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Reviews the global literature on the effects of rats on island flora and fauna, then uses New Zealand as a case study because of its four-decade history of rat eradications and many detailed and innovative studies of how rats affect native species, which include use of exclosures, local manipulations of rat populations, video surveillance, and measurements of responses following eradications. Considers particularly the Norway rat (Rattus norvegicus), Pacific rat (Rattus exulans), and ship rat (Rattus rattus). Source: National Library of New Zealand Te Puna Matauranga o Aotearoa, licensed by the Department of Internal Affairs for re-use under the Creative Commons Attribution 3.0 New Zealand Licence.

New Zealand's policy to exterminate five introduced predators by 2050 is well‐meant but warrants critique and comparison against alternatives. The goal is unachievable with current or near‐future technologies and resources. Its effects on ecosystems and 26 other mammalian predators and herbivores will be complex. Some negative outcomes are likely. Predators are not always and everywhere the largest impact on biodiversity. Lower intensity predator suppression, habitat protection and restoration, and prey refugia will sometimes better support threatened biodiversity. The policy draws attention to where predators are easily killed, not where biodiversity values are greatest. Pest control operations are already contested and imposing the policy is likely to escalate those conflicts. While “high‐profile,” a focus on predator eradication obscures the fact that indigenous habitat cover and quality continues to decline. Thus, the policy is flawed and risks diverting effort and resources from higher environmental priorities and better alternatives. Biodiversity conservation policies should be guided by cost‐benefit analyses, prioritization schemes, and conservation planning in an adaptive management framework to deliver nuanced outcomes appropriate to scale‐ and site‐specific variation in biodiversity values and threats. The success of biodiversity sanctuary‐“spillover” landscapes, habitat restoration, and metapopulation management provide the foundation to build a better policy.

In this first comprehensive review of negative effects of introduced rodents on insular small mammals, the focal species Rattus rattus, R. norvegicus, R. exulans and Mus musculus are implicated in at least 11 extinctions. Furthermore, removal experiments, eradication campaigns and control programmes provide evidence for negative effects on extant populations. While data are currently insufficient for meaningful generalisation with regard to the most threatening rodents, the most threatened small mammals, and the true extent of the problem, it is interesting that R. rattus is implicated in the majority of impacts. This may be explained by its extensive distribution and ecological plasticity. I conclude with methodological recommendations to guide data collection for impact quantification and the study of impact mechanism. This information should facilitate the prioritisation and justification of eradication campaigns, control programmes and biosecurity measures while ensuring that much-needed attention is paid to the conservation of insular small mammals.

The Black Rat (Rattus rattus) spread out of Asia to become one of the world’s worst agricultural and urban pests, and a reservoir or vector of numerous zoonotic diseases, including the devastating plague. Despite the global scale and inestimable cost of their impacts on both human livelihoods and natural ecosystems, little is known of the global genetic diversity of Black Rats, the timing and directions of their historical dispersals, and the risks associated with contemporary movements. We surveyed mitochondrial DNA of Black Rats collected across their global range as a first step towards obtaining an historical genetic perspective on this socioeconomically important group of rodents. We found a strong phylogeographic pattern with well-differentiated lineages of Black Rats native to South Asia, the Himalayan region, southern Indochina, and northern Indochina to East Asia, and a diversification that probably commenced in the early Middle Pleistocene. We also identified two other currently recognised species of Rattus as potential derivatives of a paraphyletic R. rattus. Three of the four phylogenetic lineage units within R. rattus show clear genetic signatures of major population expansion in prehistoric times, and the distribution of particular haplogroups mirrors archaeologically and historically documented patterns of human dispersal and trade. Commensalism clearly arose multiple times in R. rattus and in widely separated geographic regions, and this may account for apparent regionalism in their associated pathogens. Our findings represent an important step towards deeper understanding the complex and influential relationship that has developed between Black Rats and humans, and invite a thorough re-examination of host-pathogen associations among Black Rats.

The nematode Angiostrongylus cantonensis is a zoonotic pathogen and the etiological agent of human angiostrongyliasis or rat lungworm disease. Hawai‘i, particularly east Hawai‘i Island, is the epicenter for angiostrongyliasis in the USA. Rats (Rattus spp.) are the definitive hosts while gastropods are intermediate hosts. The main objective of this study was to collect adult A. cantonensis from wild rats to isolate protein for the development of a blood-based diagnostic, in the process we evaluated the prevalence of infection in wild rats. A total of 545 wild rats were sampled from multiple sites in the South Hilo District of east Hawai‘i Island. Adult male and female A. cantonensis (3,148) were collected from the hearts and lungs of humanely euthanized Rattus rattus, and R. exulans. Photomicrography and documentation of multiple stages of this parasitic nematode in situ were recorded. A total of 45.5% (197/433) of rats inspected had lung lobe(s) (mostly upper right) which appeared granular indicating this lobe may serve as a filter for worm passage to the rest of the lung. Across Rattus spp., 72.7% (396/545) were infected with adult worms, but 93.9% (512/545) of the rats were positive for A. cantonensis infection based on presence of live adult worms, encysted adult worms, L3 larvae and/or by PCR analysis of brain tissue. In R. rattus we observed an inverse correlation with increased body mass and infection level of adult worms, and a direct correlation between body mass and encysted adult worms in the lung tissue, indicating that larger (older) rats may have developed a means of clearing infections or regulating the worm burden upon reinfection. The exceptionally high prevalence of A. cantonensis infection in Rattus spp. in east Hawai‘i Island is cause for concern and indicates the potential for human infection with this emerging zoonosis is greater than previously thought.

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.

The pristine island ecosystems of East Polynesia were among the last places on Earth settled by prehistoric people, and their colonization triggered a devastating transformation. Overhunting contributed to widespread faunal extinctions and the decline of marine megafauna, fires destroyed lowland forests, and the introduction of the omnivorous Pacific rat (Rattus exulans) led to a new wave of predation on the biota. East Polynesian islands preserve exceptionally detailed records of the initial prehistoric impacts on highly vulnerable ecosystems, but nearly all such studies are clouded by persistent controversies over the timing of initial human colonization, which has resulted in proposed settlement chronologies varying from ≈200 B.C. to 1000 A.D. or younger. Such differences underpin radically divergent interpretations of human dispersal from West Polynesia and of ecological and social transformation in East Polynesia and ultimately obfuscate the timing and patterns of this process. Using New Zealand as an example, we provide a reliable approach for accurately dating initial human colonization on Pacific islands by radiocarbon dating the arrival of the Pacific rat. Radiocarbon dates on distinctive rat-gnawed seeds and rat bones show that the Pacific rat was introduced to both main islands of New Zealand ≈1280 A.D., a millennium later than previously assumed. This matches with the earliest-dated archaeological sites, human-induced faunal extinctions, and deforestation, implying there was no long period of invisibility in either the archaeological or palaeoecological records.

Invasive rats ( Rattus spp.) are known to strongly impact large, flightless arthropods, but impacts on wider arthropod communities are less clear, with weak and inconsistent effects on most species being reported. However, logistical challenges often impede accurate measurement of arthropod population dynamics associated with rat invasions. In two diverse mesic forests on Oʻahu, Hawaiʻi, we conducted Before-After Control-Impact monitoring to assess arthropod population changes due to continuous rat trapping for 3–4 years. While communities in trapped and control sites were significantly different even before trapping was initiated, dissimilarity of the communities generally increased after trapping. Like prior studies, we found strong evidence of rat impacts on larger, flightless native Orthoptera (both trigonidiin crickets and Banza katydids), as well as on endemic predatory Eupithecia caterpillars. Additionally, many taxonomic groups exhibited significant abundance changes at trapped sites relative to adjacent control sites in both study areas. This was true for 36–41% of groups in arboreal communities and 21–31% in ground-dwelling communities; 67–90% of these significant changes indicated relative population increases at trapped sites. However, the identities of the responding groups frequently differed between study areas and between arboreal and ground communities within each area. Species richness for some groups, particularly native species, increased modestly in response to trapping. There was little evidence for major changes in arthropod trophic structure. Our results suggest a wide range of effects of invasive rats on ground-dwelling and arboreal arthropods, and highlight the complex nature of ecological responses that can follow rat suppression in diverse terrestrial food webs.

Invasive rats are one of the world's most successful animal groups that cause native species extinctions and ecosystem change, particularly on islands. On large islands, rat eradication is often impossible and population control, defined as the local limitation of rat abundance, is now routinely performed on many of the world's islands as an alternative management tool. However, a synthesis of the motivations, techniques, costs, and outcomes of such rat-control projects is lacking. We reviewed the literature, searched relevant websites, and conducted a survey via a questionnaire to synthesize the available information on rat-control projects in island natural areas worldwide to improve rat management and native species conservation. Data were collected from 136 projects conducted over the last 40 years; most were located in Australasia (46%) and the tropical Pacific (25%) in forest ecosystems (65%) and coastal strands (22%). Most of the projects targeted Rattus rattus and most (82%) were aimed at protecting birds and endangered ecosystems. Poisoning (35%) and a combination of trapping and poisoning (42%) were the most common methods. Poisoning allows for treatment of larger areas, and poison projects generally last longer than trapping projects. Second-generation anticoagulants (mainly brodifacoum and bromadiolone) were used most often. The median annual cost for rat-control projects was US$17,262 or US$227/ha. Median project duration was 4 years. For 58% of the projects, rat population reduction was reported, and 51% of projects showed evidence of positive effects on biodiversity. Our data were from few countries, revealing the need to expand rat-control distribution especially in some biodiversity hotspots. Improvement in control methods is needed as is regular monitoring to assess short- and long-term effectiveness of rat-control. Las ratas invasoras son uno de los grupos animales más exitosos a nivel mundial que ocasionan la extinción de especies nativas y cambios en los ecosistemas, particularmente en las islas. En las islas grandes, la erradicación de las ratas es generalmente imposible y el control de población, definido como la limitación local de la abundancia de ratas, hoy en día se practica rutinariamente en muchas de las islas del mundo como una herramienta alternativa de manejo. Sin embargo, se carece de una síntesis de motivaciones, técnicas, costos y resultados de dichos proyectos de control de ratas. Revisamos la literatura, buscamos sitios web relevantes, y realizamos una encuesta por medio de un cuestionario para sintetizar la información disponible sobre los proyectos de control de ratas en las áreas naturales isleñas en todo el mundo para así mejorar el manejo de ratas y la conservación de especies nativas. Se recolectaron datos de 136 proyectos que se realizaron en los últimos 40 años; la mayoría se ubicaron en Australasia (46 %) y el Pacífico tropical (25 %) en ecosistemas boscosos (65 %) y franjas costeras (22 %). La mayoría de los proyectos estaban enfocados en Rattus rattus, y la mayoría (82 %) estaban centrados en la protección de aves y ecosistemas en peligro de extinción. Los métodos más comunes fueron el envenenamiento (35 %) y una combinación de trampas y veneno (42 %). El envenenamiento permite tratar con áreas más grandes y generalmente dura más tiempo que el trampeo. Los anti-coagulantes de segunda generación (principalmente el brodifacoum y la briomadiolona) fueron los más usados. El costo medio anual de los proyectos de control de ratas fue de US$17,262 o de US$227/ha. La duración media de los proyectos fue de cuatro años. Para el 58 % de los proyectos, se reportó una reducción en la población de ratas, y el 51 % de los proyectos mostró evidencias de un efecto positivo sobre la biodiversidad. Nuestros datos provienen de pocos países, lo que revela la necesidad de expandir la distribución del control de ratas, especialmente en algunos puntos calientes de biodiversidad. Se necesita mejorar los métodos del control, así como un monitoreo regular para evaluar la efectividad del control de ratas a corto y largo plazo.

Rodents are among the most widespread and problematic invasive animals on islands worldwide contributing to declining endemic island biota through predation and disruption of mutualisms. Identifying what rodents eat is critically important to understanding their effects on ecosystems. We used DNA metabarcoding to identify the diets of three invasive rodents in Hawaiian forests: house mouse ( Mus musculus ), black rat ( Rattus rattus ), and Pacific rat ( Rattus exulans ). These rodents primarily eat invertebrates and plants, but previous diet studies have provided only a limited understanding of the diet breadth by relying on morphological identification methods. We opportunistically collected fecal samples from rodents trapped at seven forest sites across Oʻahu, Hawaiʻi for two years. Plant and invertebrate diet items were identified from DNA extracted from fecal samples using rbc L and COI primers, respectively. Intact seeds were identified using a dissecting microscope to quantify potential contributions to seed dispersal. All rodent species ate primarily plants and invertebrates of introduced species. However, some native taxa of conservation importance were identified. Neither the rodent species nor the sites drove patterns of diet composition, suggesting that diet variation may be determined by opportunistic foraging or intraspecific variation. Black rat fecal samples contained intact seeds more frequently than house mouse samples, but surprisingly, when samples contained seeds, black rats and house mice both defecated hundreds of introduced seeds, likely contributing to seed dispersal. Conservation efforts targeting invasive rodent control should specifically include house mice and should monitor introduced prey items to prevent predation release of unwanted introduced species.