-breeding-census" model cialized habitat requirements, are more prone to struc ture, which meant that the juvenile class contained desicc ation and may prefer moister locations compared those individuals that had survived their first year. The to older animals (Bell, 1978; Newman, 1990 ; Thomson, reason for this choice was to allow the annual fertility 1996). As such, juveniles are not under consideration for rate to be the product of the annual reproductive rate and translocation. first-year survival, thereby eliminating the need to Appropriate conservation management decisions can specify these two parameters separately (Caswell, only be addressed by combining our knowledge of the 200 1 ). Time spent as a subadult before entering the life history and current status of L. hamiltoni adult stage was determined by the estimated mean and populations with predictions from population model standard deviation of the age at first reproduction, using ling. A tailor-made model constructed to simulate the L. the 'variable stage duration' approach described in hamiltoni population at the frog bank is used to compare Caswell (200 I). various hypothetical translocation scenarios, and to Projections were made over a 30-year period, the identify important assumptions and parameters relating known natural minimum life span of L. hamiltoni (un to L. hamiltoni. This will ultimately guide managers and published data).