Rodent Eradication Research Articles

Responses of an island endemic invertebrate to rodent invasion and eradication

Invertebrates dominate many terrestrial ecosystems in terms of biomass, and they also structure ecosystems through their roles as pollinators, detritivores, primary consumers, predators and prey. Invasive rodents (rats and mice) are known to have detrimental effects on many island invertebrates, although these effects are seldom quantified or ecologically understood. Here we provide evidence of the effects of invasive rats (Rattus spp.) on island invertebrate populations using a large-scale natural experiment. We investigated the effects of invasive rats on Falkland camel crickets (Parudenus spp.) in the Falkland Islands (South Atlantic) by comparing an index of camel cricket relative abundance between 18 rat-infested islands, six rat-eradicated islands and 13 naturally rat-free islands (in total, 37 islands). Our study provided two key results. First, camel crickets were up to an order of magnitude more abundant on rat-free islands than on rat-infested or rat-eradicated islands. This difference was larger in native tussac grass Poa flabellata than in other vegetation types. Second, camel cricket populations recovered after rat eradication, because the relative abundance of camel crickets on rat-eradicated islands was intermediate between those of naturally rat-free and rat-infested islands, and among rat-eradicated islands relative abundance was lowest where rats had been cleared most recently. Our results demonstrate severe suppression of a superabundant and large-bodied island endemic invertebrate by invasive rodents, and its prompt recovery after rodent eradication.

Prognosis for ecosystem recovery following rodent eradication and seabird restoration in an island archipelago

Invasive species are widespread and can have devastating effects on biota, especially insular biota. Invasive species eradications are increasingly employed to promote island recovery to preinvasion states. However, it remains unclear if additional restoration actions may be required on islands that were once heavily reliant on seabird guano for ecosystem functions. Active seabird augmentation has been suggested as necessary to exact ecosystem recovery on contemporary timescales in some cases. I use two experiments on offshore islands in Cook Strait, New Zealand, to test the hypothesis that seabird restoration will restore island ecosystem functioning following invasive rodent removal. The first is a small-scale single-island fertilization experiment that simulates seabird recovery. This experiment tested the recovery potential of offshore islands and was used to infer the density of seabirds needed to elicit ecosystem recovery. The second is a large-scale natural experiment that takes advantage of eight islands with differing rodent eradication and seabird restoration histories. I compared ecosystem functioning variables (delta 15N, C:N ratios in soil, plants, and spiders, as well as arthropod abundance and diversity) on two islands that had rodents eradicated and two islands undergoing seabird augmentation with two control islands (never invaded by rodents) and two positive control islands (currently invaded by rodents). The results suggest that islands do have the potential for recovery given nutrient amendments, but that islands with rodents eradicated and islands undergoing seabird augmentation have not recovered most of their ecosystem function. Finer, intra-island analysis showed that seabird restoration projects have the potential to speed the recovery process, but that the projects on the studied seabird restoration islands were not advanced enough to produce island-wide recovery. The results suggest that high seabird densities (5-10 burrows/m2) are needed to promote recovery to never-invaded control levels. Seabird augmentation, through chick translocation and/or social facilitation with decoys, vocalization playbacks, and/or mirrors can supplement passive seabird recovery on islands where seabirds have been extirpated or extremely reduced by invasive predators. Such restoration efforts may be necessary to promote ecosystem recovery on contemporary timescales.

Alternatives to brodifacoum and 1080 for possum and rodent control—how and why?

The risks of vertebrate pesticides to non-target animals are determined by intrinsic susceptibility, the toxicokinetics of the compounds used, and the degree and frequency of exposure. Metabolism and persistence studies coupled with field surveys have provided us with an improved understanding of the toxicokinetics and non-target effects of different anticoagulants. This has enabled improved choice of tools for island versus mainland use in New Zealand, and has stimulated the development of low-residue tools. Brodifacoum is a potent second-generation rodenticide used worldwide for commensal rodent control, and has been widely used to eradicate rodents from islands. The risks associated with ‘one-off’ application of baits containing second-generation anticoagulants for rodent eradication on islands are considered to be outweighed by the potential benefits to their ecosystems. Possums are susceptible to bait containing brodifacoum, but not to first-generation anticoagulants, hence brodifacoum has been the only alternative to 1080 that effectively targets both possums and rodents. On the mainland, contamination of wildlife and game species and secondary poisoning of non-target species has been substantial where brodifacoum has been used repeatedly. We are extending the current range of low residue alternatives to reduce reliance on brodifacoum and sodium fluoroacetate (1080). Currently, suitable alternatives available include encapsulated cyanide Feratox® and cholecalciferol paste Feracol®. Other compounds being developed as alternatives include micro-encapsulated zinc phosphate and low dose cholecalciferol and coumatetralyl, or C+C.

New Zealand’s Use of Brodifacoum in Eradication Efforts and Current Investigation of New Baits and Toxins

Author(s): Broome, Keith; Murphy, Elaine; Cunningham, Celia; Eason, Charles T. | Abstract: Rodent eradication on islands has transitioned over 30 years in New Zealand through stages of initial scepticism, to early accidental and experimental successes, and now to the current bold large-scale aerial applications over increasingly large and complex island ecosystems. Starting in the 1970s and 1980s, gaining momentum in the 1990s, and continuing to the present day, islands once occupied by rodents are now being reclaimed. Until the mid 1980s, very few islands were entirely free of animal pests. Rodent eradication on islands, using bait in stations and baits applied from the air, has been spectacularly successful. To date, more than 90 islands around New Zealand have been cleared of rodents, and brodifacoum use has had an obvious benefit on valuable island ecosystems. The tactical use of toxic bait to protect island populations of indigenous birds, reptiles, and invertebrates endangered by rats and mice continues to be refined to enable larger and more complex islands to be cleared of rodents. New bait types that are especially attractive to mice as well as rats, long-life baits, new toxins, and tunnel delivery systems will augment existing bait types to aid future eradication programmes and help prevent re-invasions. New developments will need to be ethically and socially acceptable and demonstrate clear advantages in non-target impact as well as efficacy in achieving eradication. These endeavours are part of global efforts to eradicate invasives and manage islands to protect native birds and other important fauna and flora.

Rodent Eradication on Cocos Island, Guam: Integrating Wildlife Damage Management, Resort Operations, and Non-Target Concerns

Author(s): Lujan, Dana T.; Vice, Daniel S.; Guerrero, Jesse P.; Candaso, Clifford C. | Abstract: Introduced Polynesian rats threatened native bird populations and forest habitat on Cocos Island, Guam. To eliminate the threat posed by rat populations, a rodent eradication was conducted on the 33.6-hectare off-shore island in March and April 2009. An integrated approach to eradication was implemented that included trapping, bait stations, and hand broadcast of rodenticide bait. Trapping was conducted within the resort buildings, where human activity precluded the use of rodenticides. Bait stations, employed in commensal resort settings, were designed to prevent terrestrial crabs from accessing the bait. In addition, bait station deployment and retraction methods were used to reduce impacts on daily resort operations. Non-target concerns, primarily with native forest birds during broadcast operations, supported a decision to use diphacinone, a rodenticide with low avian toxicity risk. Bait consumption by the locally threatened Micronesian starling was evident during the broadcast application, but substantial monitoring for non-target impacts revealed no mortality or sublethal effects for starlings or other potential non-targets. Eradication operations on Cocos Island present a prime example of integrated wildlife damage management, combining traditional eradication methods with novel approaches to address site-specific challenges.

Environmental Monitoring for Brodifacoum Residues after Aerial Application of Baits for Rodent Eradication

Author(s): Fisher, Penny; Griffiths, Richard; Speedy, Cam; Broome, Keith | Abstract: Aerial application of brodifacoum bait for eradication of invasive rodents from islands raises concerns about environmental contamination and adverse effects on nontarget wildlife. We summarize results of environmental monitoring for brodifacoum residues after New Zealand eradications in a fenced reserve at Maungatautari and on the offshore islands Little Barrier, Rangitoto, and Motutapu. Brodifacoum was not detected in extensive freshwater monitoring at Maungatautari, or in freshwater samples from Little Barrier Island. Residual concentrations were present in soil samples from underneath degrading bait pellets on Little Barrier, and decreased to near the limit of detection by c. 100 days after application. No brodifacoum was present in marine shellfish sampled from Little Barrier, Rangitoto or Motutapu. A range of birds, including a kiwi from Little Barrier, were considered nontarget mortalities. Nine little blue penguins found dead on beaches outside the Rangitoto/Motutapu area after baiting were considered most likely to have died of starvation, despite the detection of brodifacoum in three birds. This result highlights the critical role of post-application environmental monitoring in rodent eradications, and information gaps regarding the movement, persistence, and effects of brodifacoum in the environment.

Challenges and Unique Solutions to Rodent Eradication in Florida

Once established, invasive rodents cause significant impacts to island flora and fauna, including species extinctions. There have been numerous efforts to eradicate invasive rodents from islands worldwide, with many successes. For a number of reasons, many invasive vertebrates have become established in Florida, including several rodent species. We have implemented rodent eradication efforts on two Florida islands. Using the successful eradication strategy developed for Buck Island, U.S. Virgin Islands, we have attempted the eradication of roof rats from Egmont Key off Tampa Bay. We also are attempting to eradicate Gambian giant pouched rats from Grassy Key in the Florida Keys. On Egmont Key, we used a grid of bait stations containing diphacinone rodenticide bait blocks and hand tossing of bait blocks into thickets. On Grassy Key, we used a grid of bait stations containing a zinc phosphide bait along with intensive live-trapping. We discuss the eradication planning, efforts to minimize nontarget animal losses, and follow-up activities. We also discuss some of the difficulties encountered in each of these two different situations.

Panel Discussion – Regulatory and Policy Issues around Non-Target Mortality and Environmental Fate of Rodenticides

Author(s): Eisemann, John D.; Swift, Catherine E.; Dunlevy, Peter A.; Pitt, William C.; Witmer, Gary W. | Abstract: One of the goals of this Symposium was to bring together agricultural and conservation users of rodenticides to discuss the impacts of rodenticides on the environment, examine the current regulatory climate governing their use, and identify ways that users can reduce or eliminate these impacts. Some of the presentations in today’s symposium highlighted specific impacts, and the preceding talk described the scenario of what can happen if an issue related to pesticide impacts ends up in the courts. The three agencies that were represented on this panel (USDA, USFWS, and EPA) have regulatory oversight and enforcement authority for the use of rodenticides and/or the adverse effects resulting from the use of rodenticides. In addition, USDA and FWS are the primary federal users of rodenticides for agriculture and conservation. USDA holds the registrations for a number of rodenticide products for agricultural and conservation purposes. Panelists were asked to describe how Federal Insecticide, Fungicide and Rodenticide Act (FIFRA), Migratory Bird Treaty Act (MBTA), Endangered Species Act (ESA), National Environmental Policy Act (NEPA), the Animal Damage Control Act, and the administration of USDA’s pesticide labels apply to rodenticide adverse effects. Panelists were then asked to bring up an issue within the scope of their agency that they view as problematic for conducting eradication projects. Panelists were also asked to suggest proactive measures that the rodent eradication community can undertake to improve future rodent eradication efforts. Finally, the floor was opened to audience members for questions and comments.

Nontarget Risk and Environmental Fate of the Broadcast Application of a Diphacinone Rodenticide at Mōkapu and Lehua Islands, Hawai'i

Invasive commensal rodents such as Rattus spp. and Mus spp. imperil many threatened and endangered native species including plants, invertebrates and birds within Hawai'i and U.S. territories and possessions in the Pacific. In some cases, the eradication or control of invasive rodents could allow natural recovery and active restoration of native species and ecosystems negatively impacted. The broad scale application of rodenticides is a necessary management tool for this purpose, but it is highly controversial to the public and regulatory agencies. There is great perceived and actual risk of nontarget mortality and environmental contamination. One of the conservation uses of rodenticides registered in Hawai'i is the aerial broadcast of rodenticide bait over large areas of native ecosystems on the main Hawai'ian islands, repeated periodically to maintain reduced rodent population levels. Recognizing that the success of this program depends on public and regulatory support, a coalition of state and federal agencies and private landowners have carefully designed Hawai'i’s rodent control program to minimize short- and longterm environmental impacts. In the early 1990s, diphacinone was selected as the primary rodenticide for conservation uses in Hawai'i because of its long track record of safe and effective use in agriculture worldwide. Hawai'i’s program has 5 components: research on efficacy and environmental impacts, regulatory compliance, developing and using local technical expertise, monitoring of rodenticide impacts and native species recovery and public outreach and engagement, particularly at the community level. After many years of generating the efficacy and safety data in support of regulatory approval by the U.S. Environmental Protection Agency (EPA) and the State of Hawai'i, a diphacinone product (Diphacinone–50, Hacco, Inc., Randolph, WI) was approved in 2007 for conservation uses in the U.S. Subsequently, rodenticide pellets containing the active ingredient diphacinone at 0.005% (50 ppm) were broadcast by helicopter in February 2008 on Mōkapu and in January 2009 on Lehua. Mōkapu was the first island in the world where the aerial broadcast of this less hazardous active ingredient was used to eradicate rats. Island eradications in other parts of the world have usually used broad-spectrum active ingredients that are far more persistent and bioaccumulative, thus imparting a much higher risk to nontarget species and the environment. Monitoring of nontarget and environmental effects on Mōkapu and Lehua did not detect diphacinone residues. A number of factors, including state of Hawai'i restrictions on bait entering the ocean, led to rats surviving on Lehua. These projects demonstrate that the aerial broadcast of a rodenticide containing diphacinone can be an effective and safer tool for conservation. Hawai'i is using the results from Mōkapu and Lehua to plan future rat eradication and control projects and continue development of a long-term program.

From the Sea of Cortez to the Aleutian Islands, Alaska: General Principles Applied to Eradication Planning, Compliance, and Implementation (Abstract)

Author(s): Howald, Gregg R.; Buckelew, Stacey; Wegmann, Alex; Swinnerton, Kirsty; Samaniego, Araceli | Abstract: The removal of introduced rodents from islands can be a highly successful, and thus, important conservation tool for land managers. Non-native rodents have adverse impacts to island ecosystems. Rats alone have been implicated in approximately 40-60% of all bird and reptile extinctions globally. Introduced rodents have been removed from more than 330 islands worldwide using techniques adapted over a ~30-year time period – the use of a bait containing a rodenticide delivered by hand into bait stations or broadcast by hand or helicopter, or a combination of these techniques. Island Conservation has applied this basic approach to remove rodents from islands spanning from the deep tropics to north temperate/sub-Arctic latitudes. Across these regions, the same principle has been applied but the details customized to the local biological and sociopolitical environment. The fundamental considerations during rodent eradication planning have included: 1) delivery of a bait containing a rodenticide into every potential rat territory; 2) delivery at a time of year when the rodent population is food stressed and more likely to eat the bait; 3) minimizing or eliminating, wherever possible, risk to non-target species prone to disturbance and/or toxicant exposure; and 4) ensuring appropriate legal or regulatory compliance and acceptance by local communities. In most cases, the balance between the probability of eradication success and these considerations, particularly risk to non-target species and safe workplace seasonal time periods, are not compatible and may conflict. Thus, adjustments to the eradication plans are sometimes necessary, and are done with careful evaluation of counter-risks to eradication efficacy, non-target species, logistics, or workplace safety. We will use our projects as case studies to demonstrate the approach we take to balance the risks and mitigation, with decisions made in conjunction with our project partners. Comprehensive and transparent planning, with the involvement of experts in the eradication field, land managers, local biologists, regulators, and local community are imperative to overall project success.

Eradication of black rats Rattus rattus from Anacapa Island

Removing invasive rats from islands is a powerful conservation tool, and practitioners are now targeting larger islands for rat eradication. As they do so, they face the challenge of mitigating for potential non-target impacts on native biodiversity that may be susceptible to rodenticides. We report on the eradication of black rats Rattus rattus from Anacapa Island, California, in 2001–2002, which was the first-ever invasive rodent eradication from an entire island where an endemic rodent was present and the first aerial application of a rodenticide in North America. As a mitigation strategy we staggered the rodenticide application over 2 years and held a representative sample of the Anacapa deer mouse Peromyscus maniculatus anacapae in captivity. We also mitigated for bird species potentially susceptible to brodifacoum poisoning and monitored aspects of the terrestrial and marine environments. The free-ranging native rodent population severely declined following rodenticide applications but reintroduction and translocation efforts were successful, and the population quickly recovered to pre-eradication levels. Non-target impacts also included mortality of raptors, gulls and passerines, including high mortality of rufous-crowned sparrows Aimophila ruficeps obscura despite planned mitigation. All observed non-target impacts are expected to be ephemeral; however, further monitoring should reveal details on the dynamics of those impacts. Brodifacoum was not detected in the marine environment or in significant amounts in terrestrial soil, plants and arthropods. Seabird benefits from the rat eradication were quickly realized.

Beyond Kapiti - A decade of invasive rodent eradications from New Zealand islands

New Zealand, an archipelago of more than 2000 islands, has a terrestrial fauna especially depauperate in native land mammals. Kiore (Rattus exulans) was the first of four rodent species introduced by people. A project to eradicate invasive rats from Kapiti Island in 1996, represented a turning point in the technology, complexity and scale at which managers of natural heritage on New Zealand islands could operate. This paper includes case studies of some significant projects targeting rodents, sometimes with other introduced mammals, undertaken in the 12 years following Kapiti. Details of the methods, costs, results and outcomes are provided for Kapiti, Whenua Hou, Tuhua, Campbell, Raoul, Hauturu, Taukihepa, and Pomona islands, collectively representing a total of over 23,000 ha of habitat cleared of introduced mammals. Research and trials undertaken in the Kapiti project provided the basis for future environmental risk assessments, allowing other projects to focus on knowledge gaps. New trends in invasive species eradication in New Zealand include more challenging multi-species eradication projects, some of which are undertaken by self- funded community groups. To summarise the lessons of the New Zealand experience: a programmatic approach is recommended which will fit each eradication within a context or framework of goals for those islands; address biosecurity issues at the outset; build capability to attempt the most challenging and rewarding projects; facilitate investment in monitoring and manage expectations of stakeholders to ensure their ongoing support. Success breeds success but is never guaranteed.

Response of mice (Mus musculus) to the removal of black rats (Rattus rattus) in arid forest on Santa Cruz Island, Galápagos

A combined rat and mouse trapping grid was established in arid coastal forest on Santa Cruz Island, Galapagos over June and July 2009. Mouse traps were opened first and did not initially catch any mice. Rat traps were opened four days later and began catching rats on the second night. Mice were trapped in increasing numbers only after the rat catch-rate had declined substantially. Interference with bait at rat traps also increased. The estimated density of rats was 4.8 rats/ha and mice 32.3/ha. The results suggest mice were present but their activity and/or numbers were being suppressed by the larger rodent. This conclusion has implications for competitive exclusion of native Galapagos rodents by both introduced species. It also suggests caution for rodent abundance estimates and planned rodent eradications when two or more species are present.

Morphometric change, distribution, and habitat use of Cook Strait giant weta (Deinacrida rugosa: Orthoptera: Anastostomatidae) after translocation to Matiu-Somes Island

Giant weta (Orthoptera: Anostostomatidae) are large flightless New Zealand insects in the genus Deinacrida. They usually survive in the absence of introduced rats and many have now been transferred to islands and areas after rodent eradications. One such transfer of the Cook Strait giant weta, D. rugosa was made from Mana Island to Matiu-Somes Island in 1996 and we investigated the sex ratio, morphometrics, growth, distribution, habitat use, nocturnal activity pattern and incidence of natural injury in February, May, September and December 2007 and February 2008. Most weta found were adults (82%), more adult females (74%) were seen than males but more male juveniles (61%) were seen than females. Weta from Matiu-Somes Island had slightly longer metatibiae than those from Mana Island. Cohorts of different-sized insects become larger in most successive samples indicating a 2-year development period following eclosion. Adults were always present but were most frequently seen in February when the smallest juveniles were also found, and large juveniles were common in September and December. Weta have spread throughout the island but were abundant over the northern half of the island with few found at the southern end. Adults and large juveniles were usually found on or near the ground whereas small juveniles were above ground on vegetation. Most adult weta were seen during the fi rst two hours of darkness in February on the ground or on short grass, and only five were on bushes. Overall, 22% of adults were injured, usually with damaged antennae or legs. Cook Strait giant weta are now so well established on Matiu-Somes Island that we consider this translocation a success.

Managing invasive animals on sub-Antarctic islands

People have deliberately or accidentally introduced animals to many sub-Antarctic islands over the past 250 years. Environmental changes as a consequence of these introductions can be inferred from observations made directly before or following eradication. Broad options for managing invasive animals include prevention, eradication, sustained control or no control, circumstances usually dictating the best option. xamples of successful management of invasive animals in the sub-Antarctic region include Enderby and Campbell islands. Key challenges in furthering management of invasive animals in the region include improving biosecurity to prevent pests reaching new locations (includ­ing islands from which they may have been eradicated) and increasing the scale of islands on which eradication of rodents is attempted. Success with the Macquarie Island rabbit eradication project will greatly increase understanding and confidence in what can be done.

Bait acceptance by house mice: an island field trial

Predation by introduced house mice Mus musculus on islands is one cause of decline in native birds and has deleterious impacts on other ecological aspects. Eradication of rats (Rattus spp.) from islands of up to >10000 ha has been achieved, but for mice scale is still an issue with the largest island cleared being only 710 ha. The feasibility of eradicating mice from larger islands is being considered, and to support these assessments, we undertook a field study on Gough Island (6400 ha) to determine whether all mice would be likely to accept toxic bait. We replicated a toxic bait operation as closely as possible, in timing, probable bait density and distribution, using a bait formulation used commonly in rodent eradication operations. Baits lacked toxin but were coated with the fluorescent dye rhodamine B. Mice trapped in and around the baited areas were inspected under ultraviolet light for fluorescent marking indicative of bait consumption. Of 434 mice, 97% tested positive, including mice trapped on assessment lines up to 90 m from the closest bait. There was no difference in the proportions of unstained mice from assessment lines outside baited sites compared with mice trapped in the core baited sites, suggesting large-scale foraging movements over relatively large distances into the baited sites from surrounding, non-baited habitat. Despite the high bait densities (15.7 kg ha−1 at initial application and 7.9 kg ha−1 at second application), bait consumption rates of ~4 kg ha−1 day−1 occurred after both applications. This was much higher than expected (probably the result of large-scale movements) and meant that all baits were consumed before trapping began. Thus the 13 unstained mice trapped in the core of the baited area may have moved there after bait was consumed. Further trials are required to assess whether all unmarked mice were false negatives (not exposed to bait) or if any were true negatives (rejected bait). A separate experiment found that all 11 mice trapped in a cave had eaten bait applied aboveground around the cave’s entrances, suggesting that caves do not serve as refugia for mice and are thus unlikely to compromise an eradication attempt.

Tropical Rat Mites as newly emerging disease pathogens in rodents and man

Different mite species, generally found on various animals, may temporarily also infest man and consequently, such arthropods may be responsible for pruritic skin reactions which are often misdiagnosed. Mite dermatitis caused by the Tropical Rat Mite Ornithonyssus bacoti occurs in several small mammals and rodents under tropical and temperate climatic conditions. This mite was identified in 1923 as a cause of human dermatitis in the United States. In Europe, the Tropical Rat Mite was first detected in the German seaport of Hamburg in 1931, a site indicating the mite was carried across the oceans along with infested ship rats, probably in the times of sailing vessels. According to various observations in Germany, O. bacoti appears in wild rodents more frequently than it seems. In most cases, symptoms of mites are recognized only when they attack humans, but the diagnosis of a rat mite dermatitis requires identification of the parasite, which however is more likely to be found in the environment of its host than on the hosts’ skin itself. Here, three different outbreaks from Bavaria are reported. As a clinical example is presented the case of a 23-year-old medical student and several other residents inhabiting a rat- and mice-infested house in Munich. The arthropods originally came from an Italian restaurant and surrounding facilities. Mites were found in large number in the students’ flat. The patient was suffering from severe itching and papular urticaria. He consulted a dermatology clinic complaining of a pruritic dermatitis for at least 2 weeks. Dermatitis was misdiagnosed as allergy, treatment with the antiphlogistic agent fluocinolonacetonid remained unsuccessful. Eradication of rodents and treatment of the rooms of the house with a pyrethroid were performed to prevent reinfestations. O. bacoti is a periodic hematophageous parasite and spends relatively short time on a host. Causal therapy by antiparasitic agents on human patients is not necessary. If indicated, treatment should be symptomatic.

Invasive Rodent Eradication on Islands

Invasive mammals are the greatest threat to island biodiversity and invasive rodents are likely responsible for the greatest number of extinctions and ecosystem changes. Techniques for eradicating rodents from islands were developed over 2 decades ago. Since that time there has been a significant development and application of this conservation tool. We reviewed the literature on invasive rodent eradications to assess its current state and identify actions to make it more effective. Worldwide, 332 successful rodent eradications have been undertaken; we identified 35 failed eradications and 20 campaigns of unknown result. Invasive rodents have been eradicated from 284 islands (47,628 ha). With the exception of two small islands, rodenticides were used in all eradication campaigns. Brodifacoum was used in 71% of campaigns and 91% of the total area treated. The most frequent rodenticide distribution methods (from most to least) are bait stations, hand broadcasting, and aerial broadcasting. Nevertheless, campaigns using aerial broadcast made up 76% of the total area treated. Mortality of native vertebrates due to nontarget poisoning has been documented, but affected species quickly recover to pre-eradication population levels or higher. A variety of methods have been developed to mitigate nontarget impacts, and applied research can further aid in minimizing impacts. Land managers should routinely remove invasive rodents from islands <100 ha that lack vertebrates susceptible to nontarget poisoning. For larger islands and those that require nontarget mitigation, expert consultation and greater planning effort are needed. With the exception of house mice (Mus musculus), island size may no longer be the limiting factor for rodent eradications; rather, social acceptance and funding may be the main challenges. To be successful, large-scale rodent campaigns should be integrated with programs to improve the livelihoods of residents, island biosecurity, and reinvasion response programs.

Complexities of costing eradications

Conservation dollars are and likely always will be limiting. Spending those dollars effectively and efficiently is of pivotal importance (Wilson et al., 2006). Martins et al. (2006) present an analysis of economic costs of invasive mammal eradications on islands that they claim ‘provide conservation planners with a robust, if preliminary, estimate of the cost of any proposed eradication programme.’ Given that invasive mammal eradication is one society’s most powerful conservation tools, this is a badly needed instrument (Donlan et al., 2003). However, while we commend Martins and colleagues’ efforts and goal, the available data are simply and sadly insufficient to provide such a tool. More importantly, providing a naive tool is potentially misleading and dangerous for land managers, conservation practitioners and philanthropists who may turn to the pages of conservation journals for guidance. The costing of eradication campaigns is a complex process. As quantitatively demonstrated by Martins and colleagues, island area plays a major factor in the variation of costs. Using general linear models with a dataset that included 41 eradication campaigns, the authors conclude ‘decision makers considering potential eradication programmes need only know island area, distance from airport (at least in the New Zealand region), and species to be eradicated to make internally consistent and robust first-pass estimates of likely costs.’ In reality, a number of factors and examples strongly suggest that this is not the case. We briefly address five of those issues here. First is the issue of fixed costs. Martins and colleagues claim that due to efficiency gains in the ability to remove invasive mammals from islands, campaigns should be significantly cheaper today than 20 years ago. They go on to state ‘thus, for a hypothetical 10 km island situated 100 km from an airport, costs would decrease from US$ 251 000 if rodents were eradicated in 1983 . . . to only US$ 31 200 if it were done in 2003.’ While the authors are correct in suspecting that the detected effect in their regression model is indeed real, due to certain fixed costs of eradication campaigns, the relationship does not decrease linearly with gains in efficiency. For example, a conservative estimate of the cost of rodenticide bait alone for an aerial eradication campaign on a 10 km island is US$60 000 – twice their predicted total eradication cost (assuming $4 per kg of bait with an application rate of 15 kg ha , cost estimate from Bell Labs, USA). While a few of the rodent eradication campaigns used by Martins et al. reported total costs less than the bait cost estimate at the above application rate, these were conducted on small islands ( 1 km) where either a bait-station approach was used, or perhaps a lower application rate was used but still successful due to the small area of the island. Of the data available worldwide for application rates of past aerial broadcasts, now the most common method used in rodent eradications, the mean application rate is 17.6 kg ha , which is a conservative estimate as more than one bait application is sometimes necessary (range 10–35, n=16; C. J. Donlan, unpubl. data). Secondly, variable costs have a substantial impact on the realized cost of an eradication campaign. While Martins et al. acknowledge that local factors may be important, they fail to appreciate the impact of these factors. Important variable costs of eradication campaigns include (1) needs to mitigate for potential non-target species, (2) the techniques used (e.g. bait stations versus aerial broadcast of bait for rodent eradications), (3) the level of local capacity present, (4) amount of environmental compliance required and (5) the levels of bureaucracy. For example, the cost of campaigns for rodent eradication using aerial broadcast by helicopter depends largely on isolation (e.g. nearest port not airport as remote campaigns are increasingly boat based), while the cost of a campaign using a bait station approach depends largely on labor costs. These variable costs can influence the overall costs of eradication campaigns by an order of magnitude. Black rats Rattus rattus were recently eradicated from Anacapa Island, CA, USA (Howald et al., 2005). Anacapa Island was the first aerial-based

Costing eradications of alien mammals from islands

AbstractThe ability to estimate costs of alien species eradications is essential for a rigorous assessment of priorities for island restoration. Using a global data file from 41 islands, mostly gleaned from the ‘grey’ literature, we show that the cost of vertebrate eradications can be satisfactorily predicted if island area and species to be eradicated are known. About 72% of the variation in cost can be explained by island area, whereas, for a given area, rodent eradications are 1.7–3.0 times more expensive than ungulate eradications. Costs per hectare decrease with island size. Restricting the analysis to roughly half the data set, the relatively homogeneous half concerned with New Zealand islands, we identify two further influences on cost: date of eradication and distance to the main airport (an indicator of remoteness). For a given area, costs have declined over time but increase with island remoteness. This information therefore provides conservation planners with a robust, if preliminary, estimate of the cost of any proposed eradication programme.