Graham Nugent

Top down or bottom up? Comparing the impacts of introduced arboreal possums and ‘terrestrial’ ruminants on native forests in New Zealand

We review and contrast the impacts on New Zealands native forests of the two main types of introduced mammalian herbivore; ‘arboreal’ browsers, represented solely by the brushtail possum Trichosurus vulpecula, and ‘terrestrial’ ruminants, represented by deer (Cervidae) and goats Capra hircus. Because of their large size and sophisticated digestive anatomy, the ruminants are able to consume virtually all of the foliage of preferred native plants that is available to them and thereby exert a strong ‘top down’ regulatory effect on forest composition, particularly where they can supplement their diet with fallen leaves from the forest canopy. Possums appear to be less efficient as folivores than the ruminants as a consequence of their smaller size and simpler digestive anatomy. This may sometimes severely limit their top-down influence on plant abundance. However, possums appear able to make much greater use of foliage where they are able to combine an abundant but nutritionally inadequate leaf resource with high-quality foods such as fruit. For both possums and deer, the size and nature of supplemental food sources (i.e. other than growing foliage) appears crucial in determining both herbivore carrying capacity and the severity of their impacts on forest composition. Particularly important is whether or not the availability of the key supplemental foods is directly and negatively affected by herbivore abundance. We explore the implications of this hypothesis for herbivore management.

Unexpected consequences of control: competitive vs. predator release in a four-species assemblage of invasive mammals

Invasive species are frequently the target of eradication or control programmes to mitigate their impacts. However, manipulating single species in isolation can lead to unexpected consequences for other species, with outcomes such as mesopredator release demonstrated both theoretically and empirically in vertebrate assemblages with at least two trophic levels. Less is known about the consequences of species removal in more complex assemblages where a greater number of interacting invaders increases the potential for selective species removal to result in unexpected changes in community structure. Using a replicated Before-After Control-Impact field experiment with a four-species assemblage of invasive mammals we show that species interactions in the community are dominated by competition rather than predation. There was no measurable response of two mesopredators (rats and mice) following control of the top predator (stoats), but there was competitive release of rats following removal of a herbivore (possums), and competitive release of mice following removal of rats.

EPIDEMIOLOGY AND PATHOGENESIS OF MYCOBACTERIUM BOVIS INFECTION OF RED DEER(CERVUS ELAPHUS) IN NEW ZEALAND

AIMS This study was initiated to investigate aspects of the epidemiology, pathogenesis and transmission of tuberculosis in wild red deer, with the aim of determining whether this species may be considered a reservoir host of Mycobacterium bovis in New Zealand. METHOD One hundred and six wild red deer (Cervus elaphus) carcasses from the Castlepoint and Hauhungaroa Range areas, which are endemic for bovine tuberculosis, were examined for the presence of M. bovis infection. Samples were also examined from 46 skin test-positive farmed deer killed at two deer slaughter premises. Where possible, a standard set of tissues and excretion site samples was collected for mycobacteriological examination. RESULTS Fifty-eight infected deer were identified, and of these 28% showed no gross lesions. The prevalence of tuberculosis confirmed by culture in the wild deer was 32%. Only one of 18 deer younger than 1 year was infected. Mature deer (>2 years) were 12 times more likely to be infected than those under 1 year of age. Infected older deer were less likely to show typical gross lesions than younger animals. Mycobacterium bovis was isolated from the oropharyngeal tonsil of 34 of 56 (61%) of the infected deer, and this was the most commonly infected site. Gross lesions were found in 18 of the 34 infected tonsils and only one of these showed a purulent tonsillitis. Mycobacterium bovis was recovered from four of 53 nasopharyngeal tonsils, four of 53 oropharyngeal swabs, one of 53 tracheal and nasal swabs, and one of 46 faecal samples, but not from any urine specimens. CONCLUSION These findings suggest that significant bacillary excretion from infected deer was uncommon, and is more likely to occur in severely affected animals. This study has confirmed the importance of mucosa-associated lymphoid tissues (MALT), particularly the oropharyngeal tonsil, in the pathogenesis of tuberculosis in deer. The findings justify investigation of the hypotheses that the prevalence of tuberculosis in wild deer in New Zealand is high due to transmission of infection from possums, and that in the absence of an infected possum population, the prevalence of tuberculosis in deer is likely to be low, and spatially patchy. CLINICAL RELEVANCE The results suggest that about one quarter of infected deer show no detectable gross lesions. This implies that many infected carcasses may enter the food chain unrecognised and that the estimated sensitivity and specificity of diagnostic tests may be erroneous if there is a difference in test performance between those conducted on deer with or without gross lesions. Diagnostic sensitivity following slaughter may be improved by routine culture of oropharyngeal tonsils and careful examination of lungs for adhesions and small subpleural tubercles.

Diet and diet preferences of introduced ungulates (Order: Artiodactyla) in New Zealand

Abstract Understanding the diet and diet preferences of introduced ungulates is a crucial step towards modelling their trophic interactions and managing their impacts. We collated information on the diet of 8 introduced ungulates (Order: Artiodactyla) from 19 studies in New Zealand. A total of 185 indigenous plant genera (379 species) was recorded as eaten. Five studies assessed diet choice by 4 ungulate species (red deer, fallow deer, white‐tailed deer, and feral goats) in forests. Of the 60 plant species or genera present in two or more studies, 8 were always “preferred” (eaten more than expected from their availability) and 15 were always “avoided” (proportionately less eaten than was available); most others were “not selected” (eaten in proportion to their availability). From these data we constructed a table of preferences for commonly occurring forest plant species. Ungulates preferred woody species with low foliar lignin. Although there was no significant relationship between diet choice and foliar N, avoided species all had low foliar N. Ungulate preferences for fern species were closely related to foliar P concentrations, but not to foliar N concentrations. Since relatively few plant species were consistently preferred, we conclude that many native species have neutral resistance to ungulate herbivory, a conclusion that is contrary to the notion that the New Zealand flora is especially vulnerable to introduced herbivores.

Advances in New Zealand mammalogy 1990–2000: Deer

The rapid expansion of deer farming in recent decades has stimulated frequent introductions of new genetic material, either as live animals or as frozen semen (Pearse & Goosen 1999). This has produced a blurring of taxonomic boundaries, a process that is likely to be hastened by deliberate hybridisation between species, e.g. between red and Pere Davids deer (see below) and between red and sambar deer (Muir et al. 1997), and by inadvertent hybridisation between supposedly distinct species in captivity (e.g., red and rusa deer on some Australian deer farms: P. F. Fennessy cited in Asher et al. 1999). During 1984 and 1985, 77 Pere Davids deer (Elaphurus davidianus) were imported into New Zealand as prospective farm stock (Asher et al. 1988). Most died from malignant catarrhal fever (Orr & Mackintosh 1988). Only about 20 of their descendants survive, at Mt Hutt Station, Canterbury (D. Hood pers. comm. 2000). Although karyotypically similar to red deer (i.e., 2n = 68), Pere Davids deer are 50-60% larger than red deer, and have considerably larger hooves, a longer tail, and antlers that characteristically appear to grow backwards. Pere Davids deer are long-day (summer) breeders, whereas red deer are shortday (autumn) breeders. Gestation in Pere Davids deer is 283 days (Wemmer et al. 1989), about 50 days longer than red deer. Despite the large genetic distance between Pere Davids deer and red deer (Tate et al. 1992), hybridisation between the two species was achieved by artificial insemination (Asher et al. 1988; Fenessy & Mackintosh 1992). Between 1986 and 1991, 16 Fj hybrid calves were produced, and these were in turn used to produce over 400 maternal backcross calves (i.e., 25% Pere David, 75% red deer) that have proven to be extremely valuable in mapping the red deer genome (Tate et al. 1997). However, maintaining these populations of hybrids has proven difficult, so they are unlikely to be incorporated into the New Zealand farmed deer population. Remaining Y\ and backcross hybrids are almost wholly confined to research facilities.

USE OF RELEASED PIGS AS SENTINELS FOR MYCOBACTERIUM BOVIS

Identifying the presence of bovine tuberculosis (TB; Mycobacterium bovis) in wildlife is crucial in guiding management aimed at eradicating the disease from New Zealand. Unfortunately, surveys of the principal wildlife host, the introduced brushtail possum (Trichosurus vulpecula), require large samples (>95% of the population) before they can provide reasonable confidence that the disease is absent. In this study, we tested the feasibility of using a more wide-ranging species, feral pig (Sus scrofa), as an alternative sentinel capable of indicating TB presence. In January 2000, 17 pigs in four groups were released into a forested area with a low density of possums in which TB was known to be present. The pigs were radiotracked at 2 wk intervals from February to October 2000, and some of them were killed and necropsied at various intervals after release. Of the 15 pigs successfully recovered and necropsied, one killed 2 mo after release had no gross lesions typical of TB, and the only other pig killed at that time had greatly enlarged mandibular lymph nodes. The remainder were killed at longer intervals after release and all had gross lesions typical of TB. Mycobacterium bovis was isolated from all 15 pigs by mycobacterial culture. Home range sizes of pigs varied widely and increased with the length of time the pigs were in the forest, with minimum convex polygon range-size estimates averaging 10.7 km2 (range 4.7–20.3 km2) for the pigs killed after 6 mo. A 6 km radius around the kill site of each pig would have encompassed 95% of all of their previous locations at which they could have become infected. However, one pig shifted 35 km, highlighting the main limitation of using unmarked feral pigs as sentinels. This trial indicates use of resident and/or released free-ranging pigs is a feasible alternative to direct prevalence surveys of possums for detecting TB presence.

Big-game, small-game, and gamebird hunting in New Zealand: Hunting effort, harvest, and expenditure in 1988

Abstract A postal survey of 8639 licensed firearm owners in 1989 indicated that an estimated 117 200 ± 6300 New Zealanders (3.5% of the total population) did some hunting in 1988. An estimated 33 100 former hunters did not hunt in 1988 but thought it likely that they would hunt again in future. The survey provided useful estimates of 1988 national totals for hunting effort (4.4 million hunter days), gross expenditure (

Toward eradication: the effect of Mycobacterium bovis infection in wildlife on the evolution and future direction of bovine tuberculosis management in New Zealand

NZ100 million), and harvest (6.5 million animals). Small-game hunting dominated, involving 81% of hunters, 59% of total hunting effort, and 86% of total numerical harvest: rabbits (Oryctolagus cuniculus) and possums (Trichosurus vulpecula) each comprised 40% of the national bag. Gamebird hunting involved 48% of hunters, 19% of effort, and 11% of total harvest: ducks comprised 73% of the gamebird harvest. Although big-game hunting attracted the fewest hunters (42%) it ranked second for hunting effort (21%). Big game formed 3% of the total numerical harvest: pigs (Sus scrofa), goats (Capra hi...

Commercial Exploitation as a Pest Control Tool for Introduced Mammals in New Zealand

Abstract New Zealands bovine tuberculosis (TB) control programme has greatly reduced the burden of tuberculosis on the farming industry, from 11% of mature cattle found with TB at slaughter in 1905 to <0.003% in 2012/13. New Zealand implemented TB control measures in cattle from the mid-twentieth century, and later in farmed deer. Control was based on established methods of tuberculin testing of herds, slaughter of suspect cases, and livestock movement control. Unexplained regional control failures and serious disease outbreaks were eventually linked to wildlife-vectored infection from the introduced Australian brushtail possum (Trichosurus vulpecula), which also triggered a wildlife disease complex involving a range of introduced species. This paper reviews the progressive elucidation of the epidemiology of Mycobacterium bovis in New Zealands wildlife and farmed livestock, and the parallel development of research-led, multi-faceted TB control strategies required to protect New Zealands livestock industries from damaging infection levels. The adoption of coordinated national pest management strategies, with increasingly ambitious objectives agreed between government and industry funders, has driven a costly but very successful management regime targeted at controlling TB in the possum maintenance host. This success has led to initiation of a strategy designed to eradicate TB from New Zealands livestock and wildlife, which is considered a realistic long-term prospect.

The evaluation of indices of animal abundance using spatial simulation of animal trapping

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.