Towns, D.R. Eradications of vertebrate pests from islandsIsland around Newinvasives: Zealand: what eradicationhave we delivered and and what have management we learned?
Eradications of vertebrate pests from islands around New Zealand: what have we delivered and what have we learned?
D. R. Towns Research and Development Group, Department of Conservation, Private Bag 68 908 Newton, Auckland 1145, New Zealand.
INTRODUCTION Invasive species are now recognised as major agents (Towns et al. 2006). In the UK, a US$1.6 million attempt of global change (Mack et al. 2000; Simberloff 2003). to remove hedgehogs (Erinaceus europaeus) introduced The effects of invasive species are particularly severe on by the inhabitants of the Uist Islands of Scotland proved islands (Paulay 1994) where they are implicated in two ineffectual – at least in the initial years – largely because thirds of recorded animal extinctions (Cole et al. 2005). On animal rights activists convinced Scottish Natural Heritage the other hand, there are increasing numbers of successful to use live capture and relocation rather than kill trapping eradications, especially of introduced mammals. These (Carrel 2007; Webb and Raffaelli 2008). Such examples include exotic foxes from 40 islands covering 210,000 ha pose a dilemma. Because of the extent to which invasive in the Aleutian Islands of Alaska (Ebbert and Byrd 2002), species can disrupt ecological processes and human welfare 45 populations of introduced mammals from 29 islands (Mack et al. 2000), increasingly ambitious eradications of in northwestern Mexico (Aguirre-Muñoz et al. 2011) and these species should be attempted (Simberloff 2002). But 21 species of introduced mammals from 17 islands in the as the public profile of these attempts increases, so does Galapagos archipelago off Ecuador (Donlan et al. 2003). resistance to them, despite likely benefits to biodiversity, The upper limits of areas attempted have risen greatly since native ecosystems, and ultimately human welfare. the 1990s (Donlan and Wilcox 2008). The eradication of Since 1996, eradications requiring toxins in New mice (Mus musculus) and ship rats (Rattus rattus) is now Zealand have often been publicly notified through the being attempted on 3881 ha Rangitoto-Motutapu Island, Resource Management Act 1991 (RMA). Proposers New Zealand (Griffiths 2011), and the eradication of must compile an Assessment of Environmental Effects Pacific rats (Rattus exulans) has been achieved on 3083 ha (AEE), which is available for public submissions. The Hauturu (Little Barrier) Island, New Zealand (Towns et al. AEE and submissions are then examined by independent 2006); Norway rats (R. norvegicus) on 11,300 ha Campbell commissioners who may reject the application or place Island, New Zealand (McClelland and Tyree 2002); cats conditions on the way the project is conducted. Two key (Felis catus) on 12,800 ha Macquarie Island, Australia; biological questions often arise during this process (pers. rabbits (Oryctolagus cuniculus) on 3450 ha Norfolk Island, obs.). Firstly: “Do the benefits to biodiversity outweigh Australia; and in the Galapagos Islands, Ecuador, goats financial and short term environmental costs?” Secondly: (Capra hircus) on 458,812 ha Isabela Island, and pigs (Sus “How good is the evidence for cause and effect between scrofa) on 58,465 ha Santiago Island (Donlan and Wilcox losses of biodiversity and purported agents of decline?” 2008). Neither question is exclusive to eradication attempts Eradications on large islands are expensive and are on islands. Any attempted eradications should include likely to include sites with a high public profile, or inhabited measures of the benefits to species and ecosystems. by people. For example, since 1996 the Department of In addition, treating the eradications as large-scale Conservation has undertaken ten large and complex experiments should over time illustrate the relationship island eradication campaigns at a total cost of over NZ between introduced organisms and those that they affect $8 million (updated from Broome 2009). Among these, (Towns et al. 1997). there was intense debate within the scientific community and Maori tribal groups (iwi) over the removal of Pacific In this review I describe the outcomes of eradications of rats from Hauturu in 2004 (e.g., Kapa 2003; Towns et vertebrates from islands around New Zealand and ask how al. 2006), which incurred legal costs of at least NZ$ measured outcomes have informed our understanding of the 200,000 (Broome 2009). Elsewhere, eradication attempts effects of invasive species. I first summarise the biological have been stiffly resisted on the grounds of unacceptable benefits attributable to eradications on islands from which collateral damage or concerns from animal rights activists all vertebrate pests have been permanently removed. I then
Pages 364-371 In: Veitch, C. R.; Clout, M. N. and Towns, D. R. (eds.). 2011. Island invasives: eradication and management. IUCN,364 Gland, Switzerland. Towns: Eradications of vertebrate pests from NZ islands
Table 1 Number of invasive vertebrate populations removed from 95 islands around New Zealand, their general effects on native biota (King 2005) and the type and quality of evidence of their effects on island ecosystems. Introduced No. Single species Ops pest General diet in New Zealand Evidence for effects References Weka Between island Gallirallus 3 1 Invertebrates, reptiles, ground- comparisons; stable Harper 2007 australis dwelling birds including seabirds isotopes (seabirds) Foliage, flowers, fruit and bark of >90 Brushtail spp of native plants; extensive canopy possum 3 0 defoliation; predation of invertebrates Forest canopy recovery Atkinson 1992 Trichosurus (e.g., large snails), eggs, nestlings and after eradication vulpecula adult birds (including seabirds) Rabbit Recolonisation by Oryctolagus 12 2 Grasses and shrubs broadleaved coastal shrubs Towns et al. 1997 cuniculus after eradication Between island comparisons of plants, Whitaker 1978; Atkinson Foliage, flowers, fruit, seeds and reptiles and seabirds; 1985; Towns 1991, Pacific seedlings of forest plants; wide range exclosure experiments with 2002, 2009; Towns et al. rat Rattus 42 26 of invertebrates; lizards; eggs and plants; post eradication 1997, 2007; Pierce 2002; exulans chicks of some birds recovery of invertebrates, Campbell 2009; Campbell plants, lizards, tuatara and and Atkinson 1999, 2002; seabirds Rayner et al. 2007
Foliage, fruit, seeds and rhizomes of Observed post invasion Norway rat R. 26 11 plants; wide range of invertebrates, declines of tuatara; post- Newman 1986; Allen et norvegicus eradication responses of al. 1994; Campbell 2002 lizards; eggs and chicks of some birds forest plants Fruits of native plants; wide range of Post invasion declines of Ship rat invertebrates, lizards; eggs, chicks and invertebrates and forest Atkinson and Bell 1973; R. rattus 6 5 adults of some terrestrial and arboreal birds and bats; stable Harper 2007; Towns 2009 birds isotopes (forest bids) Between island comparisons of Newman 1994; MacIntyre House mouse 13 4 Seeds of native plants; wide range of invertebrates, post- 2001; Roscoe and Murphy Mus musculus invertebrates; some lizards and birds eradication responses by 2005 invertebrates and lizards Stoat Mustela 7 5 Invertebrates, lizards and birds; Post invasion declines of King and Murphy 2005 erminea introduced rodents and rabbits birds Fitzgerald and Veitch Invertebrates, lizards, birds (esp. Post invasion declines 1985; Fitzgerald et al. Cat 8 1 seabirds); introduced rodents and of birds; post eradication 1991; Girardet et al. 2001, Felis catus rabbits recolonisation by land and Veitch et al. 2004; K. sea birds Baird (pers comm.) Fruits and foliage of plants, wide Pig 10 1 range of invertebrates; frogs and Exclosures; recovery of Harper 1983; Coleman et Sus scrofa lizards; ground-nesting birds and their seabirds post eradication al. 2001 eggs; introduced rodents and rabbits Cattle Wide range of herbs, grasses shrubs Bos taurus 3 0 and trees None recorded Post invasion destruction Goat Fungi, ferns, grasses and broadleaved of vegetation; diet analysis; Sykes 1969; Parkes 1984; Capra hircus 10 1 shrubs and trees post eradication recovery of Brook 2002; Bellingham plant communities et al. (2010b) Dilks and Wilson 1979; Sheep 4 0 Grasses and some shrubs Post removal recovery of Meurk 1982; Meurk et al. Ovis aries native herbs and grasses 1994
365 Island invasives: eradication and management ask whether the eradications provide less obvious benefits BIOLOGICAL OUTCOMES OF ERADICATIONS through scientific knowledge, communication, political support and international uptake. Species and communities Given that eradications were designed to protect and STUDY SITES enhance depleted biodiversity, what were the benefits? Based on assessments of eradications over the last 20 Islands used here are those beyond the range of natural years, in situ recovery or subsequent translocations to recolonisation by the eradicated vertebrates. Successful islands now free of introduced mammals around New eradications are those with no recolonisation for two Zealand improved the long term prospects for at least 16 years or more after the original campaign. A few islands species of invertebrates and 76 species of vertebrates. The have occasional incursions of mammals through natural latter included two of the four species of frogs, all three dispersal, but if these are consistently eliminated on arrival, taxa of tuatara, 23 of the 80 species of lizards, 32 of the 73 the site is regarded as permanently clear and is included in taxa of terrestrial birds and 16 of the 84 taxa of seabirds the analysis. Guidance about motives for eradications was (Bellingham et al. 2010a). Furthermore, earlier eradications obtained from legal status of the land, statutory plans and of goats from Great Island (Three Kings Group) may have interviews with project managers. Evidence of the effects enabled the recovery of more than 200 species of plants of invasive species was regarded as available if accessible and up to 30 species of endemic snails (Brook 2002; P.J. with search engines such as the Department of Conservation de Lange pers comm.; Bellingham et al. 2010b). Similarly, library catalogue, Google Scholar and BIOSIS. the removal of pigs from Aorangi Island (Poor Knights Up to 2010, all invasive mammals and one species Group) likely provided benefits for numerous rare species, of bird had been removed from 95 islands; a total of 147 including 18 species of plants, five species of snails, 13 populations of 13 species of vertebrates within an area of species of insects, six species of reptiles and two species of 32,000 ha (updated from data In: Veitch and Bell 1990; birds (Towns et al. 2009b; Bellingham et al. 2010a). Clout and Russell 2006). Eradications on an additional 20 For many species, range contractions have been islands (total 4700 ha) of eight species of vertebrates have reversed after eradications as species are either returned yet to be confirmed. The most frequently eradicated species to sites they previously occupied or released into new were Pacific and Norway rats (Fig. 1), but also included ones as a conservation measure. Excluding planting for one species of out-of-range flightless predatory bird and island reforestation, translocations alone have involved at one arboreal marsupial (Table 1). Most of the remaining least 139 populations of 63 taxa of animals (Fig. 2). The species were farm animals that became feral, although results of species translocated to or between islands must domesticated livestock removed from islands retired as be treated with caution because determining the success farms were not included in these totals. Assessments of the of translocations can be difficult. If we use self‑sustaining effects of feral species were complicated by the previous populations as the minimum criterion for success (e.g., presence of stock on 20 (21%) of the islands, which in Dodd and Seigel 1991), birds have the highest proportion most cases were also cleared of forest for agriculture. of identified successful translocations to islands after pest Additionally, even the forested islands were burned during eradication 44/72 (61%). The proportion is much lower for Maori or early European history (Bellingham et al. 2010a), invertebrates 3/21 (14%) and reptiles 3/37 (8%). None of although they have now had many decades to recover. the populations of amphibians and seabirds translocated Furthermore, on 25 (26%) islands, multiple species to new islands can yet claim to have met basic criteria of terrestrial vertebrates coexisted, with potential for for success. In part, lack of data on success relates to complex interactions between them (e.g., Courchamp et the ease of locating released animals. With the exception al. 1999, 2000). On the other hand, for most of the earlier of terrestrial birds, which often have flexible and high eradications, multispecies removals were conducted over reproductive output, many invertebrates and reptiles are long time intervals, with the potential to measure responses cryptic and difficult to locate at low density. Some, such between the eradications. Finally, all of the islands are as tuatara, also have low reproductive output and late age inhabited by introduced birds such as European starlings at maturity (Cree 1994). For such species the outcome of (Sturnus vulgaris) and blackbirds (Turdus merula) whose translocations may not be measurable for years or even effects are unknown. Many such species are now found decades after release (e.g., Towns and Ferreira 2001). through the entire archipelago and are assumed to have Furthermore, aside from at least three known failures equal effects across the sample. (4%), there are also populations (all birds) that are maintained in island environments where they are unlikely
Fig. 1 Composition of 147 populations of invasive Fig. 2 Composition of 139 translocations of 63 taxa of vertebrates removed from 95 islands around New native vertebrates and invertebrates to islands cleared of Zealand. all introduced mammals.
366 Towns: Eradications of vertebrate pests from NZ islands to ever form self-sustaining populations, but where their native species resistant to grazing. Full recovery of native prospects can be improved away from introduced predators. plant communities is likely within a few decades (Meurk et Examples of these include kakapo (Strigops habroptilus), al. 1994). Likewise, after the removal of goats from Great kiwi (Apteryx spp.), takahe (Porphyrio mantelli) and hihi King Island in 1946, grazing-induced turf was 40 years (Notiomystis cincta). Here success is based on overall later replaced by early successional forest up to 2m tall, increases in metapopulations, even though contributing and reappearance in coastal forest of endemic tree species populations may be very small (see also Bellingham et al. (Wright and Cameron 1990; Bellingham et al. 2010b). 2010a). However, the spread of some endemic species has been slower than expected, largely due to the absence of birds Populations that are expanding after invasive species able to disperse large seeds. The importance of dispersers removals may carry a legacy of past problems. For was illustrated when the translocation of a small number example, when Pacific rats threatened populations of of kereru to Great King Island was rapidly followed by the northern tuatara (Sphenodon punctatus) on Hauturu, the appearance of new populations of seedlings (Bellingham remaining eight adults were taken into captivity to breed et al. 2010b). until Pacific rats were eradicated in 2004. Since 2006, over 100 tuatara raised in captivity have gradually been released (MacAvoy et al. 2007). This appears to be an exemplary Ecosystems and landscapes breeding programme but the adult tuatara on Hauturu have The removal of invasive species should, in theory, lost genetic variation, with potential attendant problems enable the recovery of ecosystems dominated by native of low fitness (MacAvoy et al. 2007; Miller et al. 2008). species (Towns et al. 2009b). In New Zealand, 47(49%) Furthermore, around 78% the released progeny were sired of the eradications were on island Nature Reserves, where by one male (Moore et al. 2008). Tuatara can take over the removal of exotic organisms is mandated in order to 10 years to reach sexual maturity and each female has protect the integrity of native ecosystems (New Zealand an annual reproductive output of about 2 offspring (Cree Reserves Act, 1977). However, measuring ecosystem 1994). Consequently, even determining numerical success responses to eradications has proved challenging. Recent or failure of the Hauturu population may take many decades. advances centred on the role of seabirds as drivers of island Establishing the genetic effects of a predation bottleneck ecosystems (Towns and Atkinson 2004; Bellingham et al. and restricted paternity on tuatara may take even longer. 2010a). On islands off northeastern New Zealand, Fukami et al. (2006) compared ecosystem processes on islands Similar problems can arise in translocated populations. with large seabird populations with those where seabirds Miller (2009) assessed the genetic heterozygosity of three are suppressed by rats. The authors found that compared populations of translocated lizards, each of which had self- with islands invaded by rats, soils on seabird islands had sustaining populations (sensu Dodd and Seigel 1991). She higher total C, total N, total P, and marine-derived δ15N, found that when the founder population is low (15), or in greater microbial CO2 production, and more abundant larger populations when there is relatively low founder herbivorous and microbe-feeding nematodes. Many macro- survival, inbreeding depression can erode genetic diversity invertebrates in the forest litter were also more abundant on sufficiently to jeopardise the long term prospects for the seabird islands, including such diverse groups as beetles, populations. collembolans and minute land snails (Fukami et al. 2006; Such problems aside, natural recovery in situ, Towns et al. 2009a). The seabird effects were also reflected recolonisations, and translocations can greatly change the in higher foliar and litter N concentrations, greater N to structure of communities on islands once invasive species lignin ratios and higher litter decomposition rates (Wardle have been removed. Some of these changes are subtle. et al. 2009). In contrast, compared with the islands invaded For example, on Korapuki Island, lizard assemblages in by rats, seabird islands had lower seedling densities and the presence of Pacific rats and rabbits were dominated lower tree basal area, reflecting the disturbance effects of by diurnal species of skinks. After the two mammals were seabird on forest vegetation (Fukami et al. 2006; Roberts et removed, dominance within the assemblages shifted as al. 2007). These observations were tested experimentally previously rare nocturnal geckos become increasingly by Jones (in press), who added fertiliser to mimic guano abundant (Towns 1991, 2002). Similar subtle effects on Maud Island, which has few seabirds. As previous of rats such as Pacific rats have been reported for plant comparative studies indicated (e.g., Fukami et al. 2006), communities. Comparisons of seedling composition on the treated sites had increased litter decomposition rates, arthropod consumer abundance, and above-ground net islands where Pacific rats are present, have been excluded 13 using cages, and have been eradicated, indicate that the rats primary productivity. Jones (2010a) also measured δ N have measurable effects on at least 11 and perhaps over and C:N ratios in soils, plants and spiders on northeastern 30 species of coastal and forest plants. These effects are islands with expanding seabird populations 12–22 years sufficiently severe to result in impaired recruitment, sex after Pacific rats had been eradicated. She found that the imbalances and declines to local extinction of canopy and two measures of N increased with time, indicating that subcanopy species (Campbell and Atkinson 1999, 2002; these islands would converge with equivalent measures on Campbell 2011). There may also be a feedback loop, uninvaded islands within about four decades. where predation on the large seeds of some plants by On some islands, the removal of invasive species of Pacific rats reduces their incidence in the canopy, thereby large herbivores has led to changes of entire landscapes. reducing visits from fruit pigeons (kereru: Hemiphaga On Campbell Island, Meurk (1982) described the rapid novaeseelandiae) and dispersal of large-fruited plants reappearance of brightly-flowered megaherbs in areas that remain (Campbell and Atkinson 2002). The extent to protected from sheep. Previous examples documented which changed seedling recruitment after release from the succession from turf to forest on Great Island after the effects of Pacific rats might change forest composition is removal of goats. Similar landscape-level changes are now as yet unclear. apparent on islands retired from grazing and planted by volunteers. For example, when farming ceased on Tiritiri More extensive changes in community structure can Matangi Island in 1971, only 11% of the original forest follow the removal of grazing species such as sheep and cover remained. By 1994, 280,000 trees had been planted goats. On subantarctic Campbell Island, removal of sheep (Rimmer 2004), and at least 60% of the island now has a from the island in 1990 was followed within four years by rapidly closing canopy of young forest (R. Renwick pers recovery of tall native grasslands, reinvasion of the old comm.). pasture by native megaherbs, and declines in coverage by
367 Island invasives: eradication and management
SCIENTIFIC AND POLITICAL OUTCOMES OF ecosystems (Broome 2009). Such aims can be difficult to ERADICATIONS communicate if the media prefers stories about the conflict or complexities generated by projects instead of their Science and communication benefits (Bremner and Park 2007). When eradications are proposed, conservation scientists are frequently asked: to provide evidence that species Surprises and failures proposed for removal have detrimental effects on native A question I have sometimes been asked at RMA species. Post-eradication studies of recovery by native hearings is: “Are there detrimental long term effects species and communities should provide strong evidence of eradications on island species?” There are few such of the effects of introduced species (Veltman 1996; Towns examples. Perhaps the most notorious is the invasion of et al. 1997; Towns 2009), particularly if only one pest native plant communities by invasive boxthorn (Lycium species was present. To examine where understanding has ferocissimum) after the removal of rabbits from Motunau advanced, I have listed all 13 species eradicated from the Island (3.5 ha) in 1963. The thickets of this spiny shrub 95 islands, identified the general effects on New Zealand became so dense they were responsible for entanglement biota based on recent reviews, and then identified how problems for nesting seabirds (Beach et al. 1997). More island studies have contributed to this information (Table often, the unpleasant surprises have been less vigorous 1). response by native species than expected. One example is The most comprehensive studies have been on the the slow spread of species with large seeds after removal effects of Pacific rats, where distributional comparisons of of goats from Great Island (see above; Bellingham et plants were the basis for hypotheses tested by exclosures al. 2010b). Another was a lack of measurable response and post-eradication responses for plants (discussed by forest birds after the removal of cats from Hauturu above). Among vertebrates, hypothesised direct effects Island (Girardet et al. 2001). This eradication did have the of Pacific rats on the eggs and hatchlings of tuatara were desired effect of reducing predation of adult Cook’s petrels confirmed when there was a pulse of tuatara recruitment (Pterodoma cookii) by cats, but the unpredicted effect of after the removal of Pacific rats from three islands (Towns increased predation by Pacific rats on Cook’s petrel chicks et al. 2007). However, there was also an unexpected until the rats were eradicated in 2004. The pressure on indirect effect, where some populations of adult tuatara petrel chicks was attributed to mesopredator release, after also showed significantly better body condition (length: removal of cats as a major predator of the rats (Rayner et al. mass) when Pacific rats were removed – presumably due to 2007). The only other negative outcomes have eventuated release from interference competition (Towns et al. 2007). from reinvasions of rats to islands. For example, three These more subtle effects also became apparent for resident species of rats were eradicated from Pearl Island (512 ha) burrowing seabirds with rapid increases of fledging success 225 m off Stewart Island. Although reinvasion by ship when Pacific rats were removed (Pierce 2002; Imber et al. and Norway rats from Stewart Island was predicted, pre- 2003; Rayner et al. 2007; Towns 2009). eradication analyses of microsatellite DNA in both island Aside from useful studies of the effects of invasions by populations indicated rare mixing between them. However, cats and sheep (Table 1), there are few detailed accounts of both species reinvaded after only nine months, with their responses after removal of some of the most widespread origins on Stewart Island verified by microsatellite DNA pest species. For example, a lack of comprehensive post (Russell et al. in press). Aside from the value of DNA eradication monitoring after the removal of pigs represents analyses, the study demonstrated that the hypothesis of a missed opportunity to inform debate about their effects on infrequent reinvasions by rats did not hold after the Pearl native species other than seabirds. For species such as ship Island rat populations were eradicated. rats, the short history since eradication may account for The Pearl Island experience did provide a useful test the lack of published information on responses by native of rat dispersal capabilities, even though the outcome species. On the other hand, there has been only one study was disappointing. For most other eradications, surprises of the responses after removal of Norway rats, despite a have been more positive, including rapid and unpredicted long time interval and numerous potential study sites. recolonisations by native species. For example, three The sparse examples of benefits of eradications species of native birds recolonised Rangitoto-Motutapu supported by peer reviewed articles in reputable scientific Islands within 12 months of aerial bait spread against journals is one reason for conflict between conservation vertebrate pests and before the full programme had been organisations advocating pest eradication and sector groups completed (R. Griffiths pers. comm.). in opposition (Towns et al. 2011). In one example, animal rights activists attempted to use court action followed by National and international support for island direct sabotage in an attempt to terminate the eradication of eradications ship rats from Anacapa Island in California. The activists The development of increasingly effective methods argued that conservation benefits did not outweigh the against invasive mammals such as rodents and cats (Veitch collateral costs to native species, the rats had been 2001; Thomas and Taylor 2002; Towns and Broome 2003) demonised and the eradication was being undertaken only has received political support at the highest levels in New because the rats were there (Towns et al. 2006, H. Jones Zealand. For example, the planning and execution of the pers comm.). Correspondents in New Zealand can hold campaign against Norway rats on Campbell Island was similar views. One recent letter to a newspaper complained cited for Innovative Practice in the New Zealand Public of this “demonising” attitude inherent in the Department Service. The proposal to remove all seven species of of Conservation’s attempts to remove hedgehogs (among invasive mammals from Rangitoto-Motutapu Islands six other species) from Rangitoto and Motutapu Islands. was announced by the then Prime Minister and Minister Public attitudes to invasive species in Scotland were of Conservation (Clark 2006). Internationally, even the shaped by awareness and education (Bremner and Park earlier successes were seen as so important that a squad 2007), which suggests that some opposition to eradications of New Zealand eradication experts was proposed to assist stems from poorly developed proposals. In New Zealand, other nations with removing threats to their biodiversity other than in rare examples where eradications were (Duffy 1994). New Zealand has become an acknowledged undertaken on small islands to test methodologies, they leader in island conservation (e.g., Rauzon 2007), and pest were all done with a view to protect threatened species, eradication was even identified as one of New Zealand’s enable the public to experience prolific native wildlife on “export industries” (Simberloff 2002). For example, New islands without introduced pests and to restore modified Zealand advice, assistance and specialised equipment
368 Towns: Eradications of vertebrate pests from NZ islands been used in such diverse locations as the Seychelles, local conservation offices. For example, of 86 reports of Falkland Islands and Western Australia (McClelland and reptile translocations around New Zealand, only 15 (17%) Tyree 2002). New Zealanders have also assisted with the were in the primary literature (Sherley et al. 2010). Given eradication of rabbits and ship rats on the French island of that the available data under-estimate achievements, the Saint Paul (Micol and Jouventin 2002), ship rats on San scientific community can only communicate to the public Pedro Mártir, Farallón de San Ignacio and Isabel Islands in rather vague views of the extent of change possible. We are the Gulf of California, Mexico (Samaniego-Herrera et al. also a long way from measuring, or even identifying, the 2009), Norway rats on Rat Island in the Alaskan Aleutians ecosystem services that ecological restoration can provide. (S. Buckelew pers comm.), pigs on Santa Cruz Island off As a first step, the more tangible measures could be assisted California (Parkes et al. in press) and goats on Lord Howe by regularly updated databases of successful eradications Island off Australia (Parkes et al. 2002). Furthermore, the (e.g., Keitt et al. 2011) and, within defined criteria for New Zealand government through NZAID supports the success, a list of the species known to benefit. Pacific Invasives Initiative, a non-government organisation In New Zealand, eradications of introduced vertebrates that facilitates capacity development and provides project from islands were to some extent viewed as experiments management advice for the eradication and control of since they effectively used a “learning by doing approach” invasive species throughout the Pacific region (http://www. that tested the technologies of removal (Thomas and issg.org/CII/tools.html). Taylor 2002; Towns and Broome 2003, Broome 2009). Unfortunately, a similar developmental approach was not CONCLUSION taken to measuring the outcomes of eradications. Had Eradications of invasive species are no longer novel; questions about the effects of specific introduced species they are increasingly ambitious and expensive, which also been identified and costs of pursuing them included in makes them increasingly difficult to justify unless there are the project from the outset, we would now be in a much unequivocal benefits (Simberloff 2002). The New Zealand stronger position to identify cause and effect. For the public has gained increasing involvement in the choice more recent eradications, especially of some species of of sites for eradications, the restoration of island systems rodents, retrospective analyses of the responses of resident cleared of pests, and in the eradication methods used species and ecosystems might still prove enlightening. But (Towns et al. 2011). This involvement increases the need for others, such as the historic removal of pigs, any but to answer questions about the outcomes of eradications, the coarsest of analyses are now obscured by interactive cost-effectiveness, and the effects of invasive species on effects and time. native species and ecosystems. Fortunately, despite the few published reports of The outcomes of eradications can be measured in two gains from eradications, funding has so far been found ways. The first involves tangible measures: the rate of for progressively more ambitious projects (e.g., Broome recovery of resident species, recolonisation by extirpated 2009). One test of the political will is whether such projects species, reappearance of species reduced to undetectable continue in the face of any public disquiet. In a recent levels, and the effectiveness of reintroductions of species example, the spread of baits against rodents was able to unable to disperse to newly available sites. The measures continue despite mistaken claims that deaths of dogs and can become increasingly complex as responses affect marine life on the Hauraki Gulf beaches were an effect of communities, ecosystems, and landscapes. There are also eradication campaigns against the seven species of pests intangible measures: the effectiveness of communicating on nearby Rangitoto-Motutapu Islands (Morton 2009; results to the scientific community and the public, political Griffiths 2011). acceptance of the methods used and benefits gained, and the Perhaps we should now invite our international export of technologies to other locations. The two groups of colleagues to fill the gaps that we have left by providing outcomes are linked. Without the intangible measures such more comprehensive and scientifically robust accounts of as political support, the management of invasive species their efforts (Table 2). For example, although the benefits cannot proceed. Although examples of all such outcomes of pest eradication in New Zealand may seem impressive, are provided here, data for some of the most straightforward they are now being matched elsewhere such as in Mexico measures were difficult to obtain. Even with a long history (Aguirre-Muñoz et al. 2011). Where New Zealand may of eradications in New Zealand, records of biodiversity still contribute is from a temporal perspective, with its gains are often buried in grey literature and reports to numerous locations where invasive vertebrates have been
Table 2 Summary of information needed on effects of the more abundant invasive species of mammals on islands around New Zealand. Species Existing knowledge Information needed Sparse available information confounded by Rabbit other introduced species Effects on island plant communities Extensive information on effects on plants, Pacific rat some invertebrates, reptiles and some seabirds; Equivalent studies for southern islands all data from northern islands Post-eradication responses of plants on one Norway rat southern island (Breaksea) and some northern Effects on invertebrates, reptiles and birds over wide islands geographic range One invasion confounded by presence of weka Ship rat (Big South Cape/Taukihepa); sparse post Effects of plants and most groups of animals over eradication data (Matiu/Somes) wide geographic range House mouse Patchy data from one island (Mana) Effects on vegetation, invertebrates and small reptiles Cat Some studies on forest and sea birds Direct and indirect effects on island ecosystems Anecdotal accounts except for seabird recovery Pig on one island (Aorangi) Direct and indirect effects on island ecosystems Goat One comprehensive study (Great King) Indirect effects on island ecosystems
369 Island invasives: eradication and management
Carrel, S. 2007. Scottish islands’ surviving hedgehogs win a reprieve. successfully removed for long periods (Jones 2010b). A The Guardian, Wednesday 21 February http://www.guardian.co.uk/ focus on the natural, social and economic benefits of environment/2007/feb/21/conservationandendangeredspecies. restoration of these island ecosystems could then become travelnews. a particularly fruitful basis for international collaboration Clark, H. 2006. Huge restoration project for Rangitoto and Motutapu. News Release, Prime Minister’s Office. www.beehive.govt.nz/release: (e.g., Mulder et al. 2011). 11 June. http://www.beehive.govt.nz/node/26100 In sum, New Zealand has a strong history of development Clout, M.N. and Russell, J.C. 2006. The eradication of mammals from New Zealand islands. In: Koike, F.; Clout, M.N.; Kawamichi, M.; De of eradication technology, high levels of national Poorter, M. and Iwatsuki, K. (eds.). Assessment and control of biological political support and international influence, but patchy invasion risks, pp. 127-141. Shoukadoh Book Sellers, Kyoto, Japan and contributions to understanding the relationships between the World Conservation Union (IUCN) Gland, Switzerland. native species and agents of decline. This understanding Cole, N.C.; Jones, C.G. and Harris, S. 2005. The need for enemy-free space: the impacts of an invasive gecko on island endemics. Biological would be improved if outcome monitoring, together with Conservation 125: 467-474. the collection of appropriate baseline data, were at the Coleman, M.C.; Parkes, J.P. and Walker, K.J. 2001. Impact of feral outset incorporated into project design and costs. pigs and other predators on macro-invertebrates, D’Urville Island. Conservation Advisory Science Notes No. 345. Courchamp, F.; Langlais, M. and Sugihara, G. 1999. Cats protecting ACKNOWLEDGEMENTS birds: modelling the mesopredator release effect. Journal of Animal Ecology 68: 282-292. This account has benefited from discussions with Courchamp, F.; Langlais, M. and Sugihara, G. 2000. Rabbits killing birds: Chris Green while we were on Korapuki Island recording modelling the hyperpredation process. Journal of Animal Ecology 69: hitherto unknown positive responses by invertebrates and 154-164. reptiles more than 20 years after the removal of Pacific Cree, A. 1994. Low annual reproductive output in female reptiles from New Zealand. New Zealand Journal of Zoology 21: 351-372. rats and rabbits. I am grateful to an anonymous reviewer Dilks, P.J. and Wilson, P.R. 1979. Feral sheep and cattle and royal and to Alfonso Aguirre-Muñoz whose suggestions led to albatross on Campbell Island; population trends and habitat changes. considerable improvements to the text. New Zealand Journal of Zoology 8: 127-139. Dodd, C.K. and Seigal, R.A. 1991. 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