Impact of predators on population recovery of saddleback Philesturnus carunculatus translocated to offshore islands in New Zealand
Published source details
Hooson S. & Jamieson I.G. (2003) The distribution and current status of New Zealand saddleback Philesturnus carunculatus. Bird Conservation International, 13, 79-95
Published source details Hooson S. & Jamieson I.G. (2003) The distribution and current status of New Zealand saddleback Philesturnus carunculatus. Bird Conservation International, 13, 79-95
The saddleback Philesturnus carunculatus is a medium-sized forest passerine endemic to New Zealand. Formerly widespread throughout mainland North and South Islands and on nearby smaller islands, its susceptibility to predation by introduced mammals such as stoats Mustela erminea and rats Rattus spp. led to its extinction on the mainland, with the North Island subspecies P. c. rufusater surviving only on one small offshore island, and the South Island saddleback P. c. carunculatus on another small island and two nearby islets. However, since the mid-1960s a series of successful translocations have resulted in the establishment of both subspecies on a number of offshore islands. As part of a broader review of the status of the species, this study compared the population trends of North Island and South Island saddlebacks that had been translocated to 20-odd offshore islands with differing suites of predators.
Details of the original translocations are provided elsewhere (see the original paper and, for example, Lovegrove 1996 for references). This study compared the average annual population increase (expressed as a percentage of final population size to control for initial differences in the number of individuals released) in populations of North Island and South Island saddlebacks translocated to islands with: i) no predators; ii) weka Gallirallus australis (a large flightless native rail); iii) Pacific rats or kiore Rattus exulans; iv) weka and Pacific rats; and v) weka, Pacific rats and Norway rats R. norvegicus. In a small number of cases, islands were included in more than one category where trend data were available before and after the eradication of a predator. Translocated populations were excluded from the analysis if: the translocation had occurred too recently, or there had been too few population censuses, to quantify subsequent trends; the island was small (< 15 ha) and carrying capacity had been reached soon after the initial release; or the island had been invaded by stoats (which can extirpate any saddleback population) since the translocation.
Annual increases of North Island (n = 2) and South Island (n = 5) saddleback populations on islands without any predators both averaged around 20%. Similarly, both subspecies fared well on islands with weka present, although the average annual increase in South Island saddleback populations (c.10%; n = 3) was lower than in those of North Island saddlebacks (c.30%; n =2). However, in the presence of Pacific rats, North Island saddleback populations increased on average by around 15% per year (n = 7), whereas the population of South Island saddlebacks on Putauhinu Island declined annually by 10% between 1984–1991 (but then increased by over 30% per year following the eradication of Pacific rats in 1997). On Inner Chetwode Island, two translocations of 30 (in 1965) and 17 (in 1970) South Island saddlebacks failed in the presence of Pacific rats and weka (equating to average annual declines of around 40%). Finally, North Island saddlebacks displayed a minimal annual population increase of 1% on Kapiti Island in the presence of Norway rats, Pacific rats and weka, but increased by 33% per year following the eradication of both rat species in 1996.
Lovegrove T.G. (1996) Island releases of Saddlebacks Philesturnus carunculatus in New Zealand. Biological Conservation, 77, 151–157.
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