Introduced mammals and models for restoration
Introduction
This paper traces the development of ecological restoration in New Zealand and discusses factors that have influenced its direction. A major factor has been the extent to which introduced animals have changed selective forces operating on the native biota. A second factor has been our improved capacity to control or remove introduced mammals. And a third factor of importance is the growing realization that there are sometimes opportunities to rebuild former trophic guilds even though restoration of the full system is not possible.
Restorative action within New Zealand landscapes was initiated by the need to restore soil and vegetative cover to disturbed land such as mining sites, road cuttings and landslides. These kinds of action can be distinguished as revegetation or rehabilitation in contrast with ecological restoration, the subject of this paper. In reality these activities are part of a management continuum (Table 1). The imperatives for ecological restoration are those of providing habitats for threatened animal or plant species and restoring particular indigenous plant-animal communities to sites depleted of their original biological diversity.
The land forming New Zealand split from the Gondwana continent, 75 or more million years ago. The country is thus continental in origin but long isolation gives it many features of an oceanic island. The only land mammals present at the time of first human contact were three species of bat. This provides a clue towards answering the often-asked question: why are the biotas of isolated islands so adversely affected by introduced mammals? The explanation appears to be that it is a direct consequence of the largely mammal-free history of these islands. Their native animals have not had sufficient time with predatory mammals to evolve either escape or aggressive behaviours that would protect them against predation. And similarly, the native plants have not had sufficient time to evolve physical deterrents, such as spines, or chemical deterrrents in the foliage, that would protect them against mammalian herbivores (Atkinson, 1989). This is not to suggest that either predatory or herbivorous vertebrates were absent from pre-human New Zealand. The country had at least four avian predators (Holdaway, 1989) including the giant but extinct Haast's eagle Harpagornis moorei. Tuatara Sphenodon spp., which still survive, are ancient sphenodont reptiles that would have been effective predators of some animals, including burrowing seabirds. Most native animals would have developed behaviours that enabled them to co-exist with these predators. The 11 species of moa, extinct ratite birds, were large herbivores up to 250 kg or more in weight. It appears that native plants did evolve some structural deterrents to reduce their browsing impact (Greenwood and Atkinson, 1977) but these features have been only partially effective as protection against introduced ungulates (Atkinson and Greenwood, 1989).
Section snippets
Restoration on islands
First attempts in ecological restoration in New Zealand were made on offshore islands of the continental shelf (i.e. islands <50 km from the mainland). There are now 22 active restoration projects on offshore islands as well as two in the Chatham Islands, 870 km east of the mainland. The longest running is that on Cuvier Island (36°28.5′ S 175°46.5′ E) which began in 1960 when goats were eradicated.
Effective ecological restoration is not possible in a New Zealand environment without control, if
Restoration on the mainland
On the New Zealand mainland (North and South Islands), ecological restoration began in the early 1970s (Atkinson, 1988) but has only become widespread during the past 5 years. There are currently 12 major projects in the North Island and six in the South Island, initiated either by the Department of Conservation or by private organisations or individuals (e.g. Wilson, 1994). Some of these endeavours are now referred to as “mainland islands” (Saunders and Norton, this volume). These 18 projects
Introduced mammals and their effects in changing selection regimes
As in most countries, the direct effects of humans together with the effects of alien species we introduced, have resulted in major environmental changes. Since human contact, invasions by alien plants and animals total more than 1600 plant species, 34 species of land mammals, 33 species of landbirds, three species of reptiles and amphibians, 20 species of freshwater fish and possibly 1500 species of insects (refs. in Atkinson and Cameron, 1993). This paper emphasizes the impact of introduced
Goals for restoration
If it is intended to re-create conditions essential for a threatened species of animal or plant to survive, repair a damaged indigenous community to an earlier condition, or re-assemble the species of a past community, goals must be clearly defined.
With a threatened species, four steps are necessary to ensure recovery: (1) identify the factors most likely to be responsible for the species' threatened status; (2) control or remove any introduced plants or animals shown to be inimical to the
Restoring the ecological/evolutionary context and descriptive models for restoration
Selection pressures can be divided into physical influences, affecting all species, and biological influences that are crucial for some species but not for others. Examples of the first category are climatic and edaphic effects as well as periodic climatic or geologic catastrophes. Selection pressures of the second category are much more species-specific and include herbivore and predator effects, and those of competitors, as well as the influence of pollinators, dispersers, pathogens and
Achieving effective restorations: difficulties and solutions
Plant–animal communities, past and present, are dynamic in two senses: (1) autogenic change consequent upon the growth and interactions of the species present; and (2) externally imposed disturbance in which climatic and geological events such as droughts, high-intensity rainstorms, flooding, fires, volcanic eruptions, landslides and earthquakes alter the structure and composition of the community. Autogenic change is to be expected as a continuous process at every restoration site. Our need
Translocations
Translocation of species is an essential tool for ecological restoration. It is defined by IUCN (1987) as the intentional release of plants or animals to the wild to establish, re-establish or augment a population. Thus it can involve introduction of a species to a new site or to a site where it was previously present. In New Zealand, translocations for conservation purposes began in 1895 with the efforts of Richard Henry, a gifted naturalist working alone from a small island in Dusky Sound,
Substituting for extinct taxa
Extinction of a species is a permanent loss of a unique entity. The idea of using a related species to substitute for this loss has occurred to a number of people though it is seldom considered seriously. However, suggestions for substitutions relating to extinct birds in New Zealand were made by Atkinson, 1988, Atkinson, 1990.
The aim of a taxon substitution in a restoration project is to establish a functional surrogate for an extinct taxon, i.e. it must aim to reinstate the nexus of
Conclusion
Restoring plant–animal communities of the past can be seen as the ultimate challenge for “ecosystem management”. But improved understanding of the restoration process, and therefore our ability to manage, is most likely to come from studies of single species and their often subtle interactions with other species and physical factors. Species selected for study may be those threatened, major components of the system or, as strongly argued by Simberloff (1988), keystone species. Questions
Acknowledgments
I am grateful to David Towns, Alan Saunders, Toni Atkinson, Mike Fitzgerald and Mike Bell for reading and discussing all or parts of the manuscript and making many suggestions for its improvement.
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