Consequences of multiple herbivores for forests
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 (forests and deer), 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).
Managers of New Zealand's indigenous forests, including the Department of Conservation, tangata whenua, private individuals and Trusts, want to know if management of herbivores is effecting changes they expect, such as maintenance of palatable tree species. In the 'Forests and deer' project, learning groups of local citizens knowledgeable about the forests are involved in setting the research agenda and in taking up the results. Information from the SORTIE/NZ model will be tested in other New Zealand forests over time.