Island Mice Project
The following table outlines the current options for invasive eradications. This is by no means a comprehensive table, but rather is intended to be a way to compare the different methods quickly. See below for an in-depth look at each method.
|Methods||Toxicants||Mechanical Methods||Biological Controls||Genetic Engineering*||No Methods Applied|
|Points of Comparison||* Speculative, as technology is not yet developed|
|Past attempts||Many. More successful for rats than mice||Not many for full eradication||Few||None||Yes|
|Monetary Costs||Millions of dollars and fixed||Thousands of dollars and fixed||Variable depending on invasive organism and treatment||Millions of dollars and likely decreasing over time||None|
|Any current regulations?||Yes||Yes||Yes||Unclear||N/A|
|Effective for eradication?||Yes, though more effective for rats than mice||No||No||Unknown||No|
Back to top
What are the types of toxicants used for eradications on islands?
Currently the most common way that rodents are removed from islands is through the application of toxicants.1 Toxicants are chemicals spread by humans, that when ingested cause death. Anticoagulants are the main chemicals found in rodent toxicants. Anticoagulants inhibit blood clotting, and death occurs over the course of several days. There are two types of toxicants, termed first and second- generation anti-coagulants, which have been utilized on islands for recent eradications. First-generation anticoagulant compounds are generally Warfarin based.2 These first generation anticoagulants have eliminated rodents on Maria and Whale Island in New Zealand.2 First-generation toxicants require multiple doses and high concentrations of active chemicals.3 Rodents have a very well developed sense of smell, and so these chemical toxicants are often detected and avoided. The above factors led to bait shyness and first generation resistance.1Hence, the types of toxicants most commonly employed are second-generation anticoagulants. These second generation anticoagulants are odorless and tasteless, and often only require a single feeding in order for death to occur.1 A necessary component to these rodenticides is the low number of required feedings because rodents will learn to avoid the bait otherwise. Brodifacoum, is a common second-generation anticoagulant now widely used for large-scale rodent eradication on islands.2
Cost of toxicants
The cost of using toxicants depends on many factors, including the regulatory process, the cost of the toxicant (distributed within food-baits), the size of the area to be treated, and the bait dispersal mechanisms utilized. Toxicants are distributed within food baits so that rodents will consume the toxicant. The cost of distributing the bait via helicopters or by hand depends on the size and scope of the eradication, and certain cases will require the release of toxicants more than once. The use of 46 tons of brodifacoum, dropped via helicopters in 2008 onto Rat Island, which is 6,424 acres in size, was around 2.5 million dollars once the project was complete.3, 4Cost is also a factor in deciding whether to use toxicants, another method, or no method. (See Chart)
Case Study of the Farallon Islands
The regulatory process will vary from case to case and country by country. One example of the regulatory process in action can be seen with the Farallon Islands, off the coast of San Francisco, in the United States. The United States Fish and Wildlife Service have been working on the regulatory documents for the removal of house mice from the islands for over ten years.5 This requires an Environmental Impact Statement (EIS). An EIS is used in the United States to explore not only the regulatory and implementation aspects of rodent eradications, but also the ecological, social, and ethical aspects of a given action.6 (See Public Opinions) The Farallon’s EIS mandates a full risk assessment, for all organisms found on the island, which includes hundreds of thousands of sea birds, a rare endemic salamander and cave cricket.
Ecological and Animal Welfare concerns
Secondary, or non-target effects can be an ecological concern, because other species may inadvertently ingest the toxicant. Brodifacoum has a fairly long half-life in organisms, but is quickly degraded in the outside environment.3 Second generation toxicants are lethal to mammals, birds and possibly other vertebrates. This means on islands that it is at times necessary to remove other organisms, and to keep them in captivity to prevent secondary consumption. Secondary consumption is mostly through predators consuming rodents that have eaten the toxicant.
The welfare of the animals ingesting the toxicant is also an ethical consideration that must occur when deciding whether to use toxicants. Organizations such as Island Conservation and The Nature Conservancy have supported the use of toxicants to remove invasive rodents from islands, as their argument is that this is for the greater benefit of the endemic animals and biodiversity of the island as a whole. Their argument is that every effort is made to prevent death to non-target animals and that these rodents cause more harm to the island with their presence. This view is about the welfare of the island ecosystem, and not the welfare of the invasive rodents. Opposing arguments from the organizations People for the Ethical Treatment of Animals (PETA) and WildCare, are that the use of toxicants on islands harms many animals (including non-targets), and that animals ingesting the toxicant experience a painful death.7 This is a view and concern for the welfare of all animals. In particular, the use of second-generation toxicants may be opposed as death is via hemorrhaging that occurs over the course of days. There is also an argument that non-toxicant and safer methods should and can be utilized for rodent eradications on islands. On the flip side, people may oppose not using toxicants, as alternative methods have either not been developed or not been proven to work. Finally, rodent death due to starvation, weather, and massive population die offs could be considered a less desirable fate.
- Mensching, D. Volmer, P. 2008. Handbook of Small Animal Practice (5th. Edition). Elsiver Publishing. Pages 1191-1196.
- Thomas, B. Taylor, R. A history of ground-based rodent eradication techniques developed in New Zealand, 1959-1993. Veit, C. Clount, M. Turning the Tide: The eradication of invasive species. IUCN, SSC Invasive Species Specialist Group. IUCN, Gland, Switzerland and Cambridge, UK. f
- Meerburg, B. Brom, F. Kijlstra, A. 2008. Perspective The ethics of rodent control. Pest Management Science. 64. Pages 1205-1211.
- Williams, Ted. 2013. Poisons used to kill rodents have safer alternatives. Audubon magazine. http://www.audubonmagazine.org/articles/conservation/poisons-used-kill-rodents-have-safer-alternatives?page=3
- USFWS Farallons
- BRAKES, C. R., SMITH, R. H. (2005), Exposure of non-target small mammals to rodenticides: short-term effects, recovery and implications for secondary poisoning. Journal of Applied Ecology, 42: Pages 118–128.
Mechanical methods include a variety of trap types that are effective tools for controlling rodent populations in small areas, but are not generally effective as a stand-alone eradication tool. When the goal is to eradicate certain rodent species from a landscape, traps are most commonly employed in conjunction with other eradication methods. From 1990 to 2011, mechanical traps have been used in nine eradication programs conducted in U.S. territory.1 Mechanical methods are typically employed in areas where the use of rodenticides may pose health hazards to humans or the environment2, and to limit the total amount of toxins released into the ecosystem3 in areas where there are highly valued or protected wildlife being monitored.3 Mechanical methods are also used as a way to monitor rodent populations, before, during, and after an eradication project1 and are common in research programs.
Types of traps
There are a variety of trap types that can be employed in rodent control and eradication programs. These can be broken down into two categories: 1) kill traps; and 2) live traps. Kill traps include snap traps, glue traps, and snares. Snap traps can be effective in small scale rodent management, however their practicality becomes an issue as the target area for rodent removal increases.4 Appropriate placement and baiting are essential for snap trap effectiveness.4
Glue traps and snares have been inefficient tools for eradication campaigns against small rodent populations, simply because the animals learn to jump over and avoid them.4 The humaneness of glue traps and other restraining traps, such as snares, have also come into question, as it may take longer than a few hours for the trapped animal to die. Furthermore this may cause the animal to panic, and attempt to escape which may lead to injury.5 These concerns limit glue traps as a viable eradication method.
Live traps can offer a non-lethal, alternative approach when there is aversion to lethal methods for controlling pest animals.4 Live traps are generally more expensive than kill traps but are also more successful than snap traps.6 However, translocated rodents are likely to cause problems in their new environment similar to those they caused in the environment they were removed from, such as disease transmission.7 In addition, rodents may not survive translocation because of stress from capture and transport.
Animal response to traps
Factors such as humidity, temperature, vegetation cover and residual odors from human handling can influence trap efficacy.5 A combination of these factors, along with the rodent’s exposure to past trapping efforts can make trapping situations unique and complex. This can result in high variability in trap success rates. Limiting residual odors is a key component to trapping efforts since the sense of smell plays a primary role in a rodent’s social, reproductive, feeding and anti-predatory behaviors. The age, sex and species of the rodent to be trapped are also important factors that influence trap success rates.5
Limitations of mechanical methods
A major limitation of mechanical methods is the inability to differentiate between target and non-target species. Although it is possible to limit non-target species mortality through the use of different types of traps, it is difficult to completely avoid non-target mortalities when using mechanical methods.2 All mechanical methods must be checked regularly, often daily, in order to be effective in placing all individuals at risk and achieve a removal rate that exceeds reproduction.1, 3 This process may be limited by the area’s size and topography, for example a rocky and mountainous landscape will limit access for individuals setting the traps. This increases the difficulty, safety risks, and amount of labor required for a trapping operation, which leads to increases in the time length and expense of the operation. The exclusive use of mechanical traps may lead to “trap-shyness” where the target species becomes wary of traps through repeated exposure and experiences. Traps are also highly susceptible to damage or disturbance by people and animals, which renders them ineffective.3 Lastly, the public may be opposed to the use of lethal methods for rodent control and kill traps may cause controversy when employed on public lands.3 (See Public Opinions)
- U.S. Department of Agriculture Animal and Plant Health Inspection Service Wildlife Services. 2008. Final Environmental Assessment Predator Control on Cocos Island, Guam. April 2008
- Lorvelec, O., and M. Pascal. 2005. French attempts to eradicate non-indigenous mammals and their consequences for native biota.Biological Invasions 7:135–140.
- Witmer GW, Pierce J, Pitt WC (2011) Eradication of invasive rodents on islands of the United States. In: Veitch CR, Clout MN, Towns DR (eds) Island invasives: eradication and management. IUCN, Gland, pp 135–138
- Witmer, G., and Jojola, S. 2006. What’s up with house mice? — a review. In Proceedings of the 22nd Vertebrate Pest Conference, Berkeley, California, 6–9 March 2006. Edited by R. Timm and J. O’Brien. University of California, Davis, California. pp. 124–130.
- Clapperton, B. K. 2006. A review of the current knowledge of rodent behaviour in relation to control devices. Sci. Conserv. 263. New Zealand Department of Conservation, Wellington.
- Hygnstrom, S.E. and Virchow, D. R., 1992. G92-1106 Controlling Rats. Historical Materials from the University of Nebraska-Lincoln Extension. Paper 1512
- Meerburg, B. G., Brom, F. W. A. and Kijlstra, A. 2008. Perspective: The ethics of rodent control. Pest Management Science, 64: 1205–1211.
What are biological controls?
Predators, parasites, and diseases have been introduced in attempts to control, contain, or eradicate invasive species. Many Invasive species are able to thrive because they are not constrained by ecological factors such as predators or disease that are normally present in their native home range. These ecological factors normally limit the size of a population. Biological controls are attempts to mimic these limiting factors where a species has become invasive.
Biological controls have been successfully implemented in the past but have also failed in spectacular fashion. A famous example of a successful biological control for an invasive species was on the mainland of California. In the 1880s the cottony cushion scale, a small insect that defoliates and kills tree branches, was accidentally introduced from Australia to California and began attacking orange groves. Eventually a scale predator, an Australian lady beetle species, was transported overseas to control the scale. The solution worked and the citrus industry in California thrived.1 When the same cottony cushion scale was discovered on the Galapagos islands an Australian lady beetle was introduced and the scale population is being successfully controlled.2
Biological controls have also failed to control invasive species. In a worst case scenario a biological control agent behaves in an unexpected manner and becomes an invasive species following its release. Many scientists are wary of biological controls for this reason. Many attempts to biologically control invasive species have occurred in Australia. Rabbits were intentionally released for hunting on Australia’s mainland in 1859 and spread across the continent within a few decades.3 The rabbit virus, myxoma, was introduced to invasive Australian rabbits in 1950, eradicating around 90% of the population.4 The population began to recover after several years and since then European rabbit fleas and rabbit hemorrhagic disease virus have been introduced to control the rabbit population. Although the rabbit population today is much lower than the pre-myxoma population the rabbits are still responsible for considerable damage to agricultural systems and biodiversity.
Among failed biological controls the cane toad (Bufo marinus) is one of the most infamous examples. The cane toad was introduced to islands around the world to control agricultural pests. In 1935 the cane toad was introduced to Australia to control sugar cane pests. The cane toad failed to control those pests, expanded across vast areas of the Australian continent and damaged the populations of several Australian species. The cane toad is toxic and kills many animals that consume it, including endemic quolls, lizards, snakes, and even crocodiles. There was a wide-range of support for releasing the cane toad onto sugar plantations in Australia. Government officials, the sugar industry, and many scientists approved of the release.5 The scientific study that proved cane toads could control sugar cane pests was not adequate and government officials did not consider the toad capable of escaping agricultural environments in Australia. The failure of the cane toad as an agricultural biological control serves as a warning to those who consider biological controls for invasive species or pest management.
Where have biological controls been used on islands in the United States?
In the United States it appears that rat populations were successfully eradicated from small islands, which today are part of Pacific Remote Islands, Marine National Monument. These eradications occurred following the introduction of cats (Felis catus). These cats are the likely cause of the eradication but the islands are so remote that the rat populations were not carefully tracked over time. The cats were later removed to prevent predation of endemic island species. Small predators such as cats are non-discriminate feeders and will consume many types of prey. For this reason cats, along with a number of other small mammals, are not strongly considered for invasive species management because these predators will also hunt protected and endangered species.
- Steinberg, T. (2002). Down to earth: Nature’s role in American history. Oxford University Press.
- Simberloff, D., Genovesi, P., Pyšek, P., & Campbell, K. (2011). Recognizing conservation success. Science, 332(6028), 419.
- Garden, D. S. (2005). Australia, New Zealand, and the Pacific: an environmental history. Abc-clio.
- Saunders, G., Cooke, B., McColl, K., Shine, R., & Peacock, T. (2010). Modern approaches for the biological control of vertebrate pests: an Australian perspective. Biological Control, 52(3), 288-295.
- Weber, K. (Ed.). (2010). Cane Toads and Other Rogue Species: Participant Second Book Project. PublicAffairs.
Other Approaches for Rodent Control
Certain species of rodents are considered agricultural pests because they can cause considerable damage to crops, stored produce, and physical damage to human appliances and buildings. Rodents can also increase the spread of diseases when they reach high population levels. However most rodent species are non-pests that play important roles in the ecosystem, with less than ten percent of these species causing serious damage to crops.1 This is why there is interest to find rodent control methods that are species specific. Two approaches that researchers have been developing for pest management that may lead to species specific control options are highlighted below.
RNAi: Regulating Gene Expression
Ribonucleic Acid interference (RNAi) is an important post-transcriptional pathway that is used in many different organisms to regulate gene expression. Information from DNA is transcribed into RNA in the nucleus of a cell. RNA then gets exported outside of the nucleus and is translated by ribosomes to construct proteins. Viruses replicate themselves by hijacking the “machinery” of a cell and inject their genetic material, which are transcribed into viral proteins. RNAi is a mechanisms cells developed that can limit the amount of RNA being transcribed inside a cell and to recognize foreign genetic material to guard itself from creating viral proteins1 see TED-ED video. Currently there are efforts to use RNAi to inhibit the production of proteins that are essential for keeping an organism alive by introducing double stranded versions of theses RNAs (dsRNAs) that encode essential proteins but are recognized as foreign RNA and destroyed. Researchers are applying this technology towards invertebrate pest management as an alternative to agricultural pesticides. There also is the potential to utilize RNAi as a form of species specific rodent control.2 This would limit the harmful effects posed to non-target species that may occur from current control methods such as toxicant baits.
Major challenges facing this technique include the stability of the double stranded RNA (dsRNA) molecules in the environment, delivery of these short interfering RNA’s (siRNA) through the intestinal wall and into the cells of targeted pests, and gauging the amount of siRNAs necessary to inhibit gene expression. Further research is needed to gain a better understanding of the mechanisms of the spread, efficiency, and persistence of the inhibiting effect and for biosafety evaluation of the risks to the environment this technology may pose. A recent study involving sea lampreys has demonstrated that the use of siRNA’s can be effective in silencing genes in vertebrates through delivery by embryonic injection and feeding.3
Immunocontacreption: immune system birth control
The attractiveness of alternative non-lethal control methods and faced with rodents extraordinary breeding capacity, there have been attempts to lower their reproduction capacity through fertility control. Some contraceptive techniques involved the use of chemicals, steroids, synthetic hormones, and viruses to induce sterility in the targeted animal population.4 The effectiveness of steroids and synthetic hormone approaches are limited due to having physiological and behavioral side effects, being non-species specific, and their need to be repeatedly administered, which is both time consuming and cost ineffective.5
Immunocontraception is a species specific approach that involves contraceptive vaccines that prevent fertility by triggering an autoimmune response against molecules and hormones that are essential for reproduction. This process works by immunizing against various proteins in sperm, testis, ovaries, as well as hormones and their receptor proteins, and has been successful at reducing fertility in a wide variety of species including mice.4 Autoimmune infertility can occur naturally through mutations in individuals that are part of wild populations, however in order for this technique to be effective as a sterilizing tool for an entire population, it needs to be introduced through the use of a genetically modified virus. This virus will contain added DNA that codes for the production of proteins that will cause the host animal’s immunes system to create antibodies that attach to sperm and egg cells, blocking fertilization. When released in the population, the virus will infect the targeted animals cells, creating millions of replicants of the virus as well as the immunocontraceptive proteins that will lead to the animal’s infertility.
The aim of this technique is not necessarily to eradicate a pest species completely from an area but instead to reduce the population to a more tolerable level. The challenges facing the effectiveness of contraceptive control of small rodents include the need to administer the anti-fertility treatment at the appropriate time and across a wide range of individuals in the target population.6 Trying to prevent rodents that are not part of the anti-fertility treated population from interacting and mating with the treated population may also limit the effectiveness of this approach.7
- Heath G, Childs D, Docker MF, McCauley DW, Whyard S. (2014). RNA Interference Technology to Control Pest Sea Lampreys – A Proof-of-Concept. PLoS ONE 9(2): e88387. doi:10.1371/journal.pone.0088387
- Dainis, A. (2012, August 12). RNAi: Sclicing, dicing and serving your cells. TED-ED. Retrieved May 13, 2014, from http://ed.ted.com/lessons/rnai-slicing-dicing-and-serving-your-cells-alex-dainis#digdeeper
- Xue,X.-Y., Y.-B. Mao, X.-Y. Tao, Y.-P. Huang, and X.-Y. Chen. (2012). New approaches to agricultural insect pest control based on RNA interference. Advances in Insect Physiology 42:73
- Chambers, L.K., Lawson, M.A., and Hinds, L.A. (1999). Biological control of rodents the case for fertility control using immunocontraception. ‘Ecologically-based Rodent Management’. (Eds G. R. Singleton, L. A. Hinds, H. Leirs and Z. Zhang.) (In Press) (ACIAR: Canberra.)
- Hardy, C.M., Hinds, L.A., Kerr, P.J., Lloyd, M.L., Redwood, A.J., Shellam, G.R., Strive, T. (2006). Biological control of vertebrate pests using virally vectored immunocontraception. Journal of Reproductive Immunology 71:102-111
- Jacob, J., Singleton, G.R., and Hinds, L.A. (2008). Fertility control of rodent pests. Wildlife Research 35, 487–493.
- Biotechnology Australia (2001) The Biotechnology On-line Secondary School Resource. www.biotechnology.gov.au. Retrieved May 13, 2014 from http://web3.narooma-h.schools.nsw.edu.au/resources/BioTechOnline/BiotechnologyOnlineCD/environment/PestSpecies/EuropeanRabbit/ControlThroughBirth/e_ControlThruBirth.htm
No Methods Applied
It is not feasible to remove invasive rodents from all islands. Prohibiting factors of an eradication campaign; can be the cost of the eradication procedure, the size and topography of the island, and the presence of human settlements (See Chart). The cost of an eradication rises with the following variables; the size of the island, the remoteness of the island, and mitigation efforts for non-target species (See Toxicants). A diverse topography can prevent eradication practitioners from adequately covering all terrain with toxicant baits or traps as cracks, crevices, and caves can introduce significant challenges. Human structures provide additional spaces and refuges that allow rodents to avoid bait efforts. There are also health concerns for humans and domestic animals when removing rodents from islands with toxicants. There are other moral, political, cultural and regulatory reasons why an eradication might be rejected (See Public Opinion). All of the aforementioned are significant considerations to consider when deciding about eradications of rodents on islands.
In some cases, these risks may be too high or present too great a degree of uncertainty for a campaign to proceed. A rodent eradication campaign was recently proposed for Lord Howe Island in the Tasman Sea off the eastern coast of Australia. The eradication effort was cancelled because the toxicants would have caused ecological and economic damages. Unlike typical eradication campaigns, Lord Howe Island has a permanent settlement of people with a significant tourist industry.1 Toxicant baits are not species specific and would likely poison livestock on the island. Although the toxicants used for rodent eradications are not hazardous to humans, several residents were concerned for their health. There was also evidence that the Lord Howe Woodhens (Gallirallus sylvestris) would consume the baits and that Pied Currawongs (Strepera graculina) would consume poisoned rodents and die as well. Scientists could not determine how many secondary and non-target impacts would occur following a bait release. If the choice is made not to conduct an eradication procedure, then there are several potential ecological outcomes that may occur on the island (See Ecology of Eradication).
- Lagan, R. 2013.100,000 Rats, 42 Tonnes of Poison — And One Slightly Nervous World Heritage Island. The Global Mail March 25, 2013.