Island prioritization for invasive rodent eradications with an emphasis on reinvasion risk

For birds and humans: challenges and benefits of rat eradication from an inhabited island (Ventotene, central Italy)

BACKGROUND

Rat eradication from islands is a very effective tool that can free entire ecosystems from the pressure of alien predators. In this study we present the case study of Ventotene (Ponziane Archipelago, central Italy), which to date is the island with by far the greatest number of human inhabitants ever freed from the negative implications of rats. Rat eradication was carried out in the framework of the Life PonDerat project, co-financed by the European Union. Besides considering the conservation benefits due to the removal of rats, we also considered the socio-economic and pathogenic impacts from introduced rats.

RESULTS

The overall economic cost of the rats was quantified to be at least €18 500 per year to the residents of the island. Several zoonotic pathogens were detected in the rat population prior to eradication. A reduction in the rodenticide distributed over time on the island was also estimated. Identifying the origin of the rat population allowed for the development of more targeted and effective biosecurity measures. The eradication effort was challenged by the presence of domestic animals and variability in support for baiting in urbanised areas.

CONCLUSIONS

The results of this study open up new perspectives on island restoration projects. We demonstrated the cost-effectiveness of the action, including ecosystem restoration, reduction of rat impacts in agricultural systems, and improving overall health and food safety. Our findings will have implications for similar interventions on other islands, potentially bringing significant benefits. © 2023 Society of Chemical Industry.

Economic costs of invasive rodents worldwide: the tip of the iceberg

Background

Rodents are among the most notorious invasive alien species worldwide. These invaders have substantially impacted native ecosystems, food production and storage, local infrastructures, human health and well-being. However, the lack of standardized and understandable estimation of their impacts is a serious barrier to raising societal awareness, and hampers effective management interventions at relevant scales.

Methods

Here, we assessed the economic costs of invasive alien rodents globally in order to help overcome these obstacles. For this purpose, we combined and analysed economic cost data from the InvaCost database-the most up-to-date and comprehensive synthesis of reported invasion costs-and specific complementary searches within and beyond the published literature.

Results

Our conservative analysis showed that reported costs of rodent invasions reached a conservative total of US$ 3.6 billion between 1930 and 2022 (annually US$ 87.5 million between 1980 and 2022), and were significantly increasing through time. The highest cost reported was for muskrat Ondatra zibethicus (US$ 377.5 million), then unspecified Rattus spp. (US$ 327.8 million), followed by Rattus norvegicus specifically (US$ 156.6 million) and Castor canadensis (US$ 150.4 million). Of the total costs, 87% were damage-related, principally impacting agriculture and predominantly reported in Asia (60%), Europe (19%) and North America (9%). Our study evidenced obvious cost underreporting with only 99 documents gathered globally, clear taxonomic gaps, reliability issues for cost assessment, and skewed breakdowns of costs among regions, sectors and contexts. As a consequence, these reported costs represent only a very small fraction of the expected true cost of rodent invasions (e.g., using a less conservative analytic approach would have led to a global amount more than 80-times higher than estimated here).

Conclusions

These findings strongly suggest that available information represents a substantial underestimation of the global costs incurred. We offer recommendations for improving estimates of costs to fill these knowledge gaps including: systematic distinction between native and invasive rodents' impacts; monetizing indirect impacts on human health; and greater integrative and concerted research effort between scientists and stakeholders. Finally, we discuss why and how this approach will stimulate and provide support for proactive and sustainable management strategies in the context of alien rodent invasions, for which biosecurity measures should be amplified globally.

Control of invasive rats on islands and priorities for future action

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.

Global patterns in threats to vertebrates by biological invasions

Biological invasions as drivers of biodiversity loss have recently been challenged. Fundamentally, we must know where species that are threatened by invasive alien species (IAS) live, and the degree to which they are threatened. We report the first study linking 1372 vertebrates threatened by more than 200 IAS from the completely revised Global Invasive Species Database. New maps of the vulnerability of threatened vertebrates to IAS permit assessments of whether IAS have a major influence on biodiversity, and if so, which taxonomic groups are threatened and where they are threatened. We found that centres of IAS-threatened vertebrates are concentrated in the Americas, India, Indonesia, Australia and New Zealand. The areas in which IAS-threatened species are located do not fully match the current hotspots of invasions, or the current hotspots of threatened species. The relative importance of biological invasions as drivers of biodiversity loss clearly varies across regions and taxa, and changes over time, with mammals from India, Indonesia, Australia and Europe are increasingly being threatened by IAS. The chytrid fungus primarily threatens amphibians, whereas invasive mammals primarily threaten other vertebrates. The differences in IAS threats between regions and taxa can help efficiently target IAS, which is essential for achieving the Strategic Plan 2020 of the Convention on Biological Diversity.

Prioritizing islands for the eradication of invasive vertebrates in the United Kingdom overseas territories

Invasive alien species are one of the primary threats to native biodiversity on islands worldwide. Consequently, eradicating invasive species from islands has become a mainstream conservation practice. Deciding which islands have the highest priority for eradication is of strategic importance to allocate limited resources to achieve maximum conservation benefit. Previous island prioritizations focused either on a narrow set of native species or on a small geographic area. We devised a prioritization approach that incorporates all threatened native terrestrial vertebrates and all invasive terrestrial vertebrates occurring on 11 U.K. overseas territories, which comprise over 2000 islands ranging from the sub-Antarctic to the tropics. Our approach includes eradication feasibility and distinguishes between the potential and realistic conservation value of an eradication, which reflects the benefit that would accrue following eradication of either all invasive species or only those species for which eradication techniques currently exist. We identified the top 25 priority islands for invasive species eradication that together would benefit extant populations of 155 native species including 45 globally threatened species. The 5 most valuable islands included the 2 World Heritage islands Gough (South Atlantic) and Henderson (South Pacific) that feature unique seabird colonies, and Anegada, Little Cayman, and Guana Island in the Caribbean that feature a unique reptile fauna. This prioritization can be rapidly repeated if new information or techniques become available, and the approach could be replicated elsewhere in the world.

The biogeography of globally threatened seabirds and island conservation opportunities

Seabirds are the most threatened group of marine animals; 29% of species are at some risk of extinction. Significant threats to seabirds occur on islands where they breed, but in many cases, effective island conservation can mitigate these threats. To guide island-based seabird conservation actions, we identified all islands with extant or extirpated populations of the 98 globally threatened seabird species, as recognized on the International Union for Conservation of Nature Red List, and quantified the presence of threatening invasive species, protected areas, and human populations. We matched these results with island attributes to highlight feasible island conservation opportunities. We identified 1362 threatened breeding seabird populations on 968 islands. On 803 (83%) of these islands, we identified threatening invasive species (20%), incomplete protected area coverage (23%), or both (40%). Most islands with threatened seabirds are amenable to island-wide conservation action because they are small (57% were <1 km(2) ), uninhabited (74%), and occur in high- or middle-income countries (96%). Collectively these attributes make islands with threatened seabirds a rare opportunity for effective conservation at scale.

Measuring connectivity of invasive stoat populations to inform conservation management

Context Effective design of conservation management programs for long-term population control requires an accurate definition of the spatial extent of populations, along with a proper understanding of the ways that landscape patchiness influences demography and dispersal within these populations. Aims In the present study, genetic techniques are used to describe the population genetic structure and connectivity of invasive stoats (Mustela erminea) across the Auckland region, New Zealand, so as to assist planning for mainland stoat control, and define potential future eradication units. Methods A sample of stoats from across the region (n = 120), was genotyped at 17 microsatellite loci, and a combination of clustering, genetic population assignment and various migration estimation methods were applied to these data. Key results Moderate population structure was observed (FST = 0.03–0.21), with five geographic populations defined by genetic clustering. Almost all individuals were correctly assigned to the location of origin, and recent migration rates among forest patches were found to be low. Conclusions It is possible to define the origin of stoats at this regional scale using genetic measures. From this, we show that the stoat incursion on Rangitoto Island that occurred post-eradication in 2010 probably came from East Auckland (P < 0.0001), whereas the 2014 stoat incursion on Motutapu Island probably originated from a population linked to the Waitakeres. Also, the Waiheke Island stoat population has minimal connection to all other populations and it is therefore a potential eradication unit. Implications The low migration rates among forest patches indicated that if thorough control is imposed on a discrete forest patch, reinvasion from other forest patches will be relatively low. Importantly, for stoat control in the region, the isolation of the Waiheke Island stoat population means that eradication here is likely to be feasible with low reinvasion pressure.