Challenges
Whilst it is difficult to predict the likelihood of success, the selected eradication techniques, toxin and bait give the LHI REP the best chance of being successful given the constraints on LHI, based on global experience developed over 30 years and more than 380 successful eradications worldwide. The success rate for mouse eradications from 1997-2014 on NZ islands using the same bait and technique is 100% or 11 from 11 attempts (Broome and Fairweather, 2016) whilst rat eradications on islands over the same period have been 98% successful (37 of 39 attempts) (DIISE 2016).
Constraints that increase the risk of failure and how they have been considered for the LHI REP are detailed below.
| Constraint | Solution |
|---|---|
| Efficacy of the bait | Brodifacoum is highly toxic to both rats and mice in minute quantities, allowing a lethal dose to be consumed in a single feed. It is also a chronic toxicant (i.e. its action is delayed) meaning the rodent does not associate any illness with the bait it has consumed. These two factors are important for avoiding the consumption of sub-lethal doses and the associated risk of bait shyness/avoidance. Trials on LHI confirmed that doses available during the REP are sufficient to kill all rats and mice. |
| Palatability of the bait and alternate food sources | The Pestoff 20R bait is specially designed to be highly palatable to rodents and this was shown on LHI even with alternate food available in the laboratory and in field conditions. The Pestoff 20R bait is much more palatable than commercial rodenticides containing Brodifacoum as these contain waxes to preserve life and taste deterrents to prevent human ingestion. Whilst there are some alternate foods sources available, food availability is generally reduced over winter when the program is implemented. Other alternate food sources (i.e waste from humans) were minimised by an altered waste management regime. |
| Access to baits, inter species competition and home ranges of rats and mice | The LHI REP was specifically designed to target both rats and mice considering the smaller home range of mice. Bait has been applied at a density that will allow all rats and mice access to a lethal dose. The second bait drop also acted as a contingency to ensure there were no gaps in the bait coverage and to target individuals that may have been denied access to bait distributed in the first application (by more dominant individuals that would be dead after the initial bait drop). |
| Island size and topography (including cliffs, crevices, caves | The aerial distribution of baits was the only realistic method of baiting a large topographically challenging island such LHI. Aerial application using a specifically designed spreader bucket was shown to be effective in delivering a toxic dose of bait to every rodent on similar large and rugged islands (i.e. Macquarie and Campbell Islands). GPS technology was used to ensure total bait coverage through the development of flight lines and ensuring 100% of aerial baiting areas were treated. The second bait drop also acted as a contingency to ensure there were no gaps in the bait coverage. |
| Permanent human population | To minimise potential risks to human health, a combination of hand broadcasting and bait stations have been used in the settlement area. This allowed coverage to be maintained including in roofs and under buildings. A clean up of island hard waste successfully removed over 400 tonnes of hard waste that was providing potential rodent habitat. Discussions with regard to access to individual properties will continue prior to implementation. Contingency options for property access are available as discussed above. |
| Potential survivors | A comprehensive rodent monitoring programme has been developed for the REP. It includes intensive monitoring particularly in the settlement area immediately after the eradication and then extending to all accessible areas across the island for two years after. This approach facilitates the early detection and removal of localised survivors but will also give a high level of confidence to allow declaration of eradication success which will be declared after two years of monitoring with no rodent activity. The detection network will include a combination of detection tools including detector dogs, chew cards, chew blocks, cameras, tracker tunnels, traps and bait stations. Response to a detection will be guided by a Technical Advisory Group (TAG) who will be on immediate standby to provide consensus advice on how to respond to any specific situation. The TAG will consist of selected experts in eradication techniques, rodent detection and rodent behaviour. |
Technical Feasibility
After completing a Feasibility Study in 2001[1], the LHIB carefully considered and evaluated the eradication of rats and mice on the LHIG. Due to developments in eradication techniques during the past 20 years, particularly the refinement of aerial baiting methods, the eradication of both rats and mice on the LHI Group in a single operation was considered technically feasible and achievable. A range of possible methods and mortality agents were considered for use in eradicating both rats and mice on LHI. The only method capable of removing every rat and mouse on LHI was aerial distribution, in conjunction with hand broadcast and bait stations where required (i.e. the settlement area), of highly palatable bait containing an effective toxicant. Assessment of other options considered and why they were unsuitable on LHI are shown in the table below.
| Eradication Technique | Suitable for eradication | Feasible for Eradication on LHI | Justification |
|---|---|---|---|
| Disease | No | No | No suitable pathogen yet developed that could eliminate all individuals. |
| Trapping | Yes | No | May be feasible for eradication on small islands, however may cause individuals to become trap shy. Size and inaccessible terrain of LHI makes this option unfeasible |
| Biological | No | No | Likely to fail to completely eradicate the target species. High likelihood of unacceptable non-target species impacts. |
| Fertility Control | No | No | No suitable fertility control yet developed that could eliminate all individuals. |
| Toxicant - Bait station / hand broadcast only | Yes | No | May be feasible for eradication on small islands. Size and inaccessible terrain of LHI makes this option unfeasible. |
| Toxicant – Aerial Broadcast only | Yes | No | Highly successful on uninhabited islands. Socially unacceptable on LHI. |
| Toxicant – Combination of Aerial and Hand Broadcast / Bait Stations | Yes | Yes | Brodifacoum in the form of Pest off 20R has been selected as the preferred toxicant on LHI considering proven success, efficacy and non-target impacts |
The eradication techniques used on LHI are neither novel nor experimental. They are the culmination of more than 30 years of development and implementation involving more than 380 successful eradications worldwide (Howald et al. 2007[1] and DIISE, 2016[2]). Systematic techniques for eradicating rodents from islands were first developed in New Zealand in the 1980s. Since then techniques have improved significantly, and eradications are now being attempted and achieved on increasingly larger and more complex islands, including those with human populations.
Aerial broadcasting of bait using helicopters has become the standard method used in eradications, particularly those on large islands (Towns and Broome 2003)[3]. This method has proven to be a more reliable and more cost-effective option than the previous ground based techniques. Depending on the nature of the area to be treated, aerial baiting has been combined with hand broadcasting of bait and the use of bait stations, particularly around areas of human habitation. The use of new tracking and mapping technologies such as global positioning systems and geographic information (computer mapping) systems has increased the efficacy of aerial-based eradication programmes.
The toxicant selected for the eradication of rats and mice from the LHIG is Brodifacoum, a second-generation anticoagulant. Brodifacoum has proven to be successful in over 226 eradications, in a variety of climatic conditions including those similar to LHI, and on all 14 eradications on islands greater than 500 ha in size. An evaluation of potential rodenticides for aerial control of rodents (Eason and Ogilvie 2009[4]) concluded that Brodifacoum was the best rodenticide for island eradications. The use of any other mortality agent would be largely experimental and pose unacceptable risks of failure. The Island Eradication Advisory Group for the Department of Conservation in New Zealandm, who are recognised as leaders in this field, are of the opinion that “there is no other alternative rodenticide on the market anywhere in the world with which we would have the same level of confidence in using to eradicate Ship Rats and mice from an island such as Lord Howe”.
[1] Howald, G., Donlan, C.J., Galvan, J.P., Russell, J.C., Parkes, J., Samaniego, A., Wang, Y., Veitch, D., Genovesi, P., Pascal, M., Saunders, A. and Tershy, B. (2007). Invasive rodent eradication on islands. Conservation Biology 21, 1258-1268.
[2] DIISE (2016): Database for Island Invasive Species Eradications accessed January 2016: http://diise.islandconservation.org
[3] Towns, D. R. and Broome, K. G. (2003). From small Maria to massive Campbell: forty years of rat eradications from New Zealand islands. New Zealand Journal of Zoology 30, 377-398.
[4] Eason, C. T. and Ogilvie, S. (2009). A re-evaluation of potential rodenticides for aerial control of rodents. DOC Research and Development Series 312. Department of Conservation, Wellington, New Zealand.