When deer must die: large uncertainty surrounds changes in deer abundance achieved by helicopter- and ground-based hunting in New Zealand forests David M. Forsyth A,G , David S. L. Ramsey A , Clare J. Veltman B , Robert B. Allen C , Will J. Allen D , Richard J. Barker E , Chris L. Jacobson F , Simon J. Nicol A , Sarah J. Richardson C and Charles R. Todd A A Arthur Rylah Institute for Environmental Research, Department of Sustainability and Environment, 123 Brown Street, Heidelberg, Vic. 3084, Australia. B Science and Technical Group, Department of Conservation, c/- Private Bag 11052, Palmerston North 4442, New Zealand. C Landcare Research, PO Box 40, Lincoln 7640, New Zealand. D Learning for Sustainability, PO Box 30108, Barrington, Christchurch 8244, New Zealand. E Department of Mathematics and Statistics, University of Otago, PO Box 56, Dunedin 9054, New Zealand. F Sustainability Research Centre, University of the Sunshine Coast, Locked Bag 4, Maroochydore DC, Qld 4551, Australia. G Corresponding author. Email: dave.forsyth@dse.vic.gov.au Abstract Context. When environmental, economic and/or social effects of wildlife are considered undesirable and need to be reduced, managers require knowledge of the effectiveness of candidate control techniques, particularly the relationship between control effort and change in abundance. Aims. We evaluated the effects of control on the abundances of introduced red deer (Cervus elaphus scoticus) and sika deer (Cervus nippon) at three New Zealand forest sites (two North Island, one South Island) in an 8-year adaptive-management experiment. Methods. We identified paired areas of 3600 ha at each site that were as similar as possible in geology, physical environments and forest composition and applied deer control (helicopter- and/or ground-based hunting) to a randomly selected member of each pair. The abundances of deer were monitored in each treatment and non-treatment area for up to 7 years by using faecal pellet counts on 50 randomly located transects. Key results. The difference between deer abundances in the treatment and non-treatment areas was significantly negative at one site, significantly positive at one site and indistinguishable at the other site. Faecal pellet abundances declined with increasing helicopter-based hunting effort but did not change with increasing ground-based hunting effort. There was evidence that aerially sown 1080 baits used for possum control in two treatment areas reduced deer abundances. Conclusions. The substantial uncertainty surrounding the relationships between deer control effort and changes in deer abundance means that managers cannot assume that the environmental, economic and/or social problems caused by deer will be alleviated with the quantum of control effort applied in the present study. Implications. Reducing the abundances of deer in forests may require substantially more control effort than is currently believed. Received 25 January 2013, accepted 27 August 2013, published online 1 October 2013 Introduction Managers often want to reduce the densities of invasive or overabundant wildlife populations so that the perceived negative environmental, economic and/or social impacts of the animals are minimised (e.g. Mack et al. 2000). However, those impacts will not be mitigated if the control fails to reduce populations to densities at which impacts are minimised (Choquenot and Parkes 2001; Nugent et al. 2001b). The efficacy of control is likely to vary with the method of control as well as with environmental factors such as habitat and climate (e.g. Veltman and Pinder 2001). It is therefore important that managers understand the effectiveness of the control techniques available to them, particularly the relationship between control effort (and hence cost) and reduction in population size or density (Hone 1990, 1994; Ramsey et al. 2012). The most reliable way to understand the effects of control techniques on populations is by experimentation (i.e. replicated and randomised assignment of treatment and non-treatment; CSIRO PUBLISHING Wildlife Research, 2013, 40, 447–458 http://dx.doi.org/10.1071/WR13016 Journal compilation Ó CSIRO 2013 www.publish.csiro.au/journals/wr