An invasive non-native mammal population conserves genetic diversity lost from its native range.

Invasive, non-native species are one of the major causes of global biodiversity loss. Although they are, by definition, successful in their non-native range, their populations generally show major reductions in their genetic diversity during the demographic bottleneck they experience during colonization. By investigating the mitochondrial genetic diversity of an invasive non-native species, the stoat Mustela erminea, in New Zealand and comparing it to diversity in the species' native range in Great Britain, we reveal the opposite effect. We demonstrate that the New Zealand stoat population contains four mitochondrial haplotypes that have not been found in the native range. Stoats in Britain rely heavily on introduced rabbits Oryctolagus cuniculus as their primary prey and were introduced to New Zealand in a misguided attempt at biological control of rabbits, which had also been introduced there. While invasive stoats have since decimated the New Zealand avifauna, native stoat populations were themselves decimated by the introduction to Britain of Myxoma virus as a control measure for rabbits. We highlight the irony that while introduced species (rabbits) and subsequent biocontrol (myxomatosis) have caused population crashes of native stoats, invasive stoats in New Zealand, which were also introduced for biological control, now contain more genetic haplotypes than their most likely native source.

Using genetic techniques to quantify reinvasion, survival and in situ breeding rates during control operations.

Determining the origin of individuals caught during a control/eradication programme enables conservation managers to assess the reinvasion rates of their target species and evaluate the level of success of their control methods. We examine how genetic techniques can focus management by distinguishing between hypotheses of 'reinvasion' and 'survivor', and defining kin groups for invasive stoats (Mustela erminea) on Secretary Island, New Zealand. 205 stoats caught on the island were genotyped at 16 microsatellite loci, along with 40 stoats from the opposing mainland coast, and the age and sex were determined for each individual. Using these data, we compare and combine a variety of genetic techniques including genetic clustering, population assignment and kinship-based techniques to assess the origin of each stoat. The population history and individual movement could be described in fine detail, with results indicating that both in-situ survival and breeding, and reinvasion are occurring. Immigration to the island was found to be generally low, apart from in 1 year where around 8 stoats emigrated from the mainland. This increased immigration was probably linked to a stoat population spike on the mainland in that year, caused by a masting event of southern beech forest (Nothofagus sp.) and the subsequent rodent irruption. Our study provides an example of some of the ways genetic analyses can feed directly into informing management practices for invasive species.

Sex-biased dispersal and a density-independent mating system in the Australian brushtail possum, as revealed by minisatellite DNA profiling.

Natal dispersal can have important effects on mammal population structure and dynamics following a local population crash. Such dispersal is of practical importance when applied to the control of pest species because dispersal may significantly, and undesirably, reduce the population recovery time following a control operation. The relative dispersal rate of the sexes is also critical because that too will affect the rate of population increase. Here, we describe a field experiment in which we reduce the density of two populations of the Australian brushtail possum, and use genetic similarity, as estimated by minisatellite DNA profiles, to investigate dispersal in the original (undisturbed) and recovering populations. Our results show that the genetic similarity within the undisturbed populations was lower between males than between females. Conversely, the genetic similarities between males and females in the two recovering populations were not significantly different, while relatedness among males was significantly higher in the recovering populations when compared with those in the pre-removal populations. These data indicate two important characteristics of dispersal in possums: (i) that dispersal in established populations is sex biased towards males; and (ii) that within the first 3 years following population control, 'the vacuum effect', whereby individuals from areas adjacent to a control area expand their home range and invade the depopulated area, is the most important factor in the re-colonization process for possums. We found no evidence that the mating system, which is polygynous, varied when the density was markedly reduced. These results indicate that drastic reductions in population density by conventional control will not affect the rate of spread of biological control agents that rely on sexual transmission for dissemination.

Molecular ecology and biological control: the mating system of a marsupial pest.

Many studies in molecular ecology have focused on the use of repeat DNA markers to determine the nature of mating systems in a wide variety of animal species. Whilst these studies typically have focused on important issues such as the evolutionary consequences of fitness variation among males, genetic studies of mating systems are potentially also important because they can generate information of significance to wider issues in wildlife management. For example, genetically modified, sexually transmitted viral diseases have been suggested as potential agents for the control of vertebrate pest species. An understanding of the epidemiology of such agents requires an intimate knowledge of the sexual contact rates between individuals of the target species. Here, we report the use of minisatellite DNA profiling to reveal the mating system in two New Zealand populations of the introduced Australian brushtail possum. The brushtail possum is New Zealand's most important mammalian pest and a species for which control by a sexually transmitted immunocontraceptive has been proposed. Encouragingly, we report considerable variation in the reproductive success of males at both study sites, with one male siring offspring from four females in one year (mean no. of offspring/reproductively successful male/year at the two sites is 1.95-2.15), while many sired none. This bias in the pattern of reproductive success among males will probably facilitate the spread of an immunocontraceptive agent and thereby increase the power of this approach to biological control.

A comparison of 3-parameter, single-species population models, in relation to the management of brushtail possums in New Zealand.

A range of published 3-parameter, single-species models are compared using possums in New Zealand as an example. Most models yield similar population growth curves and similar estimates of maximum sustainable yield (MSY). A new model of competition for refuges, and interactive models of competition for food, both yield growth curves peaked to the right. In the second case, the relationship between the shape of the curve and the parameters of the interactive model is described. Three management consequences of asymmetric (rightward-peaked) growth as opposed to logistic growth are discussed. First, where the animal is a pest a greater control effort is required to reduce density. Second, for sustained yield harvesting the best policy is an adaptive one which initially requires no knowledge of the shape of the growth curve. Third, sustainable yields may be 50% higher than those predicted by the simple logistic model.