The devastating impacts of introduced rodents on offshore islands around the world are well documented. The presence of exotic rodents on islands is one of the greatest causes of species extinction in the world. Ship rats alone are responsible for the severe decline or extinction of at least 60 vertebrate species and currently endanger more than 70 species of seabird worldwide. They suppress plants and are associated with the declines or extinctions of flightless invertebrates, ground-dwelling reptiles, land birds and burrowing seabirds. Mice have also been shown to impact on plants, invertebrates and birds.
Predation by exotic rats on Australian offshore islands of less than 1000 km2 (100,000 ha) is listed a Key Threatening Process under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act).
On LHI, rats are implicated in the extinction of five endemic bird species, at least 13 species of endemic invertebrates, and two plant species. Rodents are also a recognised as a continuing threat to at least 13 other bird species, 2 reptiles, 51 plant species, 12 vegetation communities, and seven species of threatened invertebrates on LHI.
Since the arrival of rats on LHI in 1918, residents and the Board have endeavoured to control their numbers by shooting, ratting with dogs, introducing owls and, most recently, baiting with anticoagulant poisons. Most recently, the LHIB maintained a rodent control program that aims to keep the negative effects of rodents under control, but its ongoing nature brought with it a constant financial burden and potential human health and environmental risks from ongoing presence of poison in the environment. Under the control program, neither the rat or mouse population was being limited to a level that reduced landscape scale ecological impacts.
Globally, eradication has become a powerful tool to prevent species extinctions and to restore damaged or degraded ecosystems. The biodiversity benefits of removing rodents from islands are well recognised and have been shown to be both significant and immediate. Benefits include:
- significant increases of seeds and seedlings of numerous plant species on islands after the eradication of various rodent species
- rapid increases in the number of ground lizards (e.g. geckos, skinks) following removal of rats – including a 30-fold increase in one case
- dramatic increases in the numbers of breeding seabirds and fledging success
- rapid increases in forest birds and invertebrates.
After completing a Feasibility Study in 2001, 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 LHIG in a single operation was deemed feasible and achievable. The eradication of rodents also presents an opportunity to simultaneously eradicate the introduced Masked Owl.
Eradication (rather than ongoing control) is expected to provide the following benefits:
- Removal of a key threat to many island species resulting in significant biodiversity improvement including threatened species recovery and reintroduction
- Removal of ongoing poison in the environment and associated control costs. It also removes the risk of rodent resistance to poisons
- Long term positive impacts for tourism through protection and enhancement of World Heritage values and improved visitor experience
- Increased productivity for the Kentia Palm industry
- Elimination of current health and amenity impacts from rodents.
The eradication of rodents is consistent with numerous local, state, commonwealth and international plans and obligations. Eradication of exotic rodents from high priority islands (including LHI) was the first objective in the Commonwealth Threat Abatement Plan to Reduce the Impacts of Exotic Rodents on Biodiversity on Australian Offshore islands of Less than 100 000 Hectares. The action is in accordance with the objects of the EPBC Act and the principles of ecologically sustainable development.
Failure to proceed with the REP was likely to result in continuing adverse consequences to biodiversity, World Heritage and socio-economic values through:
- Ongoing impacts to biodiversity (including potential population decreases and extinctions) as a result of rodent predation and competition.
- Continuation of the rodent control program (and the continuous presence of poison baits in the environment) essentially in perpetuity. This presented an ongoing risk of poisoning for non-target species and potential for development of rodent resistance to poison.
- Potential further degradation of World Heritage values (including endemic and threatened species) and the potential for the LHIG to be inscribed on the “World Heritage in Danger List”.
- Ongoing socio-economic impacts associated with rodents.
Eradication vs. Control
This project centres on the eradication of rats and mice from Lord Howe Island. Eradication is different to control even though they sometimes use the same tools. Control seems simple and we have been doing it on the Island for nearly 100 years but it is actually the more difficult and costly of the two strategic options.
- Managing the populations to a level where they are still there but their harm is not a significant problem. The key to this is being able to sustain the control effort because if you stop control the rodents breed back, so this is a never-ending job.
- Because it is never-ending, the costs mount up over time – both money to do the work and environmental effects of the tools being used.
- Because of the cost, control is often limited in the size of area able to be covered sustainably. Limited area equals limited benefits. The control program on LHI protected only 10% of the island.
- Because it is ongoing, controls are effectively selective breeding programs where those individuals who survive the control methds able to breed and pass on their genes. So we end up in an “arms race” inventing new tools or methods to control rodents which are no longer vulnerable to our old methods.
- From an animal welfare perspective, because it is never-ending, more rodents are killed over time than a one-off eradication project.
- Means getting every single individual all in one operation and making sure rodents are not able to return to the Island.
- To ensure we get every last one, the design must be thorough. Every step is motivated by the need to minimise the risk of leaving individuals behind – this is the cheapest and most environmentally friendly way to approach eradication because it avoids having to repeat effort in the future.
- Making sure they can’t get back means putting systems in place to check and stop rodents leaving the mainland, searching for any that may have got through and responding rapidly to any indication of invaders. These systems have the added advantage of stopping other undesirable pests as well for little extra cost.
Techniques for eradicating rodents from islands the size of Lord Howe Island have been successfully used for over 20 years, with a recent example being Macquarie Island (12,800 Hectares) in the Southern Pacific Ocean successfully eradicating all rodents and rabbits in 2014. Although it was expected that some non target species impacts would occur during the eradication program, these species are expected to recover quickly to pre project levels very quickly after the baiting program. Consequently, continuing the ongoing program of control could never have delivered the same level of biodiversity benefits as eradication.
Current and Future Impacts of Rodents
Rats and mice have been having a significant impact on the World Heritage, community and economic values of Lord Howe Island. Rodents prey on and compete with a variety of threatened animals and plants, and impact community amenity though hygiene issues and spoiling of food stuffs.