Suchergebnisse

Context Efforts to protect or restore degraded plant communities by population control of invasive herbivores frequently fail to achieve their goals. Aims We seek to quantify changes in diet of an introduced herbivore, the brushtail possum (Trichosurus vulpecula), following population control, and determine how these may contribute to variable responses in plant condition. Methods Stomach contents of possums from five areas of indigenous forest in northern New Zealand were analysed to measure diet before and after the application of possum control. Key results The contribution of fruit, and foliage of some early successional forest species, to total possum diet increased up to 27-fold following possum population control. This was accompanied by declines in consumption of the main pre-control possum foods (foliage from common canopy trees). Dietary changes were a combination of an immediate response to control (1 year) and a strengthening of these initial changes with increasing time since control. Conclusions Possums in the study areas changed diet following population control, from a diet dominated by foliage of common canopy tree species to one dominated by fruits, and foliage of uncommon early successional plants. Pest control instantaneously increased the per capita availability of all foods, and probably permitted absolute increases in some foods through plant recovery, enabling possums to substitute scarce, high-preference foods for abundant but less preferred canopy foliage. Implications Following control of a pest herbivore, dietary changes reduce benefits for the most vulnerable preferred plant foods, but enhance benefits for less favoured plants. Intense pest control can permit some recovery of highly preferred foods, despite increased per capita consumption of these foods by survivors of control.

Context
European rabbits (Oryctolagus cuniculus) are reaching plague proportions again in some parts of New Zealand as the effect of rabbit haemorrhagic disease begins to wane. Effective monitoring techniques are required to quantify the success of alternative methods of controlling rabbits, such as poisoning.

Aim
To evaluate camera traps as a method of estimating the percentage of rabbits killed in a poison control operation, and to compare results obtained from cameras with those from traditional monitoring methods (spotlight transects and vantage-point counts).

Methods
We deployed cameras and conducted vantage-point counts and spotlight transects to compare a priori statistical power. We then used these monitoring methods to estimate percentage kill from a case study rabbit-control operation using sodium fluoroacetate (compound 1080).

Key results
Cameras had good statistical power to detect large reductions in rabbit numbers (>90%) and the percentage kill estimated using cameras was comparable with spotlight transects and vantage-point counts.

Conclusions
Cameras set up at fixed sampling locations can be an effective method of quantitatively assessing rabbit population control outcomes. We recommend that ≥6 cameras per 100 ha should remain active for at least 5 days before and 5 days following control, so as to obtain reliable estimates of percentage kill.

Implications
Cameras may be preferable to conventional monitoring methods where there is insufficient area to walk or drive transects, terrain is too rugged or scrubby for transects, and there are no or few vantage points from which to count rabbits.

We apply a new algorithm for spatially simulating animal trapping that utilises a detection function and allows for competition between animals and traps. Estimates of the parameters of the detection function from field studies allowed us to simulate realistically the expected range of detection probabilities of brushtail possums caught in traps. Using this model we evaluated a common index of population density of brushtail possums based on the percentage of leg-hold traps catching possums. Using field estimates of the parameters of the detection function, we simulated the relationship between the trap-catch index and population density. The relationship was linear up to densities of 10 possums ha–1. We also investigated the accuracy (bias and precision) of the trap-catch index for possums to estimate relative changes in population density (relative abundance) under conditions of varying detection probability, and compared these results with those obtained using a removal estimate of the population in the vicinity of trap lines. The ratio of trap-catch indices was a more precise estimator of relative abundance than the ratio of removal estimates but was positively biased (i.e. overestimated relative abundance). In contrast, the ratio of removal estimates was relatively unbiased but imprecise. Despite the positive bias, the trap-catch index had a higher power to determine the correct ranking between population densities than the removal estimate. Although varying detection probability can bias estimates of relative abundance using indices, we show that the potential for bias to lead to an incorrect result is small for indices of brushtail possum density based on trapping.

The New Zealand government and agricultural industries recently jointly adopted the goal of nationally eradicating bovine tuberculosis (TB) from livestock and wildlife reservoirs by 2055. Only Australia has eradicated TB from a wildlife maintenance host. Elsewhere the disease is often self-sustaining in a variety of wildlife hosts, usually making eradication an intractable problem. The New Zealand strategy for eradicating TB from wildlife is based on quantitative assessment using a Bayesian "Proof of Freedom" framework. This is used to assess the probability that TB has been locally eradicated from a given area. Here we describe the framework (the concepts, methods and tools used to assess TB freedom and how they are being applied and updated). We then summarize recent decision theory research aimed at optimizing the balance between the risk of falsely declaring areas free and the risk of overspending on disease management when the disease is already locally extinct. We explore potential new approaches for further optimizing the allocation of management resources, especially for places where existing methods are impractical or expensive, including using livestock as sentinels. We also describe how the progressive roll-back of locally eradicated areas scales up operationally and quantitatively to achieve and confirm eradication success over the entire country. Lastly, we review the progress made since the framework was first formally adopted in 2011. We conclude that eradication of TB from New Zealand is feasible, and that we are well on the way to achieving this outcome.

Context Aerial poisoning using sodium fluoroacetate (1080) is an important but controversial technique used for large-scale control of brushtail possums (Trichosurus vulpecula) and other pests in New Zealand. The technique reliably produces near total kills of possums and rats, provided that many tens of baits (and therefore many tens of individually lethal doses) are sown for each target animal present. Aim The aim of this study was to further refine aerial 1080 poisoning by determining the effect of prefeeding, sowing rate, and sowing pattern on effectiveness. Methods Eighteen experimental treatments comprising all possible combinations of three sowing rates (1, 2, and 5 kg/ha of bait), three frequencies of non-toxic prefeed (0, 1, and 2) and two sowing patterns (parallel and cross-hatched) were applied to each of two forested areas. Treatment effectiveness was assessed from changes in the rate of interference recorded on baited cards for three species: possum, ship rat (Rattus rattus) and mouse (Mus musculus). Key results Outcomes were highly variable, ranging from increases in pest activity to near total reductions. Possum reductions were highest where one or two prefeeds were used, and at the higher sowing rates (2 or 5 kg/ha), but with some interactions between these factors. For rats, two prefeeds resulted in the highest reductions but sowing rate had no effect. For mice, post-poisoning indices were often high, indicating low effectiveness. Conclusions Some treatments were highly effective so poor kills were unlikely to have resulted from pests not encountering bait, or the bait being unpalatable. Rather they appeared to reflect sub-lethal poisoning either as a result of low acceptance (as a result of a lack of familiarity and/or satiation) or bait fragmentation. We infer that for possum and rats prefeeding helps reduce this risk of sub-lethal poisoning not only by increasing familiarity, but also (in conjunction with high sowing rates) by increasing the bait encounter rate, particularly for possums. Implications There is scope to further reduce the amount of toxic bait sown and the cost of poisoning, without compromising efficacy, by fine-tuning the balance between prefeeding and sowing rate based on which species are being targeted and, for possums, reducing bait fragmentation.

Using field data from brushtail possums (Trichosurus vulpecula), we present a method for modelling wildlife detection probabilities. Whereas detection functions typically (e.g. for distance sampling) describe the probability of direct human observations of animal subjects, we adapted this approach for cryptic species where observation depends on animals being caught in traps. Specifically, we characterised the probability of individual brushtail possums being caught by leg-hold traps in an area of farmland and native forest in New Zealand. Detection probability was defined as the per-individual, per-trap, per-night probability of a possum being captured, and was modelled as a function of home-range utilisation. Radio-telemetry was used to define the home-range distributions of 18 possums, and a combination of scanning radio-receivers and movement-activated video-cameras recorded instances when radio-collared possums encountered and stepped on the trigger of leg-hold traps (inactivated by being wired open). We estimated a 5% chance of trapping individual possums with a single leg-hold trap located in the centre of their home range for one night (median value across possums). Furthermore, this probability decreased rapidly as a function of distance, so that at 120 m from the centre of the home range there was less than a 1% chance of trapping success per possum per night. The techniques developed in this study could be applied to a wide variety of species and sampling methods.

Context
In New Zealand, the introduced brushtail possum, Trichosurus vulpecula, is a reservoir of bovine tuberculosis and as such poses a major threat to the livestock industry. Aerial 1080 poisoning is an important tool for possum control but is expensive, creating an ongoing need for ever more cost-effective ways of using this technique.

Aims
To develop geographic information system (GIS) models to better predict spatial variation in the distribution of unmanaged possum populations, to facilitate better targeting of control activities.

Methods
Relative abundance of possums and their distribution among habitat types were surveyed in a dry high-country area of the northern South Island. Two GIS-based models were developed to predict the relative abundance of possums on trap lines. The first model used remotely sensed (digital) environmental data; the second complemented the remotely sensed data with fine-scale habitat and topographic data collected on the ground.

Key results
Digital environmental factors and habitat features proved to be key predictors of relative possum abundance. In both GIS models, height above valley floor, presence of forest cover and mean annual temperature were the strongest predictors.

Conclusions
Predictive maps (projections) of relative possum abundance produced from these models can provide useful decision-support tools for pest-control managers, by enabling possum control to be targeted spatially.

Implications
Spatially targeted pest control could allow effective control activities for invasive species or disease vectors to be applied at a lower cost for the same benefit.

Sodium fluoroacetate (1080) has been used as an aerially distributed toxin against mammalian pests in New Zealand since the 1950s. Although its use for rabbit (Oryctolagus cuniculus) control ceased temporarily after the illegal release of rabbit haemorrhagic disease virus (RHDV) in 1997, there has been a recent resurgence in the use of aerial baiting with 1080 to control rabbits as the efficacy of RHDV has fallen. Current practices for rabbit control using 1080 have changed little since the 1980s, with high sowing rates and low toxin loadings commonplace. The lack of ongoing development in baiting practices for rabbit control contrasts sharply with continued improvements in the aerial 1080 baiting practices for brushtail possums (Trichosurus vulpecula) in New Zealand, such as a shift to a comparatively high 1080 loading and using much lower application rates of prefeed and toxic bait. These modifications have resulted in an overall reduction in the amount of toxin used for possum control. The disparity in these two approaches prompted a formal review of the rationale on which the current 1080 baiting practices for rabbits are based. Two issues emerged strongly. First, the current low toxin loading used (0.02–0.04% 1080 in bait) is not based on experimental optimisation in New Zealand but, rather, on research conducted several decades earlier in Australia. Second, despite long-standing concerns about the quality of carrot bait used in New Zealand, current bait manufacturing and distribution practices still produce large numbers of small sublethal fragments. Thus, the current New Zealand practice of multiple prefeeds and very high sowing rates of bait with a low 1080 loading used against rabbits seems to have resulted from the need to compensate for the low toxic loading and poor quality control of the bait (carrots). We, therefore, suggest that there is considerable potential to improve current aerial 1080 baiting practices for controlling rabbits in New Zealand. More generally, these findings also help illustrate that 'best' pest-management practice may sometimes be based on pragmatic solutions aimed at overcoming unrecognised internal constraints that are in fact avoidable. Refining and modernising vertebrate pest-control programs, so that they better meet efficacy requirements and contemporary public expectations, therefore requires understanding not just that a solution works, but also how it works.

Context Rusa deer (Cervus timorensis), originally introduced in the 1860s, are still spreading in eastern Australia. The expanding peri-urban rusa deer population in the Illawarra region of New South Wales, Australia is having undesirable impacts on human and ecological communities, but the spatial structure of this population has not been investigated. Genetic information on invasive species is potentially useful in identifying management units to mitigate undesirable impacts. Aims The aim of this study was to investigate population structure, characterise dispersal, and determine if natural and human-made landscape features affected gene flow in rusa deer invading the Illawarra region of New South Wales. Methods We used reduced representation sequencing (DArT-Seq) to analyse single nucleotide polymorphisms distributed throughout the genomic DNA of rusa deer culled during a management program. We used admixture and Principal Component Analyses to investigate population structure with respect to natural and human-made landscape features, and we investigated whether our genetic data supported the presence of sex-biased dispersal. Key results Genetic diversity was highest in the north, near the original introduction site. A railway line demarcated restricted gene flow. Surprisingly, the Illawarra escarpment, a prominent landscape feature, did not restrict gene flow. There was no evidence of sex-biased dispersal and seven individuals were identified as genetic outliers. Conclusions The genetic structure of the Illawarra rusa deer population is consistent with individuals spreading south from their introduction site in Royal National Park. The population is not panmictic, and a landscape feature associated with urbanisation was associated with increased spatial genetic structure. Outliers could indicate hybridisation or secondary incursion events. Implications Rusa deer can be expected to continue invading southwards in the Illawarra region, but landscape features associated with urbanisation might reduce dispersal across the landscape. The genetic structuring of the population identified three potential management units on which to prioritise ground shooting operations.

Context Scent-trailing dogs ('hounds') are used to hunt introduced sambar deer (Cervus unicolor) in south-eastern Australia, but little is known about this recreational hunting technique. Aim The aim of this study was to characterise hound hunting of sambar deer in Victoria, Australia. Our study had three objectives as follows: (1) to report the hunting success of the technique; (2) to estimate pursuit times and distances; and (3) to investigate the landscape features associated with kill sites. Methods Data were collected from four hound-hunting crews during 2020–2021. GPS data from collars fitted to hounds were used to quantify hunt duration (min) and hunt distance (m). Logistic regression was used to estimate the effects of deer sex and hound pack size on the probability of hunting success. We used a Bayesian multinomial regression resource selection function (RSF) to identify the characteristics of sites where deer were killed (elevation, aspect, and distances to water and roads). Key results The four hunting teams ranged in size from 2 to 10 people and from one to eight hounds. Of 136 hunts, most (88%) harvested a deer, with pursued deer escaping on the other 12% of occasions. Pursuit times and distances were highly variable, with pursuit time >60 min for 46% of hunts and pursuit distance >5 km for 30% of hunts. The probability of killing a pursued male and female deer were similar, and there was not a positive relationship between pack size and hunt success. The RSF showed that both male and female deer were more likely to be killed on steeper slopes and closer to roads. Conclusions Successful hound hunting of sambar deer involves pursuits of considerable duration and requires a network of roads, with geographical features associated with deer kills sites being related to hunter access. Implications Hound hunting may be a useful wildlife management tool for land managers, but further studies are needed to assess its efficacy for achieving management goals.

Wild animals harbour a diverse range of pathogens. In Europe and North America, cervids (Family Cervidae) can act as reservoirs for viral, prion, bacterial, and parasitic infections. Wild deer often inhabit agricultural land, therefore representing a biosecurity risk due to their potential ability to transmit diseases to livestock. Multiple studies have investigated the infection status of wild deer in Australia, mostly during the 1970s and 1980s, and deer populations have increased greatly in abundance and distribution since then. Those studies provide an important baseline for the pathogens carried by wild deer in Australia but are limited by small sample size, the small number of deer species studied, and the disease detection methods used. Recent investigations using ELISA (Enzyme-Linked Immunosorbent Assay), PCR-based assays, and next-generation sequencing have substantially increased our understanding of viral and parasitic infections in Australian deer. These studies indicate that deer may act as reservoirs for pathogens such as Pestivirus, Neospora caninum and Entamoeba bovis. The use of next-generation sequencing has led to the discovery of novel viruses such as Picobirnavirus and a novel species of the genus Bopivirus, both of which pose transmission risks for domestic animals. Recent research confirms that wild deer could be a future source of viral and parasitic infections for domestic livestock and other wildlife species.

Context In response to the toxic health threats posed by lead (Pb), there is currently a focus on transitioning to lead-free bullets for shooting wild animals. Aim We aimed to quantify the killing efficiency and animal welfare outcomes of lead-based and lead-free (copper-based) bullets for ground-based shooting of sambar deer (Cervus unicolor) in Victoria, south-eastern Australia. Methods We used shooter-collected data from recreational diurnal hunting and professional nocturnal culling during 2020–2021. Shooters recorded rifle calibre, cartridge type, bullet mass, bullet type, shooting outcomes (miss, wound or kill), shooting distance, flight distance (the distance between where the animal was shot and where it died) as an assumed positive correlate of time to incapacitation, anatomical zones struck by bullets, and frequency of bullet exit wounds. We used flight distance as our response variable, assuming that it is positively correlated with time to incapacitation. To examine the role of several predictor variables (including bullet type) potentially influencing flight distance, the dataset was reduced to those deer killed with a single thoracic shot. Key results Our data captured shooting events involving 276 deer, with 124 deer shot at with lead-based bullets and 152 with copper-based bullets. Most (87%) of the deer were killed with a single shot. The frequency of non-fatal wounding was <4% for both bullet types and there was no distinct difference in the probability of a single shot kill for deer shot with either bullet type. For those deer killed with a single thoracic shot (n = 198), there was no evidence that bullet energy or shooting distance influenced flight distance. After accounting for differences in terminal kinetic energy, the mean flight distance of deer shot with lead-free bullets (35 m) was 56% greater than that of deer shot with lead-based bullets (22 m). Conclusions Lead-based and lead-free bullets produced similar animal welfare outcomes for shooting sambar deer. Implications A transition to lead-free ammunition for shooting sambar deer would have minimal impact on efficiency or animal welfare outcomes.

Context Helicopter-based shooting has been widely used to harvest deer or control overabundant populations in Australasia, but the effectiveness and cost of this method as a deer control tool has seldom been evaluated. Aims We evaluated the effectiveness and costs of helicopter-based shooting of fallow deer (Dama dama) and chital deer (Axis axis) in eastern Australia by quantifying (1) reductions in density, (2) the relationship between numbers killed per hour and deer density (i.e. the functional response), (3) the costs of control and (4) the effort–outcome and cost–outcome relationships. Methods We evaluated the costs and effectiveness of 12 aerial shooting operations aiming to reduce fallow deer (n = 8) or chital deer (n = 4) population densities at nine sites in eastern Australia. Sites were characterised by fragmented woodland, and all but one operation aimed to reduce grazing competition with livestock. We used pre-control population density estimates and operational monitoring data to estimate the costs and outcomes of each operation. We combined data from all operations to estimate the relationship between shooting effort and population reduction, as well as costs associated with different levels of effort. Key results Population reductions for operations ranged from 5% to 75% for fallow deer, and from 48% to 88% for chital deer. The greatest population reductions occurred when effort per unit area was greatest, and the largest reductions in deer density occurred when shooting was conducted in consecutive years. The functional response of hourly kills to deer density was best described by a modified Ivlev model, with the asymptotic kill rate estimated to be 50 deer per hour. There was no support for the existence of a prey refuge, that is, a threshold population density below which no deer could be shot. Helicopter charter was the primary cost of helicopter-based shooting programs, followed by labour; firearm and ammunition costs were relatively minor. Conclusions Helicopter-based shooting can rapidly reduce deer populations over large geographic areas, but the magnitude of the reduction depends on the effort (hours of shooting) per deer per km2. Implications Aerial shooting operations should include a pre-control population survey so that (1) measurable objectives can be established, (2) the likely level of effort and cost required for objectives to be met can be estimated and planned for, and (3) the realised population reduction can be estimated.