Prioritizing rat eradication on islands by cost and effectiveness to protect nesting seabirds

To prioritize conservation actions on Italian islands we used the case study of the eradication of the Black rat Rattus rattus to protect Cory’s shearwater Calonectris diomedea and Yelkouan shearwater Puffinus yelkouan. We evaluated for each island the effectiveness of rat eradication by means of two different indices, both based on the relative importance of the island’s nesting population of the two species at the national and regional scale, but differing in the parameters set at the divisor, i.e., respectively, the number of nesting pairs in rat-free islands and the number of islands occupied by shearwaters. We estimated analytically the monetary costs of rat eradication on each island. Islands at high risk of recolonization were excluded from further analyses, while costs and effectiveness of rat eradication were compared for the remaining islands. Rat eradication was most cost-effectively carried out on the island hosting the largest colony of P. yelkouan. Eradicating rats from all the islands in the ranking provided benefits to 63.9% of the Italian population of P. yelkouan, but only to 7.1% of that of C. diomedea. Comparing costs and effectiveness of all possible island combinations, ranging from a minimum budget of 50,000 € and a maximum of 1600,000 € (i.e. the cost for eradicating rats from all the listed islands), the maximum increase in effectiveness (marginal effectiveness) fell around a relatively small budget (200,000 €). For both species, when adopting the cost/effectiveness rankings, the number of pairs protected for 1000 € of investment was significantly higher than adopting rankings of effectiveness alone, demonstrating that conservation priorities are more efficiently identified by including monetary costs in the analysis.     

Invasive rats alter assemblage characteristics of land snails in the Ogasawara Islands

Although a large number of studies have shown that rats have a serious effect on island ecosystems, questions remain regarding how their invasion alters native species diversity and assemblage patterns on islands. In particular, a long history of invasions makes it difficult to understand the impact of rat invasion. In the present study, I investigate how an increase in predation by black rats (Rattus rattus) alters the assemblage characteristics of land snails on the Ogasawara Islands in the West Pacific. Radical declines in many land snail species have occurred since 2006 due to increasing predation by black rats. The frequency of shells with predation scars was greater on larger species and on those living on the trees and in a shallower position in the litter, suggesting greater predation pressure on these species. As a result of this selective predation, large species living on trees and in shallower parts of the litter declined. Accordingly, when black rats increased on the island, small species and species with a burrowing microhabitat became dominant in the land snail assemblage. The present findings can be used to aid in predicting the effects of rat eradication and in estimating the assemblage patterns and diversity prior to rat invasion.     

Depression of tree recruitment by the Pacific rat (Rattus exulans Peale) on New Zealand's northern offshore islands

Recruitment of tree populations on islands with and without Rattus exulans Peale, and recruitment changes on islands before and after rat eradication are compared using size-class analyses. Seedling establishment in rat-proof exclosures was compared with adjacent control areas. R. exulans depress recruitment of 11 species of coastal trees among 17 species studied: Coprosma macrocarpa, Coprosma repens, Dysoxylum spectabile, Melicytus novae-zelandiae, Nestegis apetala, Pisonia brunoniana, Pittosporum crassifolium, Pouteria costata, Pseudopanax lessonii, Rhopalostylis sapida and Streblus banksii; some to the point of local extinction. Where R. exulans has reached small islands supporting dense colonies of petrels (Procellariidae, Pelecanoididae), the numbers of some plant and animal species have been greatly reduced. Successional pathways in vegetation recovering on larger islands formerly occupied by Maori, have also been altered by this rat. Similar effects by R. exulans on the plant life of Pacific islands are probable considering the length of time R. exulans has been present.     

Mechanisms underlying the failure of an attempt to eradicate the invasive Asian musk shrew Suncus murinus from an island nature reserve

The Asian musk shrew Suncus murinus is an invasive predator that has had a considerable impact on biodiversity. An eradication was attempted from Ile aux Aigrettes, a 25-ha island reserve off the coast of Mauritius as part of a wider restoration program. A total of 759 shrews were removed using up to 1112 live-traps, in six trapping sweeps of the island. After the first four sweeps, in which 705 shrews were caught, complete eradication appeared to have been achieved, with only four shrews caught in the next three months. However, thereafter shrew captures increased and after six months shrews were again being captured in large numbers. We examine three hypotheses for the failure of the eradication program: (i) low efficiency of all traps (‘low efficiency’ hypothesis); (ii) low trap efficiency in certain locations (‘refuge’ hypothesis); and (iii) presence of trap-shy individuals (‘trap-shy’ hypothesis). We constructed an individual-based simulation of the first four trapping sweeps to predict expected patterns of shrew capture for different shrew densities and capture probabilities and in the presence of trapping ‘refugia’. The observed patterns of capture were not consistent with the expected patterns of capture under the ‘low efficiency’ and ‘refuge’ hypotheses, but were intuitively consistent with the ‘trap-shy’ hypothesis. We use the results from this study to help plan future eradication programs for S. murinus.     

A snake in paradise: Disturbance of plant reproduction following extirpation of bird flower-visitors on Guam

The introduction of an alien top predator, the brown treesnake (Boiga irregularis), has resulted in severe losses of native vertebrate populations in Guam. Among these are important pollinators and seed dispersers. This study is a first attempt to document cascading effects on vertebrate-pollinated native plant species in Guam. We investigated flower visitation, seed set and germination in two native plants, the mangrove tree Bruguiera gymnorrhiza and the forest tree Erythrina variegata var. orientalis. Both species are bird-pollinated. Studies were conducted on two Mariana islands, Guam (with high density of snakes) and Saipan (with nearly no snakes). Visitation rates by birds were high on Saipan, but zero on Guam. Insects and lizards visited flowers to a low extent on both islands. Only lizards were potential effective pollinators. Seed set of both species were significantly higher on Saipan compared to Guam, and for B. gymnorrhiza, seedling recruitment was significantly higher on Saipan. Hence, these bird-pollinated species appear highly dependent on bird visitors for reproduction. The eradication of flower-visiting birds by the invasive treesnake thus secondarily results in broken mutualistic interactions, which may, in turn, result in a lower recruitment of native plants. Thus, the treesnake affects not only potential prey species, but its effects cascade through the entire ecosystem on Guam. Conservation actions should be directed towards an improved recruitment (artificial pollination, planting) of the affected plant species.     

Husking stations provide evidence of seed predation by introduced rodents in Tongan rain forests

The islands of Tonga, in Western Polynesia, lacked rodents prior to human colonization. During the past 3000 years, people have introduced three species of rats (Rattus). Seed predation by rodents structures plant communities in many parts of the world. Therefore, there is the potential for rats to have a significant impact on the regeneration of plant communities in Tonga. We found 53 husking stations—sheltered sites used by rats to strip inedible protective structures (husks) from the seeds they eat—on eight islands in Tonga. The stations contained 13,555 empty husks from at least 18 plant species, but only 165 viable seeds and seedlings. These husking stations provide evidence of seed predation by rats. Determining the extent to which seed predation threatens plant regeneration will be important for conservation of native forest remnants.     

Insular pest control within a metapopulation context

Introduced pests threaten many species and their control is generally beneficial for conservation, particularly on islands where complete eradication is possible. Unfortunately on ‘nearshore’ islands neighbouring source populations exist and unaided reinvasion is likely. Pest control programmes at these sites thus require a metapopulation context to adequately manage movements between source and sink populations. We investigated the ecology of introduced ship rats (Rattus rattus) on a nearshore island, and gene flow with adjacent mainland populations, in order to understand the metapopulation dynamics and relative levels of pest control required within the landscape. We sampled the entire population by trapping (n = 30), achieving eradication, and found a low rat density (3.2 ha⁻¹) indicative of a sink population. Seed and other plant material constituted the major dietary component of rats. Despite its proximity to mainland source populations, the island population was genetically distinct with reduced allelic diversity caused by a recent reinvasion founder effect. Genetic analyses also detected recent migrants between the populations. In contrast, two mainland populations separated by a similar distance displayed complete genetic mixing. The small water gap therefore provides a sufficient barrier to lower the migration rate to the island and delay reinvasion, which nonetheless eventually happens. In order to maintain nearshore islands pest-free, conservation management will require a metapopulation approach simultaneously focusing on both island and source population pest control.     

Current status of and threats to some parrots of the Lesser Antilles

Four species of parrots of the genus Amazona persist in the Lesser Antillean Islands of St Lucia, St Vincent, and Dominica. The universal threat of habitat destruction, particularly severe on small islands, jeopardizes the continued existence of these species. If adequate habitat can be preserved and hunting restricted or eliminated, it is likely that these parrots will survive. The imperial parrot, although the least numerous of all, seems likely to endure as the mountain forests of Dominica are at least temporarily secure. On St Vincent the wild population of Amazona parrots is probably substantial and habitat destruction is proceeding relatively slowly; yet at least two hotels are keeping these and other parrots in captivity—in numbers exceeding a dozen in one case. Proprietors may become interested in selling these birds to foreign zoos or dealers, and the capture of birds for a commercial market will be a threat to the wild populations. The hazards of this practice and possible alternatives are discussed.     

Effects of nonnative fish and habitat characteristics on lentic herpetofauna in Yosemite National Park, USA

Nonnative fishes have been introduced into naturally fishless mountain lakes worldwide, often with negative consequences for native fauna. In this study, I used data collected during a census of native herpetofauna, nonnative trout, and habitat characteristics at all lentic water bodies in Yosemite National Park (n=2655) to quantify the effect of trout introductions and habitat on the distribution of four amphibian species and two reptile species. Impacts of trout on amphibians and reptiles were characterized using generalized additive models that included as predictor variables fish presence/absence, amphibian presence/absence (only in models for the two reptile species), elevation, water depth, littoral zone and shoreline substrate composition, relative survey date, and location. All species showed significant associations with habitat characteristics, and elevation and water depth appeared particularly important in influencing distributions. In addition, distributions of the mountain yellow-legged frog (Rana muscosa) and Pacific treefrog (Hyla regilla) were strongly negatively associated with the presence of nonnative trout while those of the Yosemite toad (Bufo canorus) and Sierra newt (Taricha torosa sierrae) were unrelated to trout presence. The distribution of the mountain garter snake (Thamnophis elegans elegans) was strongly negatively associated with the presence of nonnative trout and positively associated with the presence of native amphibians. Regression results for the Sierra garter snake (Thamnophis couchi couchi) were similar except that the direct effect of nonnative trout was considerably weaker. Together, these results indicate that trout introductions have resulted in considerable alteration of Yosemite's herpetofauna. Long-term studies will be necessary to determine whether removal of nonnative trout populations, where possible, would allow these impacts to be partially reversed.     

The ecology and conservation of reptiles on Round Island and Gunner's Quion,Mauritius

The reptile communities on two islands off the north coast of Mauritius are described. One of these islands, Round Island, is the last refuge for four species: Phelsuma guentheri, Leiolopisma telfairii, Bolyeria multocarinata and Casarea dussumieri, which together with four more species form a relict herpetofauna of great scientific and conservation interest. The reptile fauna of the neighbouring island of Gunner's Quoin has been reduced to four species, probably because of habitat destruction and the effects of ship rats Rattus rattus. Round Island has also suffered habitat destruction through introduced mammals but because of its remoteness, rats are absent. Conservation of the Round Island ecosystem is discussed; removal of the remaining alien herbivores, the rabbit Oryctolagus cuniculus and conservation education about Round Island are the main recommendations.     

Assessing critical habitat: Evaluating the relative contribution of habitats to population persistence

A principal challenge of species conservation is to identify the specific habitats that are essential for long-term persistence or recovery of imperiled species. However, many commonly used approaches to identify important habitats do not provide direct insight into the contribution of those habitats to population persistence. To assess how habitats contribute to overall population viability and characterize their relative importance, a spatially-explicit population viability model was used to integrate a species occurrence model with habitat quality and demographic information to simulate the population dynamics of the Ord’s kangaroo rat (Dipodomys ordii) in Alberta, Canada. Long-term productivity (births-deaths) in each patch was simulated and iterative patch removal experiments were conducted to generate estimates of the relative contribution of habitat types to overall population viability. Our results indicated that natural dune habitats are crucial for population viability, while disturbed/human-created habitats make a minor contribution to population persistence. The results also suggest that the habitats currently available to Ord’s kangaroo rats in Alberta are unlikely to support long-term persistence. Our approach was useful for identifying habitats that did not contribute to population viability. A large proportion of habitat (39%) represented sinks and their removal increased estimated population viability. The integration of population dynamics with habitat quality and occurrence data can be invaluable when assessing critical habitat, particularly in regions with variable habitat quality. Approaches that do not incorporate population dynamics may undermine conservation efforts by under- or over-estimating the value of habitats, erroneously protecting sink habitats, or failing to prioritize key source habitats.     

Understanding the decline and extinction of monarchs (Aves) in Polynesian Islands

Understanding the decline and extinction of species has become critical to conservation biology. The five monarch species of the genus Pomarea, endemic to the southeastern Pacific, are all listed as threatened. Introduced mammals and birds are believed to be responsible for their rarefaction. We analyzed the historical and current distribution of monarchs and introduced animals and found no relation between presence of Polynesian rats (Rattus exculans) and monarch distribution. There was a highly significant correlation between the arrival of the black rat (Rattus rattus) and the decline and extinction of monarch populations. The extinction of monarch populations after colonization by black rats tended to take longer on larger islands than on smaller ones. On islands without black rats, monarchs persisted even where forests have been reduced by more than 75%. After an island was colonized by black rats the number of monarch pairs with young decreased dramatically. Eggs in artificial nests placed in sites used by monarchs were only preyed upon by black rats. No eggs were preyed upon by Polynesian rats (Rattus exulans) or introduced birds. Monarch nests were mainly placed on horizontal branches inside the canopy and were more accessible than nests of Polynesian warbler (Acrocephalus caffer), a species still locally abundant. Warbler nests were placed higher up on vertical branches near the top of trees. These studies suggest that nest predation by black rats has been the main cause of monarch decline. However observations of direct aggression of adult monarchs by introduced red-vented bulbuls (Pycnonotus cafer), especially when monarchs raise their young, suggest that introduced birds could aggravate the decline of monarch populations already weakened by black rats. We discuss the practical implications of these findings for monarch conservation.     

Damage by rats Rattus norvegicus to breeding birds on Danish islands

Occurrence of rats Rattus norvegicus was recorded in Danish islands during the period 1970–1979. Rats apparently became common during the 1960s and 1970s. Their frequency fluctuated, indicating a possible 4-year cycle. Most rat colonisations lasted a single year. Geographical differences in their frequency may be caused by regional differences in distribution of larid colonies. Rat distribution did not follow the pattern suggested by the theory of island biogeography; presence of larids may be of major importance. When rat populations in hedges, ponds and streams are at maximum population density in autumn, pregnant females emigrate to, and so colonise, islands. Flooding and the severity of the winter may cause extinction. Food supply and predation may be of importance in limiting rat numbers. Breeding birds suffer, especially those placing their nests near rat runs; rat predation on adults may have serious consequences for a long-lived species, whereas that on eggs and young is insignificant. Some larid species renest in the immediate vicinity when predated by rats; others desert their dense colonies. Rat control is successful in small islands, whereas large island populations are only exterminated with difficulty. They should be controlled to prevent predation and to keep breeding grounds usable. This should be done in late winter when population density is low and food-supply insufficient.     

Ecology and conservation of two endangered subspecies of the leopard cat (Prionailurus bengalensis) on Japanese islands

The conservation of rare species on islands is a special challenge, especially when the islands are faced with increasing human impact. Two endangered wild felids in Japan provide a contrast that is useful for examining conservation strategies. The Iriomote cat (Prionailurus bengalensis iriomotensis) is endemic to Iriomotejima Island in Okinawa, and the Tsushima leopard cat (Prionailurus bengalensis euptilurus) occurs in the Tsushima islands in Japan, although a related subspecies found in the Korean Peninsula. Population size was approximately 100 individuals for each subspecies. While the population of Iriomote cat appears relatively stable, the population and abundance of the Tsushima leopard cat has decreased in recent decades. Because of their small population sizes and restricted habitats both are listed as endangered species/subspecies in Japan’s Red List, and the Iriomote cat is listed as an endangered subspecies in the IUCN Red List. Although both are similar-sized felids living on small islands, their ecological characteristics such as food habit, habitat selection, and density differ. These differences seem to be caused by the climate, the biological environment (the species composition of fauna in each island, and the presence of competitors), and the artificial background. The threats facing these two felids are also similar. Habitat destruction, traffic accidents, and negative influences by introduced species are the principal threats to be addressed for their conservation. At the same time, there are differences in the degree of impact by each threat, in each species’ conservation programs in practice, and in the stages of progress and operation of programs. These differences are partly because they have different ecological features and partly because the social background of each island, such as human population, main industry, and historical relationship between human and wildlife, are quite different. We will compare the ecology and state of these two wild felids and discuss the different conservational situations in relation to felid ecology and human society.     

Conservation action in the Galàpagos: feral pig (Sus scrofa) eradication from Santiago Island

Introduced mammals are major drivers of extinction and ecosystem change. As omnivores, feral pigs (Sus scrofa) are responsible for wholesale adverse effects on islands. Here, we report on the eradication of feral pigs from Santiago Island in the Galápagos Archipelago, Ecuador, which is the largest insular pig removal to date. Using a combination of ground hunting and poisoning, over 18,000 pigs were removed during this 30-year eradication campaign. A sustained effort, an effective poisoning campaign concurrent with the hunting program, access to animals by cutting more trails, and an intensive monitoring program all proved critical to the successful eradication. While low and fluctuating control efforts may help protect select native species, current eradication methods, limited conservation funds, and the potential negative non-target impacts of sustained control efforts all favor an intense eradication effort, rather than a sustained control program. The successful removal of pigs from Santiago Island sets a new precedent, nearly doubling the current size of a successful eradication, and is leading to more ambitious projects. However, now we must turn toward increasing eradication efficiency. Given limited conservation funds, we can no longer afford to spend decades removing introduced mammals from islands.     

Facilitating the evolution of resistance to avian malaria in Hawaiian birds

Research has shown that avian malaria plays an important role in limiting the distribution and population sizes of many Hawaiian birds, and that projected climate change is likely to eliminate most disease-free habitat in Hawai’i in the next century. I used a modeling approach, parameterized with demographic data from the literature and the field, to examine alternate management scenarios for the conservation of native Hawaiian birds. I examined the feasibility of using management in the form of rodent control to facilitate the evolution of resistance to malaria by increasing the survival and reproduction of native birds. Analysis of demographic data from seven native species, Akepa (Loxops coccineus), ‘Akohekohe (Palmeria dolei), Elepaio (Chasiempis sandwichensis), Hawai’i’amakihi (Hemignathus virens), Hawai’i creeper (Oreomystis mana), Omao (Myadestes obscurus), and Palila (Loxioides bailleui), suggest that differences in life history cause some species to be more susceptible to local extinctions from the transmission of malaria. Modeling results demonstrated that rodent control at middle, but not high, elevations can facilitate the evolution of resistance to malaria in several species of Hawaiian birds. Advocating a management approach that encourages evolutionary change in endangered species contrasts with the traditional conservation paradigm but it may be the best strategy to reduce the impacts of one of the multiple stressors that have devastated the native bird community of Hawai’i.     

The trade in fruit bats Pteropus spp. on Guam and other Pacific islands

Residents of Guam, Mariana Islands, have imported large numbers of fruit bats Pteropus spp. from other Pacific islands during the past 10 years as a delicacy. The trade in bats began in the 1960s or early 1970s after overhunting had greatly reduced the populations of Guam's native fruit bats. The quantity of bats shipped to the island peaked in 1979 and 1980, with an estimated total of more than 24 000 animals brought in each year. From 1981 to 1984, imports fell to an estimated mean of 14 475 bats per year. During the 1970s, Guam's importers purchased bats mainly from the Caroline Islands (Palau and Yap) and other islands in the Marianas (Saipan, Rota and Tinian). Changes in availability and laws during the 1980s have caused the fruit bat trade to expand geographically, making islands outside Micronesia (Western Samoa, Tonga, American Samoa and Papua New Guinea) important suppliers of bats. Because large-scale commercial exploitation may severely deplete an island's population of fruit bats, perhaps beyond the point of recovery, we recommend that island governments be conservative in the number of bats allowed for harvest until studies on abundance and life history have been completed.     

Evaluating alternative rodenticides for island conservation: roof rat eradication from the San Jorge Islands, Mexico

Introduced commensal rats (Rattus spp.) are a major contributor to the extinction and endangerment of island plants and animals. The use of the toxin brodifacoum to completely eradicate rats from islands is a powerful conservation tool. However, brodifacoum is toxic to animals other than rats and on some islands its use may not be feasible without prohibitively expensive mitigation. As part of a regional conservation program, we experimentally tested brodifacoum and two less toxic rodenticides, diphacinone and cholecalciferol, in eradicating Rattus rattus from three small islands in the northern Gulf of California, Mexico. All three rodenticides were successful in eradicating rats, suggesting that the less toxic diphacinone and cholecalciferol may be useful alternatives to brodifacoum for some island eradication programs. However, the choice of rodenticide must be balanced between efficacy and the risks to non-target species. Applied field research is needed on less toxic rodenticides, as well as improving palatability of baits. This may prove invaluable in preventing extinctions and in restoring larger and more diverse island ecosystems.     

Extinction debt or habitat change? - Ongoing losses of woodland birds in north-eastern New South Wales, Australia

The loss, fragmentation and degradation of native vegetation are major causes of loss of biodiversity globally. Extinction debt is the term used to describe the ongoing loss of species from fragmented landscapes long after the original loss and fragmentation of habitat. However, losses may also result from habitat changes that are unrelated to fragmentation, which reduce breeding success and recruitment. Many woodland birds have declined in fragmented landscapes in Australia, probably due to loss of small, isolated populations, though the ecological processes are poorly understood. We record the progressive regional loss of two ground-foraging, woodland birds, the Brown Treecreeper Climacteris picumnus and Hooded Robin Melanodryas cucullata, in northern New South Wales, over 30 years. This has happened despite most habitat loss occurring over 100 years ago, suggesting the payment of an extinction debt. Our observations suggest that several ecological processes, caused by habitat loss, fragmentation or degradation, and operating over different time scales, have led to both species’ declines. Female Brown Treecreepers disperse poorly among vegetation remnants, leaving only males in isolated populations, which then go extinct. In contrast, Hooded Robins suffer high nest predation in fragmented landscapes, producing too few recruits to replace adult mortality. Foraging by both species may also be affected by regrowth of ground vegetation and shrubs. We found little support for a major role played by drought, climate change or aggressive Noisy Miners Manorina melanocephala. We propose that both extinction debt in the classical sense and ongoing habitat change frequently contribute to species’ decline in modified landscapes. Management to arrest and reverse such declines needs to consider these multiple causes of decline. For instance, reconnecting isolated populations may be inadequate alone, and activities such as appropriate grazing, fires and the addition of woody debris may also be required.