John P. Parkes

Management of introduced mammals in New Zealand

Abstract Of the 31 species of exotic mammals that have current wild or feral populations confirmed in New Zealand, at least 25 are actively managed as pests in at least part of their range to reduce their impacts on biodiversity and production values. This paper summarises the current legal status of introduced mammals, the strategic and tactical options to manage them as pests, and their actual management by different agencies, both as pests and as resources. We then discuss some ways in which management agencies might better integrate their actions, record how much is invested in different types of research on the different species, and summarise the main types of problems the research aims to address.

Eradicating multiple invasive species on inhabited islands: The next big step in island restoration?

Invasive species are the greatest threat to island ecosystems, which harbour nearly half the world’s endangered biodiversity. However, eradication is more feasible on islands than on continents. We present a global analysis of 1,224 successful eradications of invasive plants and animals on 808 islands. Most involve single vertebrate species on uninhabited islands, but plant and invertebrate eradications occur more often on inhabited islands. Inhabited islands are often highly modified and support numerous introduced species. Consequently, targeting a single invasive species can be ineffective or counterproductive. The impacts of other pests will continue and, in some cases, be exacerbated. The presence of people also creates regulatory, logistical and socio-political constraints. Real or perceived health risks to inhabitants, pets and livestock may restrict the use of some eradication tools, and communities or individuals sometimes oppose eradication. Despite such challenges, managing invasive species is vital to conserve and restore the unique biodiversity of many inhabited islands, and to maintain or improve the welfare and livelihoods of island residents. We present a brief case study of the Juan Fernández Archipelago, Chile, and discuss the feasibility of eradicating large suites of invasive plants and animals from inhabited islands while managing other invaders for which eradication is not feasible or desirable. Eradications must be planned to account for species interactions. Monitoring and contingency plans must detect and address any ‘surprise effects’. Above all, it is important that the local community derives social, cultural and/or economic benefits, and that people support and are engaged in the restoration effort.

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.

Setting thresholds for pest control: how does pest density affect resource viability?

Abstract Conservation in New Zealand is largely focused on reducing the impact introduced mammals have on the abundance of indigenous species. Conservation managers have a range of strategies they can employ to control these pests, but the combination that maximises conservation gains depends on the protection each strategy affords, and the scale at which it can be applied. Given a limited budget, the use of threshold pest densities to initiate pest control can increase control effectiveness by reducing opportunity costs. However, complex trophic relationships between pests and resources mean that thresholds which minimise the costs of controlling pests without reducing the viability of threatened populations to unacceptable levels will often be difficult to identify. Here we review three general consumer–resource models in the context of pest control; (1) the damage function based on the functional response of pests to resource abundance, (2) density dependent predator–prey models, and (3) interactive models. Damage functions can be used to set threshold pest densities that achieve tactical but not strategic conservation outcomes. Density dependent predator–prey models can be used to set threshold pest densities that have strategic consequences for resource conservation, but are limited in their scope where pest or resource abundance is influenced by density independent environmental perturbation. Interactive models can be used to identify thresholds for imposition of pest control that are responsive to pest density, resource abundance and prevailing environmental conditions. We advocate this modelling framework as a basis for setting control thresholds for pests in New Zealand.

Quantifying eradication success: the removal of feral pigs from Santa Cruz Island, California.

A major challenge facing pest-eradication efforts is determining when eradication has been achieved. When the pest can no longer be detected, managers have to decide whether the pest has actually been eliminated and hence to decide when to terminate the eradication program. For most eradication programs, this decision entails considerable risk and is the largest single issue facing managers of such programs. We addressed this issue for an eradication program of feral pigs (Sus scrofa) from Santa Cruz Island, California. Using a Bayesian approach, we estimated the degree of confidence in the success of the eradication program at the point when monitoring failed to detect any more pigs. Catch-effort modeling of the hunting effort required to dispatch pigs during the eradication program was used to determine the relationship between detection probability and searching effort for different hunting methods. We then used these relationships to estimate the amount of monitoring effort required to declare eradication successful with criteria that either set a threshold for the probability that pigs remained undetected (type I error) or minimized the net expected costs of the eradication program (cost of type I and II errors). For aerial and ground-based monitoring techniques, the amount of search effort required to declare eradication successful on the basis of either criterion was highly dependent on the prior belief in the success of the program unless monitoring intensities exceeded 30 km of searching effort per square kilometer of search area for aerial monitoring and, equivalently, 38 km for ground monitoring. Calculation of these criteria to gauge the success of eradication should form an essential component of any eradication program as it allows for a transparent assessment of the risks inherent in the decision to terminate the program.

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

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.

The ecological dynamics of pest-resource-people systems

Abstract Pest problems involve people who value a resource affected by a pest that is managed by people, ideally the same as those who value the resource. Management that is not inclusive of pests, resources, people, and their interactions usually fails. Mammalian pests in New Zealand are of two sorts: those that are pests in their native land (usually r-strategists), and those that are not (usually K-strategists). The impact on New Zealand resources of r-strategists tends to be periodic and acute when their densities are high. Control works best against them when it is applied once densities become intolerable, or once such densities can be predicted. The impact of K-strategists is more stable but is chronic. Control operations against them need to be sustained and regular to retain the stability of the pest-resource interaction, but to drive it in favour of the resource. The basic strategies to deal with the impact of pests are: those for which a one-off management action has a permanent benefit (e.g., ...

Detection of RNA of rabbit haemorrhagic disease virus from New Zealand wild rabbits

Rabbit numbers have returned to high levels in some areas of New Zealand following the initial spread of rabbit haemorrhagic disease virus (RHDV). We undertook to determine whether possible infection with an RHDV-related virus was interfering with the initiation of new outbreaks of rabbit haemorrhagic disease (RHD). RHDV RNA was detected by polymerase chain reaction with reverse transcription (RT-PCR) using RHDV-specific primers from tissue samples of wild rabbits that had been shot in the field. RHDV RNA was detected in 11 of 19 rabbits from an area of Otago where the rabbit population had greatly expanded and in 2 of 8 rabbits from the West Coast where outbreaks of RHD had not been previously reported. Among the 13 rabbits positive for RHDV RNA, 10 had detectable antibodies against RHDV. The nucleotide sequences of the isolates — a segment of the RHDV capsid gene — shared about 99% identity with that of the Czech strain V351 and that of a 1997 New Zealand wild isolate, but shared only about 84% identity with that of a European benign rabbit calicivirus. These results provide evidence for persistent infection of RHDV in rabbits.

Changes in immunity to rabbit haemorrhagic disease virus, and in abundance and rates of increase of wild rabbits in Mackenzie Basin, New Zealand

The evolutionary race between diseases and their hosts may lead to attenuation of the disease agent, increasing resistance in the host, or both. This is an undesirable outcome when the disease is being used as a biocontrol agent but a desired outcome when the host is valued by people. Introduced wild rabbits Oryctolagus cuniculus are a pest to agriculture and biodiversity values in New Zealand’s grasslands, particularly on the drier eastern sides of both islands. The costs to manage them using conventional control could not be sustained by landowners who since the 1980s have proposed the introduction of the viral biocontrol agents myxomatosis and then rabbit haemorrhagic disease virus (RHDV). Myxomatosis failed to establish but RHDV did establish and spread following its illegal introduction in 1997. However, since 1997, rabbit haemorrhagic disease (RHD) has become less effective for biocontrol of rabbits in New Zealand. Three lines of evidence from our four study sites in the Mackenzie Basin support this claim. First, the proportion of rabbits of all ages with antibodies to RHDV has increased in samples of rabbits shot each year since 1997. Taken alone this may simply reflect an accumulation in cross-sectional samples of seropositive older rabbits that have been exposed to infection but survived successive epizootics. Second, the proportion of young rabbits, sampled at an age when they have been exposed to a single epizootic event, that have antibodies to RHDV has also increased since 1997. This is strong evidence that something has changed in the rabbit–virus interaction. The cause of this effect remains unknown but is reflected in the third line of evidence, that the abundance of rabbits as indexed by standardised spotlight counts has increased since 1997. The rate of increase has, however, been much slower than that seen in the same populations as they recovered from conventional control before the arrival of RHD. Thus, we conclude that RHD is still an effective biocontrol but its efficacy is waning.

Thresholds in plant–herbivore interactions: predicting plant mortality due to herbivore browse damage

Patterns of herbivore browse at small scales, such as the rate of leaf consumption or plant preferences, drive the impact of herbivores on whole-plant processes, such as growth or survival, and subsequent changes in plant population structure. However, herbivore impacts are often non-linear, highly variable and context-dependent. Understanding the effect of herbivores on plant populations therefore requires a detailed understanding of the relationships that drive small-scale processes, and how these interact to generate dynamics at larger scales. We derive a mathematical model to predict annual rates of browse-induced tree mortality. We model individual plant mortality as a result of rates of foliage production, turnover and herbivore intake, and extend the model to the population scale by allowing for between-tree variation in levels of herbivore browse. The model is configurable for any broadleaved tree species subject to vertebrate or invertebrate browse, and is designed to be parameterized from field data typically collected as part of browse damage assessments. We parameterized and tested the model using data on foliage cover and browse damage recorded on kamahi trees (Weinmannia racemosa) browsed by possums (Trichosurus vulpecula) in New Zealand forests. The model replicated observed patterns of tree mortality at 12 independent validation sites with a wide range of herbivore densities and browse damage. The model reveals two key thresholds; in plant foliar cover, indicating when individual trees may be at high risk from browse-induced mortality, and in herbivore intake, leading to high rates of mortality across the whole population.