Landcare Research - Manaaki Whenua

Landcare-Research -Manaaki Whenua

Control of multiple pests in forests

Image - W Roscoe

New Zealand's land biota evolved in the near-absence of mammals. Māori introduced one new mammalian herbivore (the rat, kiore) and Europeans introduced over 25 more species, including three more rodent species, brushtail possums, and various species of deer.

Before mammals were introduced, forests had been grazed for millennia by flightless birds. These became extinct within 150 years of Māori settlement. Therefore trees that germinated in the post-moa, pre-European-mammal era (c. ad 1500-1800) grew, at least beyond kiore effects on seeds and seedlings, in an era with no grazing at all - many of these trees form the canopies of today's forests.

A key problem is that decades of research into how indigenous forest communities respond to pest management has been inconclusive, with different responses being observed under different conditions. A major reason for this unsatisfactory result appears to be that our thinking on the nature of responses is too simplistic. For example, there is no reason to expect that controlling (or even removing) introduced herbivores will necessarily result in forest recovery to some previous state not influenced by these animals.

To understand effects of multiple herbivores requires investigations of the life-history stages of trees at which they are most vulnerable. These stages include seeds (consumed by rodents and pigs), seedlings (grazed by many mammals but especially vulnerable to deer, goats, pigs), and adults (certain trees browsed by possums, and flowers and fruits by possums and ship rats). Experimental approaches are used, such as fenced exclosures and different hunting regimes.

In a collaborative project with the Department of Conservation (DOC), in the Kaweka Ranges, we are investigating the likelihood that seedlings will grow beyond the reach of sika deer and, in time, reach the canopy.

In another collaborative project with DOC, we are investigating the adequacy of seedling regeneration and the representation of palatable species in four forests nationally, each with different hunting regimes.

Because simple models of forest processes and interactions with herbivores have generally been found wanting, a more complex model is required to underpin forest management. Since 1998, a model, SORTIE/NZ, has been developed for complex forests at Waitutu, on the south coast of Fiordland.

We now know that it is important to incorporate below-ground processes in the model because introduced herbivores alter the nature of litter inputs to the soil. We also know that we need to incorporate the impacts of multiple herbivores because of their differential impacts on seeds (mice) and seedlings (red deer) in the regenerative phase of forest development. All of this needs to be understood within a context of varying fertility among forest stands because this is an important determinant of herbivore impacts. The model is underpinned by rigorous field testing and is developed in cooperation with DOC, the Institute of Ecosystem Studies (USA), The University of Cambridge (UK) and the Victorian Department of Natural Resources and Environment (Australia).


  • Glen AS, Hamilton T, McKenzie D, Ruscoe WA, Byrom AE 2012. Kiwi Apteryx mantelli population recovery through community-led trapping of invasive non-native mammals in Northland, New Zealand. Conservation Evidence 9: 22–27.
  • Ruscoe W, Dickson R, Leckie C, Hania J, Glen A 2011. Wide-scale predator control in Hawkes Bay: community involvement in conservation. In: Saunders G, Lane C ed. Proceedings: 15th Australasian Vertebrate Pest Conference, Sydney, June 20-23 2011. Pp. 113.
  • Ruscoe WA, Ramsey DSL, Pech RP, Sweetapple PJ, Yockney I, Barron MC, Perry M, Nugent G, Carran R, Warne R, Brausch C, Duncan RP 2011. Unexpected consequences of control: competitive vs. predator release in a four-species assemblage of invasive mammals. Ecology Letters 14(10): 1035-1042.
  • Ruscoe WA, Pech RP 2010. Rodent outbreaks in New Zealand. In: Singleton G, Belmain S, Brown P, Hardy B ed. Rodent outbreaks : ecology and impacts. Manila, Philippines, International Rice Research Institute. Pp. 239-251.
  • Ruscoe W, Cave S, Sweetapple P, Pech R, Barron M, Yockney I, Perry M, Carran R, Brausch C 2010. Species interactions and consequences of pest control in forest ecosystems. Protect Autumn: 13-14.
  • Ruscoe WA, Norbury G, Choquenot D 2006. Trophic interactions among native and introduced animal species. In: Allen RB, Lee WG ed. Biological invasions in New Zealand. 186 ed. Ecological Studies. 16. Berlin, Springer. Pp. 247-263.
  • Wilson DJ, Ruscoe WA, Burrows LE, McElrea LM, Choquenot D 2006. An experimental study of the impacts of understorey forest vegetation and herbivory by red deer and rodents on seedling establishment and species composition in Waitutu Forest, New Zealand. New Zealand Journal of Ecology 30(2): 191-207.
  • Ruscoe WA, Elkinton JS, Choquenot D, Allen RB 2005. Predation of beech seed by mice: effects of numerical and functional responses. Journal of animal ecology 74(6): 1005-1019.
  • Ruscoe WA 2004. A new location record for kiore (Rattus exulans) on New Zealand's South Island. New Zealand journal of zoology 31: 1-5.
  • Ruscoe WA, Wilson D, McElrea L, McElrea G, Richardson SJ 2004. A house mouse (Mus musculus) population eruption in response to rimu (Dacrydium cupressinum) seedfall in southern New Zealand. New Zealand Journal of Ecology 28(2): 259-265.
  • Choquenot D, Ruscoe WA 2003/1. Landscape complementation and food limitation of large herbivores: habitat-related constraints on the foraging efficiency of wild pigs. Journal of animal ecology 72(1): 14-26.
  • Ruscoe WA, Choquenot D, Heyward R, Yockney I, Young N, Drew K 2003. Seed production, predators and house mouse population eruptions in New Zealand beech forests. In: Singleton GR, Hinds LA, Krebs CJ, Spratt DM ed. Rats, mice and people : rodent biology and management. 96 ed. ACIAR monograph. Canberra, Australian Centre for International Agricultural Research. Pp. 334-337.
  • Choquenot D, Ruscoe WA, Murphy E 2001. Colonisation of new areas by stoats: time to establishment and requirements for detection. New Zealand Journal of Ecology 25(1): 83-88.
  • Ruscoe WA 2001. Advances in New Zealand mammalogy 1990-2000: House mouse. Journal of the Royal Society of New Zealand 31: 127-134.
  • Ruscoe WA, Goldsmith R, Choquenot D 2001. A comparison of population estimates and abundance indices for house mice inhabiting beech forests in New Zealand. Wildlife research 28(2): 173-178.
  • Choquenot D, Ruscoe WA 2000/11. Mouse population eruptions in New Zealand forests: the role of population density and seedfall. Journal of animal ecology 69(6): 1058-1070.
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