Grant Norbury

Epidemiology of rabbit haemorrhagic disease (RHD) in the South Island, New Zealand, 1997–2001

Rabbit haemorrhagic disease (RHD) was illegally released in New Zealand in August 1997 and subsequently spread by farmers and naturally over all areas inhabited by rabbits. The disease has persisted, causing either annual or biennial epidemics that appear to start each spring by infecting susceptible adult rabbits and running through to the autumn infecting young rabbits of the year. Rabbit densities have been suppressed by more than 90% in many areas where numbers were initially high and epidemics returned annually, and by about 50% in areas with low initial densities and biennial epidemics, leaving between 35% and 20%, respectively, of the survivors immunised. In some areas the disease has not reduced rabbit densities and has left more than 80% (at worst) of the survivors across all age cohorts with antibodies to RHD. A possible cause of this apparent longitudinal transmission is that seropositive adult rabbits (which may retain virus) may be infectious and pass virus on to their offspring at just the right dose and age to impart immunity. A pen trial, in which we orally dosed 9-week-old rabbits born to seropositive mothers, showed that they survived and did not seroconvert, but were fully susceptible when re-dosed at about 17 weeks. We have evidence that the possible pre-existing calicivirus does not impart immunity to RHD in at least one part of New Zealand.

Behavioral responses of two predator species to sudden declines in primary prey

Feral ferrets (Mustela furo) and cats (Felis catus) are pests in New Zealand because they carry bovine tuberculosis (Tb) and prey on native fauna. We examined their behavioral responses to large-scale poisoning of their primary prey, feral European rabbits (oryctolagus cuniculus). We monitored the movements of 70 radiocollared ferrets and 28 radiocollared cats for 2 years during pre- and posttreatment periods for 1 nontreatment (Earnscleugh) and 2 treatment (Bendigo, Grays Hills) populations in tussock grassland habitat on New Zealands South Island. There were no changes (P > 0.05) in the mean home range size of ferrets on the Bendigo site (70 ha prepoisoning and 67 ha postpoisoning), where poisoning reduced rabbit numbers by 77%, or on the Earnscleugh site (71 ha prepoisoning and 61 ha postpoisoning), where rabbit numbers remained high. However, when rabbit numbers were reduced by 99%, ferrets on Grays Hills increased home range from 85 to 230 ha (P < 0.05), and their mean distance from range center increased from 475 to 993 m (P < 0.001). While most ferrets at Grays Hills remained resident, the proportion of mobile ferrets increased from 1 of 25 to 6 of 18 (P = 0.015). For cats, treatment effects were less clear because sample sizes were low and data were highly variable. However, 3 adult cats at Grays Hills dispersed up to 9.2 km from range centers after rabbit numbers were reduced by 99%. No obvious behavioral changes of predators were observed where rabbit declines were less pronounced or where rabbit abundance remained high. This observation suggests that large changes in predator movements occur only after very large disturbances to their primary prey, but there was insufficient replication of treatments to separate treatment effects from site effects. Nevertheless, our qualitative results indicate that very large reductions in rabbit populations in tussock grassland are likely to cause a small number of predators to disperse into adjacent areas. In the absence of a clear understanding of the role of ferrets and cats in the epidemiology of bovine Tb, and of their effects on native fauna, the implications of this induced dispersal are unknown. Intuitively, however, any consequences are likely to be adverse.

Effects of predation and rabbit haemorrhagic disease on population dynamics of rabbits (Oryctolagus cuniculus) in North Canterbury, New Zealand

The impact of predation and rabbit haemorrhagic disease (RHD) on population dynamics of rabbits, and the survival of juvenile rabbits, was investigated between July 1999 and March 2000 in North Canterbury, New Zealand. Rabbit abundance and pre- and post-emergent rabbit mortality were monitored on four sites, two of which were subject to predator control. RHD spread naturally through all sites from late November to early December. Rabbit densities declined on all sites, but after the RHD epidemic, declines were significantly greater where populations of predators had not been controlled. Survival of rabbit nestlings was lower where predators were not controlled. All post-emergent radio-collared rabbits died at sites where predators were not controlled, whereas 18% of those collared at sites where predators were controlled survived to maturity. In contrast to the results from previous studies, rabbits born at the start of the breeding season had very high rates of post-emergent mortality, as they appeared to be susceptible to the RHD virus later in the breeding season. The age at which juvenile rabbits become susceptible to RHD, the timing of RHD epidemics, and the abundance of predators are likely to be important in determining survival of juvenile rabbits. This study demonstrates that predation can reduce rabbit populations to low levels, but only in combination with other factors, in this case RHD.

Invasive mammals and habitat modification interact to generate unforeseen outcomes for indigenous fauna.

Biotic invasions and habitat modification are two drivers of global change predicted to have detrimental impacts on the persistence of indigenous biota worldwide. Few studies have investigated how they operate synergistically to alter trophic interactions among indigenous and nonindigenous species in invaded ecosystems. We experimentally manipulated a suite of interacting invasive mammals, including top predators (cat Felis catus, ferret Mustela furo, stoat M. erminea), herbivores (rabbit Oryctolagus cuniculus, hare Lepus europaeus), and an insectivore (hedgehog Erinaceus europaeus occidentalis), and measured their effects on indigenous lizards and invertebrates and on an invasive mesopredator (house mouse Mus musculus). The work was carried out in a grassland/shrubland ecosystem that had been subjected to two types of habitat modification (widespread introduction of high-seed-producing pasture species, and areas of land use intensification by fertilization and livestock grazing). We also quantified food productivity for indigenous and invasive fauna by measuring pasture biomass, as well as seed and fruit production by grasses and shrubs. Indigenous fauna did not always increase following top-predator suppression: lizards increased on one of two sites; invertebrates did not increase on either site. Mesopredator release of mice was evident at the site where lizards did not increase, suggesting negative effects of mice on lizard populations. High mouse abundance occurred only on the predator-suppression site with regular production of pasture seed, indicating that this food resource was the main driver of mouse populations. Removal of herbivores increased pasture and seed production, which further enhanced ecological release of mice, particularly where pasture swards were overtopped by shrubs. An effect of landscape supplementation was also evident where nearby fertilized pastures boosted rabbit numbers and the associated top predators. Other studies have shown that both suppression of invasive predators and retiring land from grazing can benefit indigenous species, but our results suggest that the ensuing vegetation changes and complex interactions among invasive species can block recovery of indigenous fauna vulnerable to mesopredators. Top-down and bottom-up ecological release of mesopredators and landscape supplementation of top predators are key processes to consider when managing invaded communities in complex landscapes.

ECTOPARASITE AND HEMOPARASITE INFECTION IN A DIVERSE TEMPERATE LIZARD ASSEMBLAGE AT MACRAES FLAT, SOUTH ISLAND, NEW ZEALAND

A lizard assemblage at Macraes Flat, New Zealand, comprising the common skinks Oligosoma maccanni and Oligosoma nigriplantare polychroma, the endangered species Oligosoma grande and Oligosoma otagense, and the common gecko Hoplodactylus maculatus, was studied to determine the prevalence of ectoparasites and hemoparasites. The mites Odontacarus lygosomae and Ophionyssus scincorum infected all Oligosoma spp. only, and the mite Neotrombicula naultini infected only H. maculatus. The hemoparasite Hepatozoon lygosomarum infected all Oligosoma skinks, except O. n. polychroma. Oligosoma otagense had the highest infection levels of all parasites by several orders of magnitude. For all lizard species, there was no difference in mite prevalence between adult males and adult females, but juveniles were less often infected. For all skink species, there was a significant relationship between presence of the hemoparasite He. lygosomarum and infection intensity of the supposed vector, O. scincorum. It is unclear if patterns of parasite infection reflect species-specific susceptibility, host–parasite species-specific spatial ecology, or environmentally induced host physiological impairment. Considering the threatened nature of O. otagense, evidence of high parasitemia should stimulate further investigation.

The potential for biological control of stoats (Mustela erminea)

Abstract Accelerating the mortality of stoats (Mustela erminea) using biological agents, or reducing their fertility using chemosterilants or biological agents, are increasingly seen as more sustainable and more humane than trapping and poisoning. Obligate delayed implantation in fertilised female stoats of all ages allows 10–11 months for an applied biological agent or chemosterilant to interfere with gestation. Two chemosterilants (cabergoline and mifepristone) disrupt pregnancy in some species and may be effective on stoats, although they are not species‐specific and are probably more expensive than poisoning. For the longer term, more recent fertility control research has explored potentially more species‐specific options for other species based on inducing an immune response to an animals own reproductive hormones, gametes, or products from embryos. Conception will be difficult to disrupt in stoats because females are sexually mature and are mated in the nest during a short period before they are weaned. A large research effort will be required to determine which of the immunosterilants being developed could be suitable candidates for stoat control. There are fewer options apparent for using biological agents to increase stoat mortality, although species‐specific strains of canine distemper virus may be effective against stoats. The greatest impediment to controlling stoat fertility will be effective delivery of sterilants. For the foreseeable future, it will probably be necessary to rely on baits, but they are unlikely to put all target stoats at risk, and will be incapable of delivery over larger scales than at present. Before undertaking expensive field trials and development of anti‐fertility and biological agents, the effects of putative compensatory changes in demographics that may be associated with changes in stoat density should be modelled to see if the sterilisation and mortality rates that are required to achieve a given level of population control are realistic targets. Also, population control should be defined in terms of accrued benefit for wildlife by establishing the relationships between stoat densities and the viability of prey populations. Biological control of fertility or mortality may never be suitable as stand‐alone control options for stoats, particularly when some native fauna survive only if stoats are reduced to very low densities. Biological control may have greater potential when integrated with conventional control.

Trophic Interactions Among Native and Introduced Animal Species

In the absence of exotic mammals, the beech forest system is strongly driven bottom-up. The sporadic heavy seeding of beech trees results in a cascade of population increases in the native fauna, without any known reciprocal effects. In dryland ecosystems, the nutrient pulses occur annually during spring flushes of herbaceous plants. In both the little modified beech forest and the highly human-impacted dryland ecosystems, mammalian introductions have been made at both the herbivore and predator trophic levels. These exotic additions have created strong top-down effects on indigenous fauna because predator abundance (stoat, ferret, and cat) is driven mainly by exotic prey species (mice and rabbits). Predator numbers can reach levels not normally possible without the introduced prey, and this can potentially lead to extinction of the native fauna. The worst scenario for native prey occurs when mice and rabbit numbers fluctuate widely. This leads to acute bouts of predation caused by the increases in predator numbers (in the case of stoats), or as ferrets and cats switch to native species following sudden declines in rabbit abundance. We now know enough about some processes in beech forest and dryland ecosystems to build prototype models that will help to predict the wider effects of controlling introduced species, identify critical knowledge gaps, and ultimately guide management decisions to achieve desired biodiversity outcomes.

Lagomorph and sheep effects on vegetation growth in dry and mesic grasslands in Otago, New Zealand

Abstract Context. Rabbits (Oryctolagus cuniculus) are major pests in the semiarid grasslands of eastern South Island. As the benefits of a viral biological control agent (rabbit haemorrhagic disease virus) introduced in 1997 begin to wane, landowners are again faced with the costs of controlling rabbits. Aim. The study aimed to measure the extent of vegetation growth as rabbit and sheep densities changed to enable land managers to better justify decisions on pest and stock management. Methods. We investigated the relationship between seasonal growth of vegetation at two replicate sites within each of three rabbit-prone areas in Otago. At these sites, the ambient densities of lagomorphs varied naturally and in response to pest-control operations, whereas the density of domestic sheep (Ovis aries) varied according to the farmers’ husbandry needs. The vegetation was highly degraded at one of the areas studied. Rabbits were present at all three sites and hares (Lepus europaeus) at two sites. At each site, we established exclosures that excluded either all herbivores or just sheep. The observed rates of vegetation growth were modelled as a function of prevailing herbivore densities, season and site. The fitted model was used to infer expected rates of change in vegetation biomass at the sites, under varying sheep and lagomorph density indices, and seasons. Key results. In the absence of herbivores, vegetation was predicted to grow in all seasons apart from winter at the degraded sites. In the absence of sheep but in the presence of up to ∼10 lagomorphs seen per kilometre of spotlight transect, accumulation of vegetation biomass follows the same seasonal pattern as in the absence of herbivores. As lagomorph counts reach 50 per kilometre, vegetation biomass accumulates only in spring and summer and only at the least degraded sites. The maximum stocking rates for sheep for which positive biomass accumulation could be maintained under varying lagomorph densities was predicted from the model. Conclusions. At the least degraded sites some sheep could be grazed while maintaining positive pasture growth, even at high rabbit densities, except in winter if rabbit-density indices exceeded ∼30 per kilometre. At the most degraded sites, a very low density of sheep could be maintained, but only in spring and only if lagomorph density indices were below ∼10 per kilometre.

Impacts of introduced European hedgehogs on endemic skinks and weta in tussock grassland

Abstract Context. Control of introduced pest species is based on the premise that there is a relationship between pest abundance and impact, but this relationship is rarely defined. Aim. We investigated the impacts of introduced European hedgehogs (Erinaceus europaeus) on two species of small endemic skink (Oligosoma spp.) and flightless, nocturnal endemic orthopteran ground weta (Hemiandrus spp.), using an enclosure-based experimental manipulation of hedgehog density in tussock grasslands in the South Island of New Zealand. Methods. We used capture–mark–recapture methods to estimate the densities of skinks before and after exposure to a range of hedgehog densities over a 3-month period and also compared changes in indices of abundance of skink demographic groups and ground weta. Key results. Faecal analysis confirmed that hedgehogs consumed skinks and invertebrates in the enclosures. The proportional change between capture sessions in numbers of captured juvenile McCann’s skinks (O. maccanni) declined with increasing hedgehog density. Similarly, the proportional change in the numbers of ground weta encountered in pitfall traps showed a highly significant negative relationship with increasing hedgehog density. Total species abundances and numbers in other demographic skink groups did not change significantly in relation to hedgehog density. For overall skink abundance estimates, there was an apparent trend suggesting that changes in abundance were more negative with increasing hedgehog density, but this did not reach statistical significance for either skink species. Conclusions. Our results confirmed that hedgehogs are important predators of small native fauna, but suggested that highly abundant prey populations may be buffered against significant impacts. Implications. Less abundant prey and some demographic groups within populations, however, may be at significant risk from hedgehog predation.

Feeding selectivity of introduced hedgehogs Erinaceus europaeus in a dryland habitat, South Island, New Zealand

We describe the diet of introduced European hedgehogs Erinaceus europaeus (Linnaeus, 1758) in a New Zealand dryland system and provide the first quantitative analysis of food selectivity for this species. We also describe and compare the diets of nine hedgehogs and measure dietary overlap between these individuals. The most commonly eaten foods were beetles, including rare native species (in 94% of droppings), earwigs (92%), spiders (25%) and native skinks (14%). Remains of at least three skinks were found in one dropping. Earwigs and darkling beetles (Tenebrionidae) were the most preferred food types, and Hymenoptera and cylindrical bark beetles (Colydiidae) were least preferred. Consumption of most foods mirrored their availability. Most individuals’ diets conformed to the pattern seen at the population scale, with 21–54% of dry faecal mass derived from beetles and 13–39% from earwigs. One animal frequently consumed large amounts of fruit. Dietary overlap between pairs of individuals was high (mean Horn’s index, 0.84). This may be the result of limited opportunities to diversify in a very moisture-limited and low diversity habitat.