The deliberate and accidental relocation of birds and mammals around the globe has been and continues to be integral to patterns of human population spread, settlement and economic development. The outcomes of these relocations have been and may be irreversible, with long-term positive and negative implications for ecological, economic and recreational values. Therefore, for each such relocation there are benefits and/or costs which may not always be predictable but which can be realistically assessed and evaluated. A critical point may have been reached in society's recognition of the value of global biodiversity and its wish to mitigate threats to this. Therefore the OECD invited 16 leading scientists in the area of wildlife ecology to review the existing knowledge about vertebrate introductions, and to formulate recommendations for handling such introductions on a sound scientific basis in the future. These recommendations include a proposal to regulate all introductions of exotic vertebrates, to consult neighbouring countries and the public about such introductions, to adopt a combined white-list/black-list approach, and to establish national advisory committees for recommending actions in these questions.

Introduced animals, particularly mammals, continue to extinguish indigenous species in many parts of the world. This effect is greatest on oceanic islands but continental biotas are also affected. Case histories are used to illustrate difficulties of separating alien animal effects from other extinction agents, and of predicting outcomes. Further research is needed on synergistic and flow-on effects that may follow establishment of a new alien species. Combatting the introduction problem is seen as primarily one of attitude rather than of scientific understanding. It is recommended that in each country: risk analyses are made to identify problem species likely to be introduced; preventive measures are maintained against invasions by these aliens; contingency plans are established for rapid responses to invasions; intensively managed refuge areas are created to protect vulnerable indigenous species from alien species already present; further research is coupled with these actions; information on the introduction problem is disseminated more widely within countries; and international sharing of information on preventive and control measures is promoted more vigorously.

Molecular techniques show that hybridization can be a major conservation problem. Introgression can lessen genotypic variety and break up gene complexes coadapted to local environments. Gamete wastage can be a problem for hybridizing species even when gene flow does not occur. Birds and mammals introduced for game purposes have hybridized with native populations, a process often facilitated by habitat modification. Further, translocation and stock enhancement programs can lead to loss of well-adapted genotypes. These problems increase as populations become fragmented and isolated, because finding mates is harder and habitat differences that bar reproduction are obliterated. What to do about hybridization is sometimes not obvious. Even if the problem is seen as important, often little can be done. Culling may be impossible because of difficulties in recognizing introgressed individuals. Wildlife biologists and managers should work to prevent introductions and translocations if hybridization problems cannot be ruled out. Further, wildlife professionals should educate the public about evolutionary aspects of introductions and translocations, including hybridization and introgression.

This paper includes an estimate of what we need to know for predicting the spreading rates of introduced animal and plant species. The models although catching the principal aspects of the spreading process, still prove to be too simple. Yet even in their initial state of development, they require an amount of biological and environmental information that cannot be reasonably supplied. However, as such, they can indicate the kind of information needed to make statistical prediction reliable. These models also indicate the limits of prediction beyond which extrapolations from simpler models should not be made. It thus appears that the limits of prediction are very restrictive about the fate of species accidentally or deliberately introduced into an area.

Introduced populations of birds and mammals may have genetic characteristics differing from those of naturally occurring populations. Such populations are often created by translocation of small numbers of individuals. This leads to founder effects and subsequent genetic drift, often resulting in larger differences in allozyme patterns between introduced populations than between naturally established populations. In many cases, a large proportion of alleles has been lost a few generations after the introduction. Under certain conditions, the mean level of heterozygosity is also severely reduced. Theoretically, a reduction in the number of alleles in a population will result in a lowered potential to track environmental changes, but there is scant evidence for this in introduced wildlife. Likewise, evidence is lacking for inbreeding depression occurring in introduced populations of birds and mammals in the wild. Finally, some conclusions are drawn concerning management strategies for wildlife introductions with respect to genetic considerations.

Conventional control of two of New Zealand's principal vertebrate pests, the brushtail possum Trichosurus vulpecula and the European rabbit Oryctolagus cuniculus may not be sustainable on economic or social grounds. Biological control through the deliberate release of debilitating pathogens or vectored fertility control offers a potentially long-term, target-specific and humane option. However, the release of exotic pathogens, possibly genetically modified, into New Zealand raises major social and political concerns on both national and international levels which need to be weighed carefully against economic and conservation gains.

Factors that determine whether commercial exploitation of introduced mammals in New Zealand provides a useful method for reducing their densities and therefore their impacts on native biota are examined. The history of commercial harvesting of three introduced species, red deer Cervus elaphus, Himalayan thar Hemitragus jemlahicus and possums Trichosurus vulpecula is described. It is then assessed why the conservation outcomes of this harvesting have differed for these three species and an attempt is made to define some general rules about where and when commercial exploitation is a useful pest control tool. Commercial harvesters of red deer for game meat and byproducts have harvested over 2 million deer since 1960 and reduced the national population from over 1 million to a current population size of ca 250,000 deer, a 75% reduction overall. Current annual harvests average ca 20,000 deer, with annual variations explained largely (r² = 0.89) by the price of venison. Commercial harvesting of thar for game meat between 1971 and 1982 killed at least 39,000 thar and reduced the population by over 90% to <5,000 animals. After the peak harvests before 1976, low annual harvests of only a few hundred animals were able to be sustained as thar were killed as bycatch of the deer industry - but the harvest was stopped between 1983 and 1994 because of pressure from recreational hunters. Commercial exploitation of possums for fur began in 1921, with over 56 million skins being exported. The annual harvest is correlated with the price of furs. Compared with deer or thar, the prices paid per possum are low, and possums are much more abundant (ca 60 million) and ubiquitous pests. The annual harvests of possums have therefore been variable and never sufficient to have more than locally significant effects on population densities.

Thirty-one species of mammals have established wild populations in New Zealand since humans arrived ca 1,000 years ago. Some are implicated in the extinctions among the native biota, many are still causing changes to native ecosystems, and a few cause significant economic losses. This paper summarises the general nature of pest problems and the current framework for managing pests of conservation values in New Zealand. It then discusses some options to better integrate within and between the policy, strategic and tactical levels of the management framework. Appropriate integration will improve outcomes when dealing with pest species affecting many different conservation resources.

This paper reviews the impacts of three species of introduced mammalian predators on native fauna in Australia. The feral cat Felis catus, introduced over 200 years ago, is linked with early continental extinctions of up to seven species of mammals, regional and insular extinctions of many more species of mammals and birds, and the failure of management programs attempting to reintroduce threatened native species to parts of their former ranges. Evidence for cat-impact is largely historical and circumstantial, but supported by observations that afflicted native species are, or were, small (<200 g) occupants of open habitat and hence likely to be especially vulnerable to cat predation. The red fox Vulpes vulpes was released successfully in 1871. Its subsequent spread into all except parts of arid and tropical Australia coincided with local and regional declines of medium-sized (450 – 5,000 g) mammals, birds and chelid tortoises. The fox has also created recent failures of many management attempts to recover threatened native species. Unequivocal demonstration of fox-impact has been obtained in removal experiments, especially on rock-wallabies Petrogale lateralis. The dingo Canis lupus dingo, introduced 3,500–4,000 years ago, probably caused the extinction of the thylacine Thylacinus cynocephalus and Tasmanian devil Sarcophilus harrisii on mainland Australia. It effectively suppresses extant populations of large mammals, such as kangaroos, and emus, over large areas. Impacts of all three predators are wrought primarily by direct predation. Negative impacts appear to be increased in spatially fragmented forests where native species are restricted to remnant vegetation, and in arid landscapes when native species become restricted temporarily to scattered oases during drought. Alternative prey, especially rabbits Oryctolagus cuniculus, enhance negative impacts on native species by supporting large populations of the predators. It is concluded that feral cats and especially foxes have major negative impacts on certain small and medium-sized native vertebrates in Australia, whereas dingoes have major negative impacts on large species. Dingoes could have positive effects on smaller native species if they significantly suppress populations of foxes and cats. Further quantification of both the direct and indirect impacts of the three predators on native fauna is needed, and should be obtained from experimental field studies.

The American mink Mustela vison, raccoon dog Nyctereutes procyonoides and raccoon Procyon lotor are introduced carnivores in Europe. The first American minks were brought to European fur farms in the late 1920s, when also the first raccoons were released, and raccoon dogs were released in the 1930s. The numbers of American minks and raccoon dogs increased rapidly, and today they are widely distributed in Europe. The range of the raccoon is restricted to Germany and adjacent countries and Belorussia. Minks are generalist predators whereas raccoon dogs and raccoons are omnivorous. Thus, e.g. birds and their eggs form part of the diet of these species. Minks are known to have caused damage to colonial ground-nesting sea-birds. Some bird populations have, however, adapted to the presence of mink in a few years. The American mink may also have played a role in the decline of the European desman Galemys pyrenaicus in Spain and the water voleArvicola terrestris in England. It may also compete with the otter Lutra lutra, European mink Mustela lutreola and polecat M. putorius. Male American minks can mate with female European minks whose embryos may die. Hence, the American mink may be one of the reasons behind the decline of the European mink. Raccoon dogs can be locally harmful to waterfowl colonies and frogs, but their overall significance to the native fauna seems to be slight. Little is known about the predation or competition of the raccoon with the native fauna in Europe. The raccoon dog and raccoon can, however, be vectors of rabies; e.g. in Finland, the raccoon dog was the main vector of rabies during the epizootic in 1988–1989. The raccoon dog can also be a vector of e.g. sarcoptic mange and trichinosis.

This paper describes the ecological and economic consequences of introduced bird and mammal species for fauna, flora and habitats, for forestry and agriculture and also for other parts of the land developed and cultivated by man. The results presented are based on enquiries to various institutions for the environment, nature conservation, agriculture, forestry and game as well as on the analysis of scientific data from various sources including the author's. Allochthonous bird and mammal species occurring in Germany are mentioned. Examples of interspecific competition for resources between allochthonous and autochthonous species, of changes to habitats by exotic species, of interbreeding and of the introduction of diseases and parasites are given. Economic damage to agricultural crops, to fruit-growing, viniculture, forestry, parks and to inshore waters is outlined. Bird species mentioned include the ring-necked parakeet Psittacula krameri, Canada goose Branta canadensis, swan goose Anser cygnoides, pheasant Phasianus colchicus. Mammal species covered include the muskrat Ondatra zibethicus, nutria Myocastor coypus, rabbit Oryctolagus cuniculus, raccoon Procyon lotor, raccoon dog Nyctereutes procyonoides, mouflon Ovis musimon, fallow deer Dama dama and sika deer Cervus nippon.

The muskrat Ondatra zibethicus is one of the semi-aquatic rodents that has been introduced into many areas of the world. It is regarded as a most successful vertebrate invader. In northwestern Europe, it was introduced to Finland in 1919 and to the Kola Peninsula in 1931. From these areas, it spread ‘naturally’ to Sweden and Norway, respectively. Here the history of the introduction, dispersal, population development, impact on plant communities, impact on man, and indirect and direct effects on other animals in northwestern Europe are reviewed. The biological features of the muskrat are discussed in relation to the attributes of other vertebrates that are regarded as successful invaders.

Introduced mammals and birds have had a high rate of success in the Nordic countries. Out of 18 species new to the area, 14 (78%) have established a population in at least one country. American mink Mustela vison, raccoon dog Nyctereutes procyonoides, muskrat Ondatra zibethicus, white-tailed deer Odocoileus virginianus, Canada goose Branta canadensis and pheasant Phasianus colchicus have been the most successful exotics. Ecological effects caused by the newcomers include: displacement of European mink Mustela lutreola and beaver Castor fiber by American counterparts, changes in aquatic vegetation patterns caused by muskrat grazing, and locally heavy predation pressure on colonial waterbirds by the American mink. From now on, introductions of new species should be avoided, at least until proven harmless.

Captive breeding and release of the ring-necked pheasant Phasianus colchicus became very intensive in Hungary in the 1960s. In the peak year of 1977 over one million birds were harvested which was a 4-fold increase compared to the late 1960s. In the late 1970s about one million birds were released annually. In the 1980s the number of harvested pheasants started to decrease in spite of the fact that more and more birds were released (about 1,8 million in 1990). The spring population size of the pheasant has decreased from nearly 2.5 million in the late 1970s to 700,000 in 1995. The most important factor responsible for the decline is the loss of favourable habitat due to large-scale agriculture. Moreover, the increased proportion of released birds into the female pheasant population lowers the number of recruits per adult hen as has been shown in England. The mortality of wild hens and their progeny might also increase because of increased predation pressure and diseases e.g. botulotoxicosis - which both result from the increased density due to the massive releases. It is now evident that improving habitat quality would be a better way to manage pheasant as well as other wildlife populations in Hungary, and a management program towards that end has been initiated.

The raccoon Procyon lotor has been naturalised in Germany since 1927. The most dense populations exist in the states Hessia, Northrhine-Westphalia and Lower Saxony followed by Bavaria, Thuringia, Brandenburg and Mecklenburg-Vorpommern. By 1985, the raccoon had crossed the borders to Germany's neighbouring countries with the exception of the Danish and Polish borders. The factors limiting population growth are the numbers of suitable resting and breeding places - for which raccoons prefer oaks and beeches - and food supply. The raccoon has not been shown to negatively affect any indigenous wildlife species. In Hessia, plant matter, invertebrates and vertebrates each make up about one third of the diet of the raccoon. Mammals, birds, reptiles, amphibians and fish are important in winter and spring. Invertebrates can be found in considerable amounts all year long, with the maximum percentages found in summer. Plants are the main source of food in autumn. The raccoon has not yet become a problematic species, not even in urban areas. However, cases of rabies in raccoons in Germany have been documented, although seldom compared to fox Vulpes vulpes, marten Martes spp. and deer, e.g Cervus elaphus, Capreolus capreolus and Dama dama. Raccoons are also known as carriers of infectious diseases such as canine distemper, panleucopenia, Aujeszky's disease, canine parvovirus and canine adenovirus. The first case of human infection by a parasite, the ascarid Baylisascaris procyonis, from a pet raccoon was reported in 1991. The extermination of the raccoon population is not considered to be feasible. The yearly hunting bag of raccoons in Germany comprises 2,000–3,000 individuals.