David R. Towns

Intercepting the first rat ashore.

A single Norway rat released on to a rat-free island was not caught for more than four months, despite intensive efforts to trap it. The rat first explored the 9.5-hectare island and then swam 400 metres across open water to another rat-free island, evading capture for 18 weeks until an aggressive combination of detection and trapping methods were deployed simultaneously. The exceptional difficulty of this capture indicates that methods normally used to eradicate rats in dense populations are unlikely to be effective on small numbers, a finding that could have global implications for conservation on protected islands.

Invasive mammal eradication on islands results in substantial conservation gains.

Significance Global conservation actions to prevent or slow extinctions and protect biodiversity are costly. However, few conservation actions have been evaluated for their efficacy globally, hampering the prioritization of conservation actions. Islands are key areas for biodiversity conservation because they are home to more than 15% of terrestrial species and more than one-third of critically endangered species; nearly two-thirds of recent extinctions were of island species. This research quantifies the benefits to native island fauna of removing invasive mammals from islands. Our results highlight the importance of this conservation measure for protecting the worlds most threatened species. More than US

Testing island biosecurity systems for invasive rats.

21 billion is spent annually on biodiversity conservation. Despite their importance for preventing or slowing extinctions and preserving biodiversity, conservation interventions are rarely assessed systematically for their global impact. Islands house a disproportionately higher amount of biodiversity compared with mainlands, much of which is highly threatened with extinction. Indeed, island species make up nearly two-thirds of recent extinctions. Islands therefore are critical targets of conservation. We used an extensive literature and database review paired with expert interviews to estimate the global benefits of an increasingly used conservation action to stem biodiversity loss: eradication of invasive mammals on islands. We found 236 native terrestrial insular faunal species (596 populations) that benefitted through positive demographic and/or distributional responses from 251 eradications of invasive mammals on 181 islands. Seven native species (eight populations) were negatively impacted by invasive mammal eradication. Four threatened species had their International Union for the Conservation of Nature (IUCN) Red List extinction-risk categories reduced as a direct result of invasive mammal eradication, and no species moved to a higher extinction-risk category. We predict that 107 highly threatened birds, mammals, and reptiles on the IUCN Red List—6% of all these highly threatened species—likely have benefitted from invasive mammal eradications on islands. Because monitoring of eradication outcomes is sporadic and limited, the impacts of global eradications are likely greater than we report here. Our results highlight the importance of invasive mammal eradication on islands for protecting the worlds most imperiled fauna.

Purposes, outcomes and challenges of eradicating invasive mammals from New Zealand islands: an historical perspective

Rats continue to invade rat-free islands around the world, and it remains difficult to successfully intercept them before they establish populations. Successful biosecurity methods should intercept rats rapidly, before they can establish a population. Current island biosecurity practice employs techniques used for high-density rat eradication, assuming that they will be equally effective on low-density invaders. However, such approaches are often untested. Adult male Norway rats (Rattus norvegicus) were individually released onto forested rat-free islands in New Zealand to test methods of detecting and eliminating a single invader. Only half the rats released were caught within a two-week timeframe, although the mean time to interception was just under 14 days. Permanent island biosecurity surveillance systems performed better than contingency responses. Success rates were higher on islands where complete coverage could be obtained, although surveillance systems using multiple devices eventually detected most invading rats. For some rats a change of methods was necessary. Single invading rats left a rat-free island despite the presence of excessive natural food resources. With surveillance systems comprising an array of tested island biosecurity devices, and where necessary a contingency response using alternative methods, it should be possible to maintain islands as rat-free even when they have a high reinvasion rate.

Conservation of New Zealand Lizards: A Fauna Not Forgotten but Undervalued?

Abstract Context. Invasive mammals have been removed from at least 100 offshore islands around New Zealand, covering a total area of around 45 000 ha. Aims. To review the outcomes of eradications, the statutory and social environment in which the eradications were conducted, and the lessons provided for future work. Methods. Native species to benefit from the eradications were identified, as were the reasons for the eradications and the agencies responsible. Examples are provided using case studies. Key results. Three loosely linked work streams were revealed: research into efficient baits and baiting methods, threatened species-led projects nested within priorities for species recovery and supported by legislation, and community-led projects instigated by restoration societies. At least 180 populations of 14 species of invasive mammals were removed. Numerous species of native plants, invertebrates and more than 70 species of terrestrial vertebrates are recovering or are likely to recover as a result of the eradications. Partnerships have been formed with Māori and innovative projects developed with community groups. Conclusions. Eradications of invasive mammals are aggressive conservation actions that can have wide benefits for biodiversity but can also be controversial, technically demanding and expensive. Implications. Eradications are multi-scale problems. If they are to gain public acceptance, evidence is needed in support. This evidence can include understanding the detrimental effects of invasive species, the likely responses of native biodiversity, and the benefits ensuing from their recovery. However, the way this evidence is gained and communicated will also require deep understanding of nuances in regional political and cultural environments.

Burrowing seabird effects on invertebrate communities in soil and litter are dominated by ecosystem engineering rather than nutrient addition

Conservation actions are heavily influenced by value judgements and cultural perceptions. The huge lizard fauna of New Zealand played a prominent role in the worldview of the first people (Māori), but appeared to be poorly appreciated by European settlers. Early legislation to protect native fauna explicitly excluded lizards, which remained unprotected until the 1980s. After legislated protection was extended to lizards, the distribution and status of the rarest species have become increasingly understood. About 83 % of the fauna is currently regarded as threatened or at risk. Of the five species included in early recovery plans, two have improved status, two have declined and then improved as management improved and one has declined in listed status, although at least one new population has been established. There is increasing evidence that immediate threats faced by many species can be resolved, particularly those that inhabit islands. Longer-term threats may include genetically compromised populations, extremely protracted monitoring issues and climate change. Threats to progress with lizard conservation also involve socioecological problems such as social attitudes to lizards versus birds and competition for financial resources. Solutions may involve alliances between agencies, academic institutions and Māori organisations, with the latter now including statutory co-management agreements.

The Ecology of New Zealand’s Lizards

Vertebrate consumers can be important drivers of the structure and functioning of ecosystems, including the soil and litter invertebrate communities that drive many ecosystem processes. Burrowing seabirds, as prevalent vertebrate consumers, have the potential to impact consumptive effects via adding marine nutrients to soil (i.e. resource subsidies) and non-consumptive effects via soil disturbance associated with excavating burrows (i.e. ecosystem engineering). However, the exact mechanisms by which they influence invertebrates are poorly understood. We examined how soil chemistry and plant and invertebrate communities changed across a gradient of seabird burrow density on two islands in northern New Zealand. Increasing seabird burrow density was associated with increased soil nutrient availability and changes in plant community structure and the abundance of nearly all the measured invertebrate groups. Increasing seabird densities had a negative effect on invertebrates that were strongly influenced by soil-surface litter, a positive effect on fungal-feeding invertebrates, and variable effects on invertebrate groups with diverse feeding strategies. Gastropoda and Araneae species richness and composition were also influenced by seabird activity. Generalized multilevel path analysis revealed that invertebrate responses were strongly driven by seabird engineering effects, via increased soil disturbance, reduced soil-surface litter, and changes in trophic interactions. Almost no significant effects of resource subsidies were detected. Our results show that seabirds, and in particular their non-consumptive effects, were significant drivers of invertebrate food web structure. Reductions in seabird populations, due to predation and human activity, may therefore have far-reaching consequences for the functioning of these ecosystems.

Invasive rodents have multiple indirect effects on seabird island invertebrate food web structure

The current distribution of New Zealand lizards has been influenced by past geological events, habitat destruction and introduced predators. Perhaps as a reflection of long isolation, and at least 20 million years of climatic and geological change, the proportion of lizard species that are habitat generalists is relatively low, and high levels of sympatry exist in many environments. Variable strategies in resource partitioning may enable the high sympatry among lizard species, enabling division of resources spatially, temporally and/or trophically. However, overlap in resource use exists, such as honeydew sources, indicating potential competition among and within species. Compared to lizards elsewhere, the New Zealand lizard fauna has some unusual traits, with many species that reside in relatively cool environments, including some that appear to be alpine specialists, some skinks being active at night and Naultinus geckos being day active (geckos are globally dominated by nocturnal species). The lizards of New Zealand are highly opportunistic, responding to fluctuating abundance and availability of dietary items throughout the year. Both taxa feed primarily on arthropods, with plant-derived material, other reptiles and carrion also present in the diet; skinks also eat other invertebrates. Both the skinks and geckos of New Zealand disperse seeds and are probably pollinators for some plants. Some New Zealand lizards form aggregations and/or family groupings, and parental care may also be present. Some species use habitat differently in the presence of predators, which include native and introduced birds and invertebrates, native reptiles and introduced amphibians and reptiles.

Deciding when to lend a helping hand: a decision-making framework for seabird island restoration

Burrowing seabirds that nest on islands transfer nutrients from the sea, disturb the soil through burrowing, damage tree foliage when landing, and thereby modify the surface litter. However, seabirds are in decline worldwide, as are their community- and ecosystem-level impacts, primarily due to invasive predatory mammals. The direct and indirect effects of seabird decline on communities and ecosystems are inherently complex. Here we employed network analysis, as a means of simplifying ecological complexity, to better understand the effects seabird loss may have on island invertebrate communities. Using data on leaf litter communities, we constructed invertebrate food webs for each of 18 offshore oceanic islands in northeastern New Zealand, nine of which have high seabird densities and nine of were invaded by rats. Ten network topological metrics (including entropy, generality, and vulnerability) were compared between rat-invaded and uninvaded (seabird-dominant) islands. We found that, on rat-invaded islands, the invertebrate food webs were smaller and less complex than on their seabird-dominated counterparts, which may be due to the suppression of seabird-derived nutrients and consequent effects on trophic cascades. This decreased complexity of food webs due to the presence of rats is indicative of lower ecosystem resistance via lower trophic redundancy. Our results show that rat effects on island ecosystems are manifested throughout entire food webs, and demonstrate how network analysis may be useful to assess ecosystem recovery status as these invaded islands are restored.

Stakeholders and social networks identify potential roles of communities in sustainable management of invasive species

Following the removal of an introduced species, island restoration can follow two general approaches: passive, where no further intervention occurs and the island is assumed to recover naturally, and; active, where recovery of key taxa (e.g. seabirds) is enhanced by manipulating movement and demography. Steps for deciding between these techniques are: (1) outlining an explicit restoration goal; (2) building a conceptual model of the system; (3) identifying the most effective management approach; and (4) implementing and monitoring outcomes. After decades of island restoration initiatives, retrospective analysis of species’ responses to active and passive management approaches is now feasible. We summarize the advantages of incorporating these analyses of past restoration results as an initial step in the decision-making process. We illustrate this process using lessons learned from the restoration of seabird-driven island ecosystems after introduced vertebrate eradication in New Zealand. Throughout seven decades of successful vertebrate eradication projects, the goals of island restoration have shifted from passive to active enhancement of island communities, which are heavily dependent on burrow-nesting petrel population recovery. Using a comparative analysis of petrel response to past predator eradications we built a conceptual model of petrel recovery dynamics and defined key site and species characteristics for use in a stepwise decision tree to select between active or passive seabird population management. Active restoration techniques should be implemented when seabird populations are absent or declining; and on islands with no nearby source colony, small remnant colonies, highly altered habitat with shallow soil and slopes, and with competitive species pairs. As we continue to restore complex island communities, decision-making tools using a logical, step-wise framework informed by previous restoration successes and failures can aid in increasing understanding of ecosystem response.