Land Vertebrates As Invasive Species on Islands Served by the South Pacific Regional Environment Programme
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Atkinson and Atkinson: Land vertebrates as invasive species Land vertebrates as invasive species on islands served by the South Pacific Regional Environment Programme Ian A. E. Atkinson and Toni J. Atkinson Ecological Research Associates of New Zealand Inc., PO Box 48-147, Silverstream, Upper Hutt 6430, New Zealand Abstract mals. These are goats, pigs, cattle, cats, dogs, three species of rats, and mice. Some other introductions This report reviews available information on the ad- either capable or potentially capable of serious nega- verse effects of 14 alien vertebrates considered to be tive impacts are at present restricted to a few islands. ‘significant invasive species’ on islands of the South These are rabbits, brown tree snakes, mongooses, Pacific and Hawaii, supplementing our own experi- musk shrews and cane toads, all of which could be ence with that of other workers. The biological char- spread further by human activity. These 14 alien ver- acteristics of each of these invasives are outlined to- tebrates are referred to in this report as “significant gether with what is known of their impacts on native invasive species”. The report outlines the biological plants and animals. The most widespread are goats, characteristics of each, their distribution among is- pigs, cattle, cats, dogs, mice, and three species of rats. lands of the SPREP region, and what is known of Island groups are listed in alphabetical order with their impacts on native plants and animals of these summary comments on their habitats, status with re- islands. We stress, however, that this list of 14 spe- spect to damage by these invasives, and potential for cies is based on what we know at present and does restoration where known. not mean that other species not covered in detail are More information is needed on the impacts of necessarily insignificant in their effects. If New Zea- invasives on indigenous species and habitats so that land had been reviewed, for example, sheep would priorities can be decided for controlling or eradicat- have been included as a significant invasive species; ing those posing the most serious threats. Preventing donkeys are very significant in the Galapagos Islands; the further spread of invasive vertebrates and keep- and both Hawaii and New Zealand have suffered sig- ing relatively unmodified islands free of invasives nificant conservation losses from deer. Effects of in- are discussed. Island governments need to protect troduced birds on the native plants and animals of islands from further invasions for reasons of eco- islands in the Region are still not clarified. nomic interest and human health as well as main- Information in this report is derived largely from taining conservation values. A specific recommen- searching the literature but is supplemented by di- dation is made for establishing an effective strategy rect communication with other workers in this field that will reduce or prevent further establishment of as well as the experience of the writers. Detailed in- invasive animals. Suggestions are made for restoring formation is lacking for some of the invasive spe- particular islands; searches should be made to identify cies discussed, particularly musk shrews, cane toads, those islands where it appears feasible to restore and and introduced birds. Information for the islands is maintain indigenous vegetation and wildlife. also uneven in detail. The checklist of islands pub- 1. Introduction lished by Douglas (1969) has been invaluable but the naming of some islands remains unclear and some This review of the invasive land vertebrates present of the island areas she quoted are incorrect. We have on islands of the South Pacific Regional Environ- not listed islands where we have no information on ment Programme (SPREP) region is part of a larger invasive species. review of the invasive plants and animals that have We are sure that many of the information gaps in the affected these islands adversely. Numerous alien spe- island section of this report can be filled by either cies of mammals, birds, reptiles and amphibians have communication with knowledgeable residents or been introduced to this region. through visits by an informed person. In the time Some of these have become widespread and exerted available, we have not been able to find what we re- serious negative impacts on native plants and ani- gard as adequate information for several groups of 19 Invasive species in the Pacific islands: Caroline Is, Chesterfield Is, D’Entrecasteaux Mauna Loa volcano, Hawaii, than sheep browsing. Reefs (northwest of New Caledonia, not to be con- Anson visited Tinian Island, Northern Mariana Is- fused with the D’Entrecasteaux Islands of eastern lands, in 1742 and attributed the island’s park-like New Guinea), Gambier Is, Loyalty Is, Marquesas Is, scenery to overgrazing by cattle (Walter and Robins Marshall Is, New Caledonia, Palau Is, Santa Cruz Is, 1974). In New Caledonia, Barrau (1981) considered Solomon Is, Tonga, Tuamotu archipelago, Tuvalu, that open-range cattle grazing had destroyed much and Vanuatu. New Guinea and its surrounding islands of the pre-colonial landscape in which savanna wood- are more difficult and may require communication lands had been present. with a larger group of people, if not a field study, to obtain a more complete picture. Prevention/control Well maintained fences can give adequate protection Islands under French control may not have had suf- from cattle to areas of native woody vegetation. Dogs ficient attention because French is not our first lan- and shooting are a standard method of control. guage. Question marks alongside dog, cat, pig, or mouse Current researchers and control experts indicate that these animals are likely to be present in B.D. Bell, Wildlife Management International, PO Box 14-492, the island group and are likely to be free-ranging or Wellington, New Zealand. feral. But we have not been able to find written con- firmation of this belief. 2.2 Feral goat (Capra hircus) We were asked to include Hawaii as a source of ex- Characteristics amples although these islands are outside the SPREP Feral goats on Pacific islands are derived from do- region. Impacts of invasive animals have been more mestic goats (originally native to Asia), introduced extensively studied in Hawaii than elsewhere in the to an island for their milk or released as potential Pacific. food for people marooned by shipwrecks. Goats usu- ally move in groups that can build to large herds as 2. Ecology of significant invasive their numbers increase. Home ranges, i.e. the areas animals searched for food, can vary from 100 m to 20 km in 2.1 Cattle (Bos taurus) width. Males are bearded and have a strong smell during the breeding season. They do not defend ter- Characteristics and spread in the Pacific ritories and wander more widely than females (Rudge Cattle of various breeds (originally native to Asia) 1990). Studies in New Zealand show year-round have been introduced to a number of islands in the breeding; a population recovering after control was Pacific as the basis for a meat industry. They are par- able to double in about two years (Rudge and Smit ticularly important in Hawaii and New Caledonia. 1978). Impacts on conservation values Grasslands, scrub and forest are all used extensively by goats as feeding areas. Goats particularly like open Unless well contained by adequate fences, cattle areas in which to rest or sun themselves, but at night, wander into native vegetation wherever suitable food or in bad weather, they may seek shelter under rocky is available. If unchecked this can result in the for- outcrops or other places where they can keep their mation of feral herds roaming wild through exten- fur dry. In Hawaii, goats are present on all main is- sive areas of country. In Hawaii, feral cattle are lands from low to high altitudes, where they destroy present in remote forests, subalpine scrub and on in- or damage communities in both dry and wet zones accessible lava flows (Tomich 1969). Scott et al. (in (Stone 1984). Stone 1984) regarded domestic and feral cattle as the “single most destructive agent to Hawaiian ecosys- Spread in the Pacific tems, particularly to koa forests”. Regeneration of Feral goats have become established on at least 9 is- young koa (Acacia koa) trees is completely sup- land groups within the SPREP region. pressed in some forests of Hawaii (Baldwin and Fagerlund 1943). Cattle grazing was considered to Impacts on conservation values be a primary factor in the decline of an endemic le- Goats have been recognised as “the single most de- guminous vine (Vicia menziesii) on the island of structive herbivore” introduced to the islands of the Hawaii (Warshauer and Jacobi 1982). Snowcraft world (King 1985). By eating young trees in a forest (1983) found that cattle browsing was more destruc- they prevent the replacement of adult trees that form tive to mammane (Sophora chrysophylla) forests on the forest canopy. In extreme cases, such as on 20 Atkinson and Atkinson: Land vertebrates as invasive species Isabela Island in the Galapagos group, a mosaic of Spread in the Pacific low forest and an open woodland has been replaced Pigs have been introduced to the great majority of by grassland and depleted woodland or other kind of island groups in the Pacific. Their spread began with non-forest vegetation, within a 10-year period (F. colonisation of these islands by Polynesian and Cruz, pers. comm.). Goats were present on Cuvier Micronesian peoples and continued with Spanish Island, east of Auckland, New Zealand, between 1910 settlement in the 17th century and the subsequent in- and 1960, when they were eradicated. A 36-year flux of other European peoples. However, pigs in- study of forest recovery since the eradication showed troduced by early seafarers have often been displaced that more than two-thirds of the tree and shrub spe- by domestic pigs that have gone wild.