Extinction Vulnerability in Two Small, Chronically Inbred Populations of Chatham Island Black Robin Petroica Traversi

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Extinction vulnerability in two small, chronically inbred populations of Chatham Island black robin Petroica traversi A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University by Euan S Kennedy Lincoln University 2009 Abstract of a thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy, Lincoln University, New Zealand Extinction vulnerability in two small, chronically inbred populations of Chatham Island black robin Petroica traversi Euan S Kennedy ABSTRACT New Zealand ecologists and wildlife managers have assumed traditionally that threatened insular endemic bird species are less susceptible to the predicted genetic consequences of declines and inbreeding. Conservation has relied on treating deterministic extinction pressures to trigger population recoveries, with few measures taken to minimise stochastic genetic threats to long-term viability. This study tested that assumption empirically by exam- ining extinction vulnerability in two small, critically threatened Chatham Island black robin populations protected from external threats on habitat-restricted islands but not man- aged purposefully for genetic health. Analysis of the factors eliminating the species’s original island populations indicated loss through habitat change and mammalian predation within perhaps no more than two to three decades of human contact. Loss was undoubtedly hastened by intrinsic extinction-proneness (low reproductive output, forest-interior prefer- ence, sensitivity to habitat quality, intolerance of open spaces), greater in black robins than in other Petroica species in New Zealand and Australia. Low productivity in ancestral sur- vivors prevented spontaneous recovery after translocation to better habitat, requiring inten- sively assisted breeding to raise population sizes quickly. Strong growth in response to management did not suggest a moribund species but post-management growth rates were significantly slower. Analysis of demographic data from 1980-81 to 2001-02 showed breeding effort to be comparable in all populations but breeding failures of varying sever- ity at different life-history stages affected natural recovery in each. The worst affected suf- fered from briefer adult life-spans and more severe failure late in the breeding cycle. Gen- erally colder sub-canopy air temperatures suggested adverse environmental pressure on this population. Extant populations are highly inbred and severely depleted genetically following successive bottlenecks (prolonged historically) and recovery from a single-pair i event between 1979-80 and 1982-83. At current small sizes, accumulating levels of relat- edness are anomalously high. Contrary to perceptions that black robin recovery indicates normal fitness, viability was shown to be threatened by inbreeding depression expressed cumulatively in the breeding cycle as low juvenile production and survival, thus aggravat- ing a principal vulnerability. Declines did not result during the study period and effects were limited to reproductive fitness costs in black robin fathers only. There was no evi- dence of harmful effects on survival probability or from maternal inbreeding but an emerg- ing kinship effect on yearling production is possible. The restricted costs in such highly inbred populations (relative to less inbred congeners) suggest historical purging of genetic load. Work is needed to clarify this and the extent to which inbreeding could reduce re- sponses to new extinction pressures. Recent population declines may elevate levels of inbreeding further. Genetic management is strongly recommended, including habitat expan- sion to maximise population sizes and prevent further cross-breeding with a sympatric congener. Management and monitoring recommendations apply valuable lessons derived from a critique of past practices. In summary, findings give qualified support to the hy- pothesis of reduced susceptibility to inbreeding depression in threatened insular endemic birds. Findings do not support the assumption implicit in black robin management historically and in New Zealand wildlife management practice generally that stochastic genetic threats are unimportant risks to viability. KEY WORDS: Chatham Island black robin, Petroica, population, extinction, vulnerability, viability, decline, inbreeding depression, insular, endemic, genetic, translocation, bot- tleneck, productivity, survival, recovery, hybridisation ii ACKNOWLEDGEMENTS This project began on a wintry Mangere Island in 1978 when I first encountered the black robins. There were seven. On the same visit, I joined the privileged company of the many black robin workers whose earnest labours over more than four decades to save this bird from extinction have made my research project possible and worthwhile. An enormous quantity of the data I have used is theirs. I have variously blessed and cursed them for the quality of their observations and records, but I am profoundly grateful for their work, their society and their company in the field. During this project, I have been supported by good friends, family and colleagues. Without them, this work would not have come about or have been completed. Kerry-Jayne Wilson, my principal supervisor, has never faltered in her support, patience and uplifting good humour, though I have often given her cause to do so. Her generosity with knowledge, advice and time has been immense. The magisterial megaphone has been necessary more than once, but if she ever doubted my ability to do this subject justice, she was compassionate enough not to show it. Professor Richard Duncan, my associate supervisor, has guided me implacably and with great clarity and understanding well beyond my comfort zone in quantitative research. His unqualified encouragement has sustained me through many crises of confidence. I thank you both for your mentoring and support. Associate Professor Ian Jamieson, my external advisor at Otago University, has been unstintingly kind with his knowledge, time and frequent access to publications in preparation. I have benefited immeas- urably from his lucid, patient explanations of conservation genetics. Any errors committed in trans- lating his advice are mine alone. The Department of Conservation has assisted this project in countless ways. Mike Cuddihy, Con- servator in Canterbury Conservancy, encouraged this project and supported it financially in its ten- der early stages. I am also genuinely grateful to Amanda Baird, Hilary Aikman, Dave Houston, Denise Fastier, Delia Small, Antje Leseberg, Mala Nesaratman, Dr Rod Hay, Dr Adam Bester, Andy Grant, John Kearvell, Peter Hiemstra, Keri Ford, Raewyn Burt and others I have acknowl- edged in the thesis for information, file-hunting, field records, recollections of past events, graphics or help in the field. Similarly, I could not have come this far without the help of the splendid people in the Department’s Chatham Islands Area Office. In Christchurch, Wendy Osborne and Megan Hiatt preserved my fragile dignity at formatting time. Rod Morris, Brian Bell, Ian Hogarth, Ron Nilsson, Rodney Russ and John Kendrick, past black robin colleagues, have helped with recollections and allowed me to copy personal records, some from the very early days. Don Merton not only shipped all of his records to me, which was invaluable, but gave me the benefit of his in- comparable black robin knowledge on numerous occasions. iii At Lincoln University, my sincere thanks to Alison Lister and Dr James Ross who gave me thesis- saving help with statistics, and Mike Bowie who got me up and running with the temperature data- loggers. To Associate Professor Sue Worner, my express thanks for help with interpreting the temperature data. My warm appreciation to Bernadette Mani and Jane Edwards for their willing and welcome assistance with postgraduate advice and scholarship applications all these years. And to Professor Harvey Perkins, my great friend, thank you once again for your timely mentoring and encouragement. The staff at Archives New Zealand in Wellington and Christchurch were simply marvellous in the privileged access they allowed me to official files and other indispensable black robin records. Without their help, this project would have been very significantly compromised at the start. I thank Sheena Townsend at Otago University for help with stubborn pedigree software at a critical moment, and Olly Rudd at Natural History New Zealand for assistance with video records of the early black robin habitats. I cannot thank historian Bill Carter enough for sharing his valuable research with me and hearing out my own interpretations of historical events on the black robin islands. I hope I have been able to reciprocate by sharing discoveries of my own. To the generous people of Chatham and Pitt Islands, in whose hands ultimately the black robin’s future lies, my sincere gratitude

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