Quiterie Duron

Aaron B Shiels

Eric Vidal

Control of invasive rats on islands and priorities for future action

Keywords: island conservation, poison, Rattus exulans, Rattus norvegicus, Rattus rattus, rodent pest control, traps Control de Ratas Invasoras en conservación en islas, control de plagas de roedores, trampas, veneno, Rattus exulans, Rattus

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.

Resumen: 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.

Introduction

Three species of rats have been introduced by humans to over 80% of the planet's island groups [START_REF] Atkinson | The spread of commensal species of Rattus to oceanic islands and their effects on avifaunas[END_REF], and they are now a major group of invasive species that cause native species' extinctions and ecosystem change [START_REF] Towns | Have the harmful effects of introduced rats on islands been exaggerated?[END_REF]. Rattus rattus (black rat or ship rat), Rattus norvegicus (Norway rat or brown rat), and Rattus exulans (Pacific rat, Polynesian rat, or kiore) are the most damaging rat species, particularly for insular ecosystems [START_REF] Towns | Have the harmful effects of introduced rats on islands been exaggerated?[END_REF]. In a recent review of global impacts of rodent pests, R. rattus was ranked the main rodent pest species, R. norvegicus was second, and R. exulans was fourth [START_REF] Capizzi | Rating the rat: global patterns and research priorities in impacts and management of rodent pests[END_REF]). These 3 omnivorous rats are opportunistic foragers that prey on and affect a broad range of plants [START_REF] Pender | Large-scale rodent control reduces pre-and post-dispersal seed predation of the endangered Hawaiian lobeliad, Cyanea superba subsp. superba (Campanulaceae)[END_REF][START_REF] Shiels | Biology and impacts of Pacific island invasive species. 11. Rattus rattus the Black Rat (Rodentia: Muridae)[END_REF], birds [START_REF] Vanderwerf | Rodent control decreases predation on artificial nests in O'ahu 'elepaio habitat[END_REF][START_REF] Jones | Severity of the effects of invasive rats on seabirds: a global review[END_REF], reptiles [START_REF] Towns | Changes in habitat use by lizards on a New Zealand island following removal of the introduced Pacific rat Rattus exulans[END_REF]), invertebrates (St Clair 2011;[START_REF] Ruscoe | Effects of spatially extensive control of invasive rats on abundance of native invertebrates in mainland New Zealand forests[END_REF], and overall ecosystem functioning [START_REF] Fukami | Above-and below-ground impacts of introduced predators in seabird-dominated island ecosystems[END_REF][START_REF] Towns | Predation of seabirds by invasive rats: multiple indirect consequences for invertebrate communities[END_REF].

The management strategy needed to slow biodiversity loss on an island due to rats depends on the time since introduction. Preventing new introductions or the prompt removal of newly introduced species is the most effective and economical strategy [START_REF] Simberloff | Impacts of biological invasions: what's what and the way forward[END_REF]. However, rats have already invaded and established on many continents and islands, sometimes thousands of years ago [START_REF] Ruffino | Invasive rats and seabirds after 2,000 years of an unwanted coexistence on Mediterranean islands[END_REF][START_REF] Capizzi | Rating the rat: global patterns and research priorities in impacts and management of rodent pests[END_REF]). Thus, conservation professionals are challenged with protecting or restoring native communities and ecosystems [START_REF] Howald | Invasive rodent eradication on islands[END_REF][START_REF] Keitt | Best practice guidelines for rat eradication on tropical islands[END_REF]. Eradication of invasive rat populations is one of the most effective conservation tools on small or medium-sized islands [START_REF] Howald | Invasive rodent eradication on islands[END_REF]). To date, 447 successful rat eradications on 416 islands have been reported (DIISE Partners 2016), and most have had positive effects on native biodiversity [START_REF] Veitch | Turning the tide of biological invasion: the potential for eradicating invasive species[END_REF][START_REF] Towns | Purposes, outcomes and challenges of eradicating invasive mammals from New Zealand islands: an historical perspective[END_REF]. Many factors affect whether an island-wide rat eradication is possible or can be successful, including the presence of human residents and their cooperation throughout the project, island size and latitude, and rat-removal methodology [START_REF] Howald | Invasive rodent eradication on islands[END_REF][START_REF] Holmes | Factors associated with rodent eradication failure[END_REF].

Where rat eradication from islands is not possible, rat population control is largely used as an alternative [START_REF] Bomford | Eradication or control for vertebrate pests?[END_REF][START_REF] Armstrong | Strategic rat control for restoring populations of native species in forest fragments[END_REF]. In this context, rat-control is best defined as the local limitation of rat abundance. Unlike eradication projects, for which best practices for successful operations are well studied [START_REF] Howald | Invasive rodent eradication on islands[END_REF][START_REF] Keitt | Best practice guidelines for rat eradication on tropical islands[END_REF], rat-control projects in island natural areas (defined as noncrop lands with minimal human disturbance) are less recognized, and the effects of control on rat populations and biodiversity remain poorly studied [START_REF] Clayton | Management of animal and plant pests in New Zealand-patterns of control and monitoring by regional agencies[END_REF][START_REF] Ruscoe | Effects of spatially extensive control of invasive rats on abundance of native invertebrates in mainland New Zealand forests[END_REF]. Despite routine performance of rat-control on many of the world's islands, the motivations, techniques, and levels of success for such projects have not been examined. We sought to partially fill this knowledge gap by analyzing rat-control projects in island natural areas worldwide. We examined the extent and duration of rat-control projects; the species most targeted; motivations for control; methods of control (e.g., poisoning, trapping, and spatial and temporal extent); nontarget species risks and mitigations; project success rates and measures to evaluate success; and the economic costs of control techniques. This analysis of completed and ongoing projects to control invasive rats should help scientists and stakeholders to prioritize research and management actions to benefit native species and ecosystems where rat eradication may not be an option.

Methods

We obtained rat-control information for our review from the literature, a questionnaire survey, and relevant websites. We compiled data from published peer-reviewed literature by searching Web of Science (1980Science ( -2015) ) and Google Scholar. We searched titles and abstracts for the following words and combination of words: island; rodent, rat; and control, management, removal, poisoning, trapping. We excluded articles with the words eradication and crops. Relevant papers published in conference proceedings such as the Vertebrate Pest Conference (1992 to 2014) and the International Conferences of Eradication of Island Invasives (2002Invasives ( , 2011) ) were also included. We distributed a questionnaire (English and French) to people involved in rat-control through the web lists Aliens-L, Pacific Invasive Initiative, and Islands-L and through our personal networks (Supporting Information). A description of the study and the disclosure that information from the questionnaire would be used in a review paper accompanied the questionnaire (Supporting Information). We also searched the internet to glean information and documentation from rat-control and management websites. We believe that obtaining rat-control information through these multiple methods improved the thoroughness of our review and helped minimize the inevitable biases of such surveys and searches.

We reviewed the documents and questionnaires with the aim to synthesize the information collected on ratcontrol projects. Each project corresponded to one management operation in a clearly defined area. Sometimes, more than one article or questionnaire covered the same project; consequently, we compiled all the data collected for a particular project in a single entry in the data set. A Mann-Kendall test was used to determine whether the number of control projects increased significantly across years. We used Wilcoxon tests to assess whether the size of the treated area and the duration of the projects differed according to the method of control. We drew Kaplan-Meier survival curves of projects, compared their estimators among control methods with Cox proportional hazards regression models, and used Breslow χ 2 to test for significance. Then, we used a linear model to explore relationships between the cost of control (per year and per hectare) and the size of the treated area and between cost and control method (trapping, poisoning, and combined trapping and poisoning). Continuous variables were ln+1 transformed to meet normality assumptions. All statistical analyses were implemented with R version 2.15.3.

Results

Up to October 2015, we collected data on 136 different projects from 63 articles, received 50 responses to the questionnaire, and collected information from several web pages on rat-control projects on islands (Supporting Information). Almost all results reported are based on the 136 projects; when this was not the case, n is specified.

Distribution of Rat-Control

Most rat-control projects (46%) occurred in Australasia, and particularly in New Zealand (40%) (Fig. 1a). Twenty-five percent were in the tropical Pacific, most on U.S. islands, principally Hawaii (14%). Remaining projects were in the Caribbean Sea (9%), Indian Ocean (7%), North Atlantic (5%), and Mediterranean Sea (5%). Twenty-one countries (65 islands) had ratcontrol projects. Three countries contained 72% of the projects: New Zealand's North (n = 41) and South Islands (n = 8); Hawaii's Oahu (n = 10), Kauai (n = 3), Big Island (n = 3), and Maui (n = 3); and New Caledonia's Grande Terre (n = 3). Forty-nine percent of projects were in tropical or subtropical zones and the other half were in temperate zones. Most projects were conducted in forest ecosystems (65%) or coastal strands (22%) (Fig. 1b).

The number of projects in island natural areas increased slightly over time (Mann-Kendall test: tau = 0.815, p = 0.008) (Fig. 2). The earliest project was initiated in 1951 on Nonsuch Island (5.7 ha), Bermuda, to protect Bermuda Petrel (Pterodroma cahow) around their nesting site [START_REF] Carlile | A review of four successful recovery programmes for threatened sub-tropical petrels[END_REF]. However, after technological advances in eradication products and methods, several rat-control operations on small islands became eradication projects [START_REF] Pascal | Improving the breeding success of a colonial seabird: a cost-benefit comparison of the eradication and control of its rat predator[END_REF].

Targeted Species

Rattus rattus, R. exulans, and R. norvegicus were the species targeted for control. Eighty-one percent of the projects targeted at least R. rattus, 20% at least R. exulans, and 21% at least R. norvegicus. Fifty-one percent of control projects targeted only R. rattus, few targeted R. exulans alone (<1%) or R. norvegicus alone (1.5%), but 30% aimed to control 2 or 3 sympatric species simultaneously. The high frequency of rat-control for R. rattus is not surprising because it is the main rodent pest species on islands [START_REF] Capizzi | Rating the rat: global patterns and research priorities in impacts and management of rodent pests[END_REF], especially forested areas of islands [START_REF] Shiels | Biology and impacts of Pacific island invasive species. 11. Rattus rattus the Black Rat (Rodentia: Muridae)[END_REF]). Of 106 projects for which there was information, 55% targeted other invasive species simultaneously. Thirty-five of these projects were in New Zealand, where possums (Trichosurus vulpecula), stoats (Mustela herminea), weasels (Mustela nivalis), ferrets (Mustela furo), and feral cats (Felis catus) were controlled concurrently with rats. Other species controlled concomitantly with rats included invasive plants (e.g., Pennisetum setaceum), mice (Mus musculus), hedgehogs (Erinaceus europaeus), mongooses (Herpestes auropunctatus), pigs (Sus scrofa), goats (Capra hircus), birds (Pycnonotus cafer and Acridotheres tristis), land crabs (e.g., Gecarcinus spp.), and hermit crabs (e.g., Coenobita clypeatus).

Motivations for Rat-Control

Motivations were often unclear or unspecified in the literature and thus were obtained from responses to the questionnaire (n = 50). For 36% of projects, the impact of the rat population on natural resources was not known or assessed before rat-control began in an area, and ratcontrol was conducted based on the negative impacts on natural resources in other similar areas. Thirty-eight percent of projects included a research phase before ratcontrol was implemented. For the remaining projects, 26% did not specify whether there was a research phase.

Most of the projects (82%) aimed to mitigate risks to species and endangered ecosystems, especially birds (71%) (Fig. 3a). Of these projects (n = 96), most sought to protect landbirds (56%) and seabirds (34%) (Fig. 3b). Nearly, 49% of the projects were established solely for bird conservation. For landbirds, some projects controlled rats only around trees that contained nesting birds, whereas others controlled rats in a larger area for the protection of native species other than landbirds. For seabirds, control focused mainly on breeding colonies. Fewer projects (10%) included reptile protection, and only one protected a specific reptile, the endangered giant tortoise (Chelonoidis nigra duncanensis) of Pinzon Island, Galápagos. Invertebrate conservation was the aim of 11% of the projects. However, only 2% were initiated solely for the purpose of invertebrate conservation (endangered snails in Hawaii, New Zealand, and New Caledonia) (Fig. 3c). Plant protection was included in 20% of projects, yet only 2% were developed specifically to save plant species. For instance, rat-control (26 ha) allowed conservation of an endangered Hawaiian lobelia tree (Cyanea superba) [START_REF] Pender | Large-scale rodent control reduces pre-and post-dispersal seed predation of the endangered Hawaiian lobeliad, Cyanea superba subsp. superba (Campanulaceae)[END_REF]). Additional projects reduced rat consumption of Apetahia raiateensis, Pouteria grayana, and Kadua raiateensis in Raiatea, French Polynesia (F. Jacq, personal communication). Among the 97 projects set up to protect species and ecosystems, 21% were initiated to protect ecosystems as a whole, and most of which were forests. Nearly, 13% of the control projects had species reintroduction plans (Fig. 3a), mostly for birds. For example, the North Island Robin (Petroica australis longipes) was successfully reintroduced where rats were managed in 5500 ha of Puketi Forest, New Zealand (D. John and I. Wilson, personal communication). Twenty-five percent of projects were scientific experiments (Fig. 3a), 10% of which were planned specifically to answer scientific questions.

Control Methods

Applying poison (35%) and combining trapping and poisoning (42%) were the most common methods used to control rats in island natural areas (Fig. 2). Few projects (15%) used only trapping. No other methods (e.g., fertility control) beyond trapping and poisoning were mentioned in the literature reviewed, on websites, or in survey responses. Among trapping projects (n = 77), 35% used snap traps (Victor, 11%; Ka Mate, 6%; and Doc 200, 4%); 25% used live traps, and 7% used recently developed automatic self-resetting Goodnature A24 rat traps. A total of 104 projects used poison. We classified poisons into 4 groups following [START_REF] Howald | Invasive rodent eradication on islands[END_REF]: secondgeneration anticoagulants (i.e., brodifacoum, bromadiolone, flocoumafen, and difenacoum), first-generation anticoagulants (i.e., diphacinone, pindone, coumatetralyl, warfarin, and chlorophacinone), acute toxins (1080 [sodium monofluoroacetate]), and cholecalciferol. Anticoagulants were classified as first-or second-generation according to their potency and when they were developed [START_REF] Eason | Assessment of risks of brodifacoum to non-target birds and mammals in New Zealand[END_REF]. Second-generation anticoagulants (mainly brodifacoum and bromadiolone) were the most used poisons (51%) followed by first-generation anticoagulants (30%) (mainly diphacinone and pindone) and acute toxins (15%; mainly 1080).

Survey results (50 respondents) showed that 76% of projects assessed whether a nontarget species was at risk. Among these 38 projects, nontarget species (e.g., feral pigs, bats, birds, snails, and crabs) were at risk in 76% of projects, and in 58% of projects the nontarget species at risk were native species. For trapping (n = 77), only a few studies (16%) specified whether trap boxes were used to reduce incidental capture of nontarget species; trap boxes were used in 6% of projects and not used in 9%. In 5% of the projects, traps were placed in trees to limit access by nontarget species. Bait stations were used to deliver rodenticide in 82% (n = 104) of projects. Hand sowing of poison was used in 6% of projects, and poison bait was applied directly into seabird burrows in just one project. Aerial broadcasting of bait was used in 11% of projects, all in New Zealand.

Size of Control Area and Duration of Project

The area treated for rat-control ranged from 0.03 to 80,000 ha. The median surface area treated with trapping was 30 ha (interquartile range Project duration median and mean were 4 years (IQR 9) and 7.7 years (SD 8.7), respectively, and projects varied widely from 1 day in Silver Peak, New Zealand, to 55 years in Lord Howe Island. The 1-day operation was a single application of 1080 applied aerially as part of a scientific experiment in 2011 [START_REF] Schadewinkel | Effects on South Island robins (Petroica australis) from pest control using aerially applied 1080 poison[END_REF]). The 55-year R. rattus suppression at Lord Howe Island was to protect an endemic palm species (Howea forsteriana) [START_REF] Auld | Disruption of recruitment in two endemic palms on Lord Howe Island by invasive rats[END_REF]. Project duration depended on the type of control method used (Breslow χ 2 = 7.0, p = 0.031, df = 2, n = 109) and the motivation for control (Breslow χ 2 = 40.2, p < 0.001, df = 4, n = 112) but did not depend on the size of control area (Breslow χ 2 = 3.23, p = 0.072, df = 1, n = 88). Although suppressing rats with poison was more frequent than trapping, the duration of control projects was significantly longer for poisoning than for trapping alone (Wilcoxon test: W = 592.5, p = 0.003) (Fig. 4). Moreover, combining trapping with poisoning did not result in longer lasting projects (W = 1105, p = 0.669). Project duration was longer for projects aimed specifically at mitigating risks for endangered ecosystems and birds (Wilcoxon test: W = 1693, p = 0.001).

Rat-Control Evaluation

Rat and native-biodiversity monitoring were incorporated in 62% and 58% of the projects, respectively. Of the 84 projects incorporating rat monitoring, at least 45% included rat trapping, 36% tracking tunnels, 12% capturemark-recapture, 8% chewing blocks, 6% bait consumption estimates, and 5% were based on other indirect methods, including monitoring of seeds for rat predation and camera observations of artificial nest predation. The types of native biodiversity monitoring (n = 70) that took place during rat-control were mainly for birds (68%), followed by invertebrates (10%), plants (8%), reptiles (1%), and bats (1%). Bird monitoring (n = 54) was mostly based on nesting or breeding success (48%) and on bird counts (20%). Plant monitoring included fruit and seed consumption experiments or monitoring seedlings.

Contrary to eradication, during a control operation, there is no obvious endpoint (i.e., where all rats are eliminated). Therefore, success is not easily defined and projects require constant control efforts. For 58% of the projects, rat population reduction was reported, and for 51%, positive effects on biodiversity were observed. However, 29% of projects did not provide information about whether or not rat populations were reduced, and 46% did not provide information on the effects of ratcontrol on biodiversity. From the questionnaire (n = 50), 6 operations were considered unsuccessful by survey respondents due to "important collateral damages" (e.g., creating trails for rat-control efforts), "inadequate planning," a lack of cost-effectiveness, or discovery that "rats were not the principal threat." Forty-nine percent (n = 106) of projects mentioned had simultaneous control of other invasive mammals, but there was no report of confounding influences from controlling other invasive mammals on the likelihood of successful rat control.

Rat-Control Costs

Twenty-six (52%) survey respondents provided information on economic costs. The cost of control per year varied widely according to the scale of the project ($500-$226,989): with a mean of $39,766 (SD 56,494) and a median of $17,262 (IQR 32,090). (All monetary units are in U.S. dollars.) The annual cost per treated hectare also fluctuated: mean $843 (SD 2086) and median $227 (IQR 503) per hectare. The larger the control area, the more cost-efficient the control operation (p < 0.0001, R² = 0.559) (Fig. 5). There were no differences in costs per unit area relative to the method of control (p = 0.329; poison vs. trapping vs. trapping+poisoning). When asked about the part of the rat-control operation that was the most expensive, most people (78%) provided answers. Of the 39 responses to questions about cost, the most expensive part of the operation was personnel (62%), followed by materials (33%), and travel (5%). The most expensive phase of the project (n = 37) was generally fieldwork (84%) and more rarely evaluation and data analyses (8%), preparation for fieldwork (5%), or conception of the project (3%). Of the 37 projects with information on the people involved, 35% identified the participation of volunteers in fieldwork.

Discussion

Biodiversity and Hotspot Protection

Most island rat-control projects are concentrated in a few countries such as New Zealand, United States, or overseas territories associated with other developed countries. Nonetheless, many more islands, often in tropical areas and in developing countries, belong to biodiversity hotspots where invasive rats reside [START_REF] Atkinson | The spread of commensal species of Rattus to oceanic islands and their effects on avifaunas[END_REF]. Considerable efforts are underway to understand rat effects and manage them in tropical ecosystems [START_REF] Harper | Invasive rats on tropical islands: their population biology and impacts on native species[END_REF][START_REF] Keitt | Best practice guidelines for rat eradication on tropical islands[END_REF]. Due to limited economic capacity and often social instability, less-developed countries cannot always effectively protect ecosystems [START_REF] Fisher | Poverty and biodiversity: measuring the overlap of human poverty and the biodiversity hotspots[END_REF]. The necessity to transfer knowledge, expertise, and funding to developing countries will be an essential priority for the future. As highlighted by [START_REF] Simberloff | Impacts of biological invasions: what's what and the way forward[END_REF], an effort must be made to provide invasive-species research aimed at improving risk assessment and prioritizing actions. During the last decade, several people worked on prioritizing islands for eradication of a of vertebrates (e.g., [START_REF] Dawson | Prioritizing islands for the eradication of invasive vertebrates in the United Kingdom overseas territories[END_REF] or rodents specifically [START_REF] Harris | Island prioritization for invasive rodent eradications with an emphasis on reinvasion risk[END_REF]. Because current technology does not allow for many large islands to have rats eradicated, rat-control operations will continue to be common and may be prioritized over rat eradications. Benefits to species, ecosystems, and humans, and longterm sustainability of a control project, may all factor into rat-control prioritization decisions.

Ecosystem Protection

Bird conservation is currently the greatest motivation for rat-control in island natural areas. Rat-control operations are often implemented without preliminary studies at the candidate site but are often justified by an a priori understanding of rat impacts on the species of interest or similar species. Rats predominately affect native species and ecosystems negatively, yet the magnitude of such site-specific impacts can differ widely [START_REF] Harper | Invasive rats on tropical islands: their population biology and impacts on native species[END_REF]. Thus, the level of impact must be considered before the large and often long-term commitment to control rats can be made. Benefits to entire ecosystems, rather than one to a few species, may also result from ratcontrol. Such ecosystem-level effects of rat-control are often more difficult to realize and study, but they are encouraged for future conservation on islands [START_REF] Simberloff | Impacts of biological invasions: what's what and the way forward[END_REF][START_REF] Ruffino | Assessment of invasive rodent impacts on island avifauna: methods, limitations and the way forward[END_REF]. Furthermore, considering the effects of invasive rats on the entire ecosystem prior to implementing a rat-control program helps prevent unwanted consequences such as mesopredator-or competitor-release effects [START_REF] Caut | Avoiding surprise effects on Surprise Island: alien species control in a multitrophic level perspective[END_REF]). Because of these complex interactions, the control of other invasive species (e.g., mice, mustelids, possums, and cats) concurrently with rat-control generally enhances restoration results [START_REF] Saunders | Ecological restoration at mainland islands in New Zealand[END_REF][START_REF] Zavaleta | Viewing invasive species removal in a whole-ecosystem context[END_REF]). There may be problems with rat-control if native species develop dependence on invasive species. For instance, invasive rats can be crucial pollinators [START_REF] Pattemore | Invasive rats and recent colonist birds partially compensate for the loss of endemic New Zealand pollinators[END_REF]) and seed dispersers (Shiels & Drake 2011) when native plants have lost their native pollinator or disperser.

Cost-Efficiency and Sustainability

Rat-control projects are generally not very long lasting (< 8 years), even when they show positive effects on native biodiversity. Dedication to long-term protection of species or ecosystems is necessary because rat populations reinvade the control areas. A key challenge is to determine how rat-control operations can be sustained. Project duration and cost do not depend only on the size of control area, but also on the methods used, political environment, and the reasoning for launching rat-control.

Rat-control is generally viewed as expensive due to the required field personnel, yet additional, substantial costs include materials (e.g., bait and traps) and travel to the control area. In most cases, rat-control is implemented over extended periods even though the islands are small enough to make rat eradication feasible [START_REF] Pascal | Improving the breeding success of a colonial seabird: a cost-benefit comparison of the eradication and control of its rat predator[END_REF][START_REF] Russell | Predator-free New Zealand: conservation country[END_REF]. As technology improves, the size of the largest rat-eradicated area also increases, and the possibility of rat eradication must now be considered on islands previously thought to be too large or complex for eradication. If eradication is not possible, an extensive plan of rat-control should be developed prior to beginning rat suppression. At minimum, a plan should include the annual cost, methods to evaluate rat suppression and benefits to native species, and a clear goal and timeline based on the monitoring results (e.g., 80% rat reduction Conservation Biology Volume 31, No. 4, 2017 and 50% increase in target native species in 2 years) that enables project evaluation and justifies continuation or modification (e.g., [START_REF] Norbury | Density-impact functions for terrestrial vertebrate pests and indigenous biota: guidelines for conservation managers[END_REF].

Recurrent management costs of rat-control need to be reduced. Community involvement in such conservation projects may be one solution to the cost problem [START_REF] Lund | Towards a more balanced view on the potentials of locally-based monitoring[END_REF]. Working with volunteers may allow the treatment of larger areas, reduce costs, and raise environmental awareness among citizens. [START_REF] Bryce | Turning back the tide of American mink invasion at an unprecedented scale through community participation and adaptive management[END_REF] report reduced recolonization of 10,570 km 2 by invasive American mink (Neovison vison) in Scotland after they involved 186 volunteers in control of this species. In New Zealand, where conservation volunteering is frequent [START_REF] Butler | Paradise saved: the remarkable story of New Zealand's wildlife sanctuaries and how they are stemming the tide of extinction[END_REF], a large control project called Ark in the Park was initiated in 2002 in the North Island. The success of this project relied on strong volunteer participation. Today, over 120 people help manage rats by trapping and poisoning 2100 ha of forest. This project showed positive effects on biodiversity and allowed for the reintroduction of native birds [START_REF] Bellingham | Ark in the park, restoration plan[END_REF].

Improved Control Methods

Improving the cost-efficiency of control methods and reducing nontarget species risks are 2 key priorities for future rat-control projects. Our findings revealed that controlling rats across large areas was more cost-efficient by unit area than controlling rats across smaller areas. Although poisoning is often preferred over trapping as a more cost-effective way to remove rats from large areas [START_REF] Russell | Review of rat invasion biology: implications for island biosecurity[END_REF], we found that there was no difference in total project costs between trapping and poisoning.

Second-generation anticoagulants (mainly brodifacoum) are currently the most used poisons because of their high potency. Although only a few grams of bait must be ingested before causing death, there are multiple days from first feeding before the rat dies, and this is considered inhumane by some [START_REF] Hoare | The impact of brodifacoum on non-target wildlife: gaps in knowledge[END_REF]. There are also risks to nontarget native species when rat poisons are used, although the risks vary according to the frequency of application and the way bait is applied [START_REF] Hoare | The impact of brodifacoum on non-target wildlife: gaps in knowledge[END_REF][START_REF] Pitt | Non-target species mortality and the measurement of brodifacoum rodenticide residues after a rat (Rattus rattus) eradication on Palmyra Atoll, tropical Pacific[END_REF]. In a recent rat eradication on Palmyra Atoll, the use of brodifacoum bait resulted in numerous deaths of birds and fishes, and brodifacoum residue was present in soil, freshwater, marine water, and in 84% of animal carcasses analyzed. However, this operation used approximately 4-8 times the amount of bait used in other studies because of high bait removal by native land crabs, and researchers did not assess overall nontarget mortality as a proportion of resident populations [START_REF] Pitt | Non-target species mortality and the measurement of brodifacoum rodenticide residues after a rat (Rattus rattus) eradication on Palmyra Atoll, tropical Pacific[END_REF]. Similar to islandwide rat eradications, the environmental benefits of rat suppression through the use of toxic bait should outweigh the negative effects of the bait on nontarget species and soil and water contamination.

Broadcast application of poison bait by hand or aircraft for rat-control is generally uncommon, partly because the bait is readily accessible to both target and nontarget species. However, in New Zealand, aerially broadcasting poison bait by helicopter is practiced to control rodents and other pests in large or relatively inaccessible areas. Even if rat bait stations are used to reduce nontarget access to poison bait, the discrimination between rats and smaller animals, and native and non-native rats, may be difficult or even impossible [START_REF] Courchamp | Mammal invaders on islands: impact, control and control impact[END_REF]. Testing for nontarget bait take in novel environments prior to setting up a control project is essential, especially for hand-or aerial-bait dispersal. Unlike rat-eradication projects that typically use poison bait applied 1-3 times over a few weeks or months [START_REF] Howald | Eradication of black rats from Anacapa Island: biological and social considerations[END_REF], ratcontrol generally requires a regular poison bait supply for years or indefinitely. Therefore, the threats to native species from bait consumption may be much longer lasting than with rat eradications.

Rat trapping, which is a toxin-free rat-control method, may be preferred to poisoning by local human populations. When animals such as pigs are hunted, local people may be concerned about the risks of secondary poisoning (toxin accumulation in animal tissues). Ratcontrol through large-scale trapping effectively reduces rat populations in New Zealand [START_REF] King | Invasive European rats in Britain and New Zealand: same species, different outcomes[END_REF]) and reduces rats and protects native species in Hawaii [START_REF] Pender | Large-scale rodent control reduces pre-and post-dispersal seed predation of the endangered Hawaiian lobeliad, Cyanea superba subsp. superba (Campanulaceae)[END_REF]. Unfortunately, covering large areas with traps is challenging; the largest area we found for controlling rats with trapping was about 210 ha. Automatic self-resetting rat traps and toxin applicators have been proposed and tested (e.g., Goodnature A24 rat traps and Spitfire Connovation). Goodnature traps have the capacity to kill 24 times, and Spitfires traps can kill up to 50-100 times, before the trap needs to be serviced. However, these devices are expensive and their effectiveness has been mixed [START_REF] Campbell | The next generation of rodent eradications: innovative technologies and tools to improve species specificity and increase their feasibility on islands[END_REF][START_REF] Carter | Controlling sympatric pest mammal populations in New Zealand with selfresetting, toxicant-free traps: a promising tool for invasive species management[END_REF]. Nonetheless, if proven effective and reliable, they could be a good investment for long-term rat-control and reduce labor costs [START_REF] Blackie | Innovative developments for long-term mammalian pest control[END_REF][START_REF] Campbell | The next generation of rodent eradications: innovative technologies and tools to improve species specificity and increase their feasibility on islands[END_REF].

Research on alternative rat-control methods is recommended to uncover more cost-effective and less dangerous ways to remove rats humanely while limiting risks to the environment and nontarget species [START_REF] Meerburg | The ethics of rodent control[END_REF]. [START_REF] Littin | Welfare aspects of vertebrate pest control and culling: ranking control techniques for humaneness[END_REF] propose a decisionmaking process to select the control technique based on the technique's efficiency and humaneness. [START_REF] Campbell | The next generation of rodent eradications: innovative technologies and tools to improve species specificity and increase their feasibility on islands[END_REF] provide a list of innovative technologies and tools that may improve future eradication methods, and some of them may also be used for continuous ratcontrol (e.g., norbormide poison, genetic engineering, and fertility control). With long-term rat-control, rats may develop behavioral or genetic resistance to control tools [START_REF] Brunton | Behavioural resistance towards poison baits in brown rats, Rattus norvegicus[END_REF][START_REF] Pelz | The genetic basis of resistance to anticoagulants in rodents[END_REF] requires more research and experimentation to improve the efficacy of long-term control.

Project Evaluation and Adaptive Management

Around 29-45% of the projects we reviewed did not report monitoring of the rat population or biodiversity recovery following rat-control; a lack of appropriate monitoring makes the success of the project very difficult to assess by any means beyond anecdotal observations. Implementing a monitoring plan to assess the effectiveness of rat-control is essential and represents one of the biggest areas of need for current and future rat-control projects. Rat-population reduction can be monitored easily with rat trapping or rat activity measures such as tracking tunnels and chewing blocks [START_REF] Blackwell | Rodent density indices from tracking tunnels, snap-traps and Fenn traps: Do they tell the same story?[END_REF]. The confirmation of a positive effect of rat-control on biodiversity recovery can be based on increases in native species abundance or reproductive success. An additional challenge in determining the effectiveness of some rat-control projects was that other invasive mammals were concurrently being controlled, so positive effects of control were not always identifiable. To best evaluate a rat-control operation, we recommend adoption of a before-after-control-impact experimental design, where monitoring occurs on the treatment and nontreatment areas before and after the beginning of a control operation [START_REF] Reddiex | Control of pest mammals for biodiversity protection in Australia. II. Reliability of knowledge[END_REF]. this approach may not be possible in all cases (e.g., resource constraints and inability to ethically abstain from rat suppression in a conservation site for sole comparison to a rat-control site), it is the most definitive way to assess rat-control efficacy. At minimum, a before-and-after monitoring approach is necessary, but typically this is only valid for assessing initial (short term) efficacy of the control implementation. These types of evaluations are important to justify the initiation and continuation of rat-control projects, including justification to financers [START_REF] Ferraro | Money for nothing? A call for empirical evaluation of biodiversity conservation investments[END_REF] and local community members.

Ideally, rat-control monitoring results should be evaluated continuously and associated methods should be adapted to improve the cost-benefit ratio of the ratcontrol project. Evaluation will help determine how much suppression effort is needed to reduce the rat population to a point that benefits native biodiversity. This adaptive-management approach is highly favored for managing biological systems in the presence of uncertainty [START_REF] Westgate | Adaptive management of biological systems: a review[END_REF]. Adaptive management is a trial-and-error process that requires learning constantly about control effects on rat populations and on native biodiversity. Adaptive management may include the type of control method used (e.g., type of poison and trap), frequency of control sessions, and grid arrangement and size. Considering the complexity and the costs of such ratcontrol, monitoring the operation to decide to continue, adjust methods, or cease the operation seems essential.

Conclusions

Based on the 136 operations to control invasive rats on islands we reviewed, we identified actions for improving the effectiveness of rat management and native species conservation. We highlighted some priorities such as enlarging the worldwide distribution of invasive rat-control, targeting control efforts to protect entire ecosystems, increasing project cost-efficiency and sustainability, improving control methods, and evaluating the control operations to enhance adaptive management. Improving accessibility to data from rat-control projects worldwide will help transfer knowledge, expertise, and funding opportunities among projects and may help determine the causes of project successes and failures. We encourage researchers and stakeholders to monitor the effects of their rat-control operations and to make their methods and results accessible so that rat-control management can be improved.

Figure 1 .

 1 Figure 1. (a) Worldwide distribution of rat-control projects in island natural areas and (b) ecosystems in which rat-control occurs.

Figure 2 .

 2 Figure 2. Number of rat-control projects by method in island natural areas worldwide.

Figure 3 .

 3 Figure 3. Motivations for rat-control projects: (a) number of projects relative to element protected or other reasons for control, (b) bird groups targeted for protection, and (c) invertebrate groups targeted for protection.

  [IQR] = 61 ha, mean [SD] = 49.0 ha [60.4], minimum = 0.5 ha, maximum = 210 ha, n = 17) and was significantly smaller (Wilcoxon test: W = 481.5, p < 0.001) than areas treated by poisoning (median = 716 ha, IQR = 1934 ha, mean = 4269.4 ha [14171.5], minimum = 1 ha, maximum = 80,000 ha, n = 32). The largest area was in the Hauhungaroa Range (North Island, New Zealand) in August 2000, where 1080 was applied aerially to 80,000 ha (P. Sweetapple, personal communication). When trapping was the only method used, the largest area treated was about 210 ha of Halfmoon Bay on Stewart Island, New Zealand (K. Bunce, personal communication). Conservation Biology Volume 31, No. 4, 2017

Figure 4 .

 4 Figure 4. Duration of rat-control projects by method (short vertical lines are individual projects). The total number of projects (all methods combined) is 109.

Figure 5 .

 5 Figure 5. Annual estimated cost per hectare for rat-control projects relative to the total size of control area.

  . Combining or alternating control methods may lessen the possibility of rat resistance to control mechanisms, but this field

	Conservation Biology
	Volume 31, No. 4, 2017

Acknowledgments

We are grateful to the many people who answered the questionnaire or provided information on rat control projects. Funding for Q.D. PhD thesis came from the "Direction du Développement Economique et de l'Environnement" of the Northern Province of New Caledonia. We also thank anonymous reviewers for their helpful feedback in reviewing this manuscript.

Supporting Information

The emails and questionnaire sent to people involved in rat-control projects (Appendix S1) and the list of the 136 rat-control projects collected for this review (Appendix S2) are available online. The authors are solely responsible for the content and functionality of these materials. Queries (other than absence of the material) should be directed to the corresponding author.


Sylvain Deville

Céline Viazzi

Christian Guizard

An Ice-Structuring Mechanism for Zirconium Acetate

The control of ice nucleation and growth is critical in many natural and engineering situations. Yet, very few compounds are able to interact directly with the surface of ice crystals. Ice-structuring proteins, found in certain fishes, plants and insects, bind to the surface of ice, thereby controlling their growth. We recently revealed the icestructuring properties of zirconium acetate which are similar to those of ice-structuring proteins. Being a salt, and therefore different from the proteins having ice-structuring properties, its ice-structuring mechanism remains unelucidated. Here we investigate this ice-structuring mechanism through the role of the concentration of zirconium acetate and of the ice crystal growth velocity. We then explore other compounds presenting similar functional groups (acetate, hydroxyl, or carboxylic groups). Based on these results, we propose that zirconium acetate adopts a hydroxy-bridged polymer structure which can bind to the surface of the ice crystals through hydrogen bonding, thereby slowing down ice crystal growth.

Introduction

Very few compounds are able to interact directly with ice, due to the very subtle structural differences between water molecules and the ice surface. Yet, such interaction is often critical in many natural or engineering phenomena, to prevent, restrict or control ice growth. Compounds known in biology as antifreeze proteins are currently found in certain living organisms (fishes, plants, insects) where their interaction with the ice crystals prevents fatal freezing-induced damage 1 . This ensures survival at low temperature and provides an evolutionary advantage. Understanding the exact mechanism by which they prevent or control the growth of ice crystals is still a major challenge in biology.

Classical antifreeze proteins exhibit three characteristic behaviors, (1) thermal hysteresis (from a non-colligative reduction in the freezing point, thus the name antifreeze), (2) ice recrystallization inhibition, and (3) ice structuring (change in the morphology of ice crystals). This has led to alternative names such as thermal hysteresis proteins and ice structuring proteins (ISPs) 2,3 , to identify groups of proteins that may not exhibit all three of these properties. The current explanation is that ISPs bind to a specific family of crystals planes thereby restricting their growth and resulting in facetted ice crystal growth. An anchored clathrate mechanism has been recently proposed 4 for the binding.

The control of ice crystal growth is also of interest for problems as diverse as texture control in frozen food and deserts 5 , cryopreservation of cells and tissues, cryosurgery adjuvants, and materials engineering [6][7][8] . Inspired by ISPs, ice-structuring compounds have been developed as a cheaper and more convenient alternative. Among the structural requirements are the presence of hydrophobic and hydrophilic regions in the structure, long range order, and the absence of charged functional groups 9 . Such compounds are currently commercially available and used, although they are still less efficient than ISPs. All ISPs discovered to date are long organic macromolecules, often with an amphiphilic structures.

We recently revealed how a radically different compound, zirconium acetate (ZRA), exhibits ice-structuring properties similar to that of ISPs 10 . ZRA is a metallic salt, and salts were not known to exhibit such properties. The ice-structuring properties of ZRA suggest that it somehow interacts directly with the ice crystals at the atomic level. The presence of additional compounds (binder, dispersant) or particles is not required to observe the ice-structuring properties.

In materials science, the growth morphologies of ice crystals can be used to control the pore morphologies in ice-templated materials, where the pores are a replica of the ice crystals [START_REF] Deville | Influence of Particle Size on Ice Nucleation and Growth During the Ice-Templating Process[END_REF] . Generally, the growth behavior is dependent on a number of process parameters, such as the suspension viscosity and pH [START_REF] Lu | Optimization of a Nanoparticle Suspension for Freeze Casting[END_REF] , the particle concentration, the zeta potential [START_REF] Zou | Effect of Suspension State on the Pore Structure of Freeze-Cast Ceramics[END_REF][START_REF] Deville | Ice-templated porous alumina structures[END_REF] and the growth velocity of the ice crystals [START_REF] Waschkies | Control of Lamellae Spacing During Freeze Casting of Ceramics Using Double-Side Cooling as a Novel Processing Route[END_REF][START_REF] Nishihara | Synthesis of silica-based porous monoliths with straight nanochannels using an ice-rod nanoarray as a template[END_REF][START_REF] Deville | Freezing as a path to build complex composites[END_REF][START_REF] O'brien | Influence of freezing rate on pore structure in freeze-dried collagen-GAG scaffolds[END_REF][START_REF] Gutiérrez | Poly(vinyl alcohol) Scaffolds with Tailored Morphologies for Drug Delivery and Controlled Release[END_REF] . Additives can be incorporated to modify these characteristics, although their influence is mostly unpredictable and dependent, to some extent, on the suspension and ceramic powder properties [START_REF] Munch | Architectural Control of Freeze-Cast Ceramics Through Additives and Templating[END_REF] .

We take advantage of the ice templating effect to investigate the ice-structuring mechanism by which ZRA controls ice crystal growth. We explore the influence of ZRA concentration and ice crystal growth velocity. We then assess whether other compounds presenting similar functional groups -acetate, hydroxyl and carboxylic groups-exhibit the same properties. Based on these results, we propose an ice-structuring mechanism similar to that of ISPs. Figure 1. Influence of ZRA concentration on the pores morphology. The black regions (pores) are replicas of the ice structure. Scale bar 50 μm. The respective concentrations (in g.l -1 of Zr) are a: 0, b: 4.5 g.l -1 , c: 9 g.l -1 , d: 18 g.l -1 , e : 45 g.l -1 , f : 180 g.l -1 , g : 271 g.l -1 , h : 453 g.l -1 . Cooling rate 5°C.min -1 .

Experimental methods

The following compounds were bought from Sigma-Aldrich (St. Louis, MO, USA): zirconium acetate, zirconium acetate hydroxide, Pluronic F-127, pentaerythritol, D-sorbitol, xylitol, dextrin, beta-lactose, D-quinic acid, myo-inositol, acetic acid and propanoic acid. Yttrium and barium acetate were bought from Alfa-Aesar (Ward Hill, MA, USA). The pH of the suspension was adjusted using addition of two different acids (HCl or HNO3) and two different bases (NaOH or KOH), to rule out the possible influence of the ionic species. The suspension preparation, freezing and freeze-drying methods, briefly summarized here, are identical to that of reference 10.

The suspensions are prepared by mixing the ceramic powder or the polymer in aqueous suspension, the zirconium acetate and the binder in distilled water. Experiments are carried out with yttria-stabilized zirconia (TZ8Y, Tosoh, Japan) as a powder. The zirconium acetate tested, is commercially available (Sigma-Aldrich, St. Louis, MO, USA). The amount of zirconium acetate in the suspension corresponds to a concentration of zirconium provided by the zirconium acetate (g/L). Zirconium acetate is first mixed with distilled water. The solid loading of the suspension is 55wt%. The binder is then added, preferably after having been dissolved in water, and the ceramic powder is finally added. The suspension is then ball-milled for 10 hours.

The suspension is poured into a PTFE mould and cooled from the bottom, using a liquid-nitrogen cooled copper rod. The cooling rates are adjusted through a thermocouple and a ring heater placed around the copper rod. Details of the experimental setup can be found in previous papers, such as reference 14,17. This setup provides a fine control of the ice growth velocity, along the temperature gradient direction. Once freezing is completed, the samples are freeze-dried for at least 48 hrs in a commercial freezedryer to ensure a complete removal of the ice crystals. A binder removal step is performed with the following cycle: temperature rise at a rate of 600°C/h up to 500°C, steady stage of 1 hour at 500°C, temperature decrease to room temperature. Ceramic samples are densified by a high temperature sintering treatment, with a temperature rise at a rate of 600°C/h up to 1350°C, steady stage of 3 hours at 1350°C, temperature decrease at a rate of 600°C/h to room temperature. Samples are cut, perpendicular to the direction of the temperature gradient, using a low-speed diamond saw, and cleaned in an ultrasound bath to remove the debris. SEM observations were performed using either a TM1000 from Hitachi or a Nova NanoSEM 230 from FEI.

Results and discussion

In the following sections, the reader should keep in mind that the pores are a replica of the ice crystals. The templating of ice crystals is thus used to investigate the morphology and characteristics of the ice crystals.

This process has already proved to be extremely consistent and robust. Provided that the formulation of the suspension is the same, using the same cooling setup and cooling rate yield structures with the same morphology. No variations of morphology are found across samples obtained in the same conditions. This consistency is obtained because of the equilibrium conditions obtained during the unidirectional solidification [START_REF] Deville | In Situ X-Ray Radiography and Tomography Observations of the Solidification of Aqueous Alumina Particles Suspensions. Part II: Steady State[END_REF] . After the initial nucleation and growth stages (transient regime) [START_REF] Deville | In Situ X-Ray Radiography and Tomography Observations of the Solidification of Aqueous Alumina Particle Suspensions-Part I: Initial Instants[END_REF] , a stable equilibrium regime is obtained. This approach is very different to the ones usually used in the investigation of ice-structuring properties of ISPs, where nucleation and growth of ice is carefully controlled and isolated, single crystals are observed.

Influence of zirconium acetate concentration

Pore morphology is extremely dependent on the concentration of ZRA in solution. Without ZRA, the pores exhibit a convoluted structure, highly dendritic and with a significant surface roughness (fig. 1a). Such dendritic structure is similar to that already reported in the literature. Above a concentration threshold (18 g.l -1 )(fig. 1d), faceted pores are obtained, as reported previously10. Faceting occurs over a large range of concentration above the threshold and increasing the concentration has little effect over the porous structure (fig. 1d, 1e and1f). Faceting progressively degrades when the ZRA concentration is very high, greater than 200 g.l -1 (fig. 1g and1h).

A low concentration of ZRA (9 g.l -1 ), below the concentration threshold required for faceting (18 g.l -1 ), impacts the pores morphology and provides additional clues on the ice crystal structuring mechanism which are discussed in the next section. The pore structure is dramatically different from that obtained without ZRA. Just below the threshold (fig. 1c), the pores exhibit a dual structure: some faceting can be observed on the outer part of the pores, while the inside reveals a spiral-like structure.

Influence of ice crystal growth velocity

The ice growth velocity, resulting from the imposed cooling rate and temperature gradient, has a dramatic effect on the pores, and yields different morphologies. Faceting is found for cooling rate lower than 15°C min -1 . For fast cooling -15°C min -1 -, honeycomb-like structures are obtained with homogeneous, hexagonal pores of a few micrometers (fig. 2b). Decreasing the cooling rate to 5°C min -1 triggers elongated growth of the faceted pores along one direction (fig. 2c). In cross-sections perpendicular to the solidification direction, the thickness of the pores remains essentially constant, while their length increases. Further decrease of the cooling rate to 2°C min -1 or below yields faceted microstructures with heterogeneous pore size (fig. 2d-2f). Conversely, increasing the cooling rate to above 20°C min -1 yields microstructures with pores exhibiting faceted surfaces on their outer part and spiral features in their inner part (fig. 2a), a behavior similar to that observed when the ZRA concentration is near the faceting threshold. Adjacent and opposed growth spirals, possibly due to pairs of dislocation, can be observed in figure 2a.

Growth mode

Local observations (fig. 3) reveal details about this spiral structure. Such structures are typical of a spiral growth mode which occurs around a screw dislocation or in the presence of instabilities of the crystal edge relative to the crystal surface which also lead to hopper morphologies [START_REF] Minkoff | Instability criteria for growth of a hopper crystal related to spiral eutectic morphology[END_REF] . In addition, we observe that the outside of the crystal -hence, the inside of the ceramic spiral-is facetted, while small dendritic features can be observed along the spirals (fig. 3b), a situation that can be found when the driving force is the greatest along the edges or corners of the crystal. This is possibly another indication that the ZRA is modifying the growth driving force, a behavior similar to that of ice-structuring proteins.

The progressive degradation of the faceting at high ZRA concentration (fig. 1g and1h) can be understood as a modification of the solvent. ZRA is provided in acetic acid solution. For high ZRA concentration, the solvent is not pure water anymore, but a mix of water, acetic acid and ZRA. It is thus possible that new phases are formed in this system, though we have no knowledge of the corresponding phase diagram. The crystals formed could thus be crystals of a phase comprising water and acetic acid, and eventually ZRA in some form. The crystal structure is likely not hexagonal anymore, explaining the loss of the facetted structure. The role of acetic acid and compounds with similar functional groups is further explored later in this paper. The evolution of the pore morphology with the ZRA concentration or growth velocity is reminiscent of the usual relationships between crystal growth rate and driving force reflecting an evolution of the interface morphology from rough to smooth (fig. 4) and therefore a change of the growth regime or of the rate-controlling process. Dendritic (figure 1), polyhedral (figures 1d and 1e) and hopper (figure 3) structures are observed.

The concentration of ZRA and the cooling rate thus have a similar effect on the crystal growth regime. Although the relationship between growth rate and driving force (undercooling) is well-known in crystal growth, the mechanism controlling the undercooling in presence of ZRA is still unknown. The evolution from a rough to a smooth interface implies a change of rate-controlling process. Rough interfaces occur when the diffusion of molecules or atoms to the growth surface is the rate-controlling process. For smooth interfaces, the adsorption of atoms or molecules at the surface is the ratecontrolling process. Adsorption of zirconium acetate could thus explain the current observation. The functional groups of zirconium acetate should thus play a critical role in the ice-structuring mechanism.

Assessing the role of functional groups

Zirconium acetate is a compound radically different from the known ice structuring compounds. We investigated the role of the functional groups to assess their influence on the ice structuring mechanism. The ZRA used in this study is provided in an acetic acid solution; three kinds of functional groups are thus present. We tested compounds respectively acetate, hydroxyl and carboxyl groups, these groups being present, respectively, in ZRA and acetic acid (fig. 5). All compounds were tested in a large concentration range, 1-20 wt.% and in the pH range 2-8.

The closest compound to ZRA is probably zirconium propionate, but is insoluble in water and thus could not be tested. Zirconium hydroxyl acetate has also a very similar structure to zirconium acetate, and it does not induce any faceting (fig. 6c). Neither do acetates such as yttrium acetate or barium acetate (fig. 6a and6b). We investigated the role of carboxyl groups with acetic and propanoic acid. None of these compounds induce any faceting (fig. 6d and6e). Some compounds with hydroxyl groups, such as polyols, are known antifreeze compounds. Examples include D-sorbitol, xylitol or pentaerythritol. In addition, we also tested dextrin, be-ta-lactose, D-quinic acid, myo-inositol, and a block copolymer (Pluronic F-127). Faceted structures have not been obtained with any of these compounds (fig. 6f to 6m). All polyols seem to have a similar effect (fig. 6g to 6m), yielding dendritic structures, similar to that obtained with sucrose in previous experiments [START_REF] Munch | Architectural Control of Freeze-Cast Ceramics Through Additives and Templating[END_REF][START_REF] Launey | Designing highly toughened hybrid composites through nature-inspired hierarchical complexity[END_REF] . The block copolymer is apparently the only one exhibiting an ice-structuring effect, the corresponding structures having much smaller pores (fig. 6f), corresponding to much small ice crystals. It might therefore effectively decrease the ice crystal growth rate, though it does not induce any faceting of the ice crystals.

Influence of poly(vinyl alcohol)

PVA is routinely added in ceramic processing as a binder and was used here in the processing of the samples. The antifreeze properties of non-peptide polymers, including poly(vinyl alcohol) (PVA), poly(ethylene glycol) (PEG), poly(acryclic acid), poly(Lhydroxyproline) or poly(L-histidine), have been investigated [START_REF] Inada | Growth control of ice crystals by poly(vinyl alcohol) and antifreeze protein in ice slurries[END_REF][START_REF] Budke | Ice recrystallization inhibition and molecular recognition of ice faces by poly(vinyl alcohol)[END_REF][START_REF] Wowk | Vitrification enhancement by synthetic ice blocking agents[END_REF][START_REF] Inada | Substitutes for Antifreeze Proteins : Potential Applications in Ice Slurry Systems[END_REF][START_REF] Knight | Nonequilibrium antifreeze peptides and the recrystallization of ice[END_REF] . They have been found to exhibit some or all of the three antifreeze mechanisms of antifreeze proteins: recrystallization inhibition of ice, thermal hysteresis, and ice crystal morphology control. The role of PVA on the observed ice structuring properties must be carefully considered. PVA is of particular interest, as it is the only non-peptide polymer that has been reported to cause thermal hysteresis. In addition, dynamic ice structuring properties have also been observed, with the growth of faceted crystals [START_REF] Budke | Ice recrystallization inhibition and molecular recognition of ice faces by poly(vinyl alcohol)[END_REF] . Being known for its antifreeze properties, PVA has naturally been tested in ice templating. Although it has been found to limit the growth of ice crystals, faceted ice-templated structures have not been observed [START_REF] Gutierrez | A Biocompatible Bottom-Up Route for the Preparation of Hierarchical Biohybrid Materials[END_REF][START_REF] Zuo | Effect of polyvinyl alcohol additive on the pore structure and morphology of the freeze-cast hydroxyapatite ceramics[END_REF][START_REF] Pekor | The Effect of Polyvinyl Alcohol on the Microstructure and Permeability of Freeze-Cast Alumina[END_REF] . Its ice-structuring properties appear therefore to be less prominent than that of ZRA. Salts, including acetates, have been reported to enhance the effects of these macromolecules in binding to ice [START_REF] Li | Enhancement of insect antifreeze protein activity by solutes of low molecular mass[END_REF] , but do not have ice structuring activity on their own. Although ZRA could therefore possibly increase the antifreeze activity of the binder (PVA or PEG) used in ice-templating, ZRA was found to exhibit ice-structuring properties on its own. No differences were observed when the binder was incorporated into the suspension 10 . Thus, the ice-structuring properties of ZRA cannot be explained by the presence of PVA. The antifreeze properties of PVA can contribute to that of ZRA but are not predominant.

Structure and ice-structuring mechanism of ice structuring compounds

ZRA seems to exhibit strong functional similarities to ice-structuring compounds. To better understand the ice-structuring mechanism on ZRA, we briefly review the current understanding of ISPs. ISPs control the growth of the ice crystals at the atomic level. Some commercially available ice-structuring compounds can prevent or slow down the growth of the ice crystals, used particularly in food engineering [START_REF] Feeney | Antifreeze proteins: Current status and possible food uses[END_REF][START_REF] Petzold | Ice Morphology: Fundamentals and Technological Applications in Foods[END_REF] , but their action does not seem to be as specific as ISPs. Although a wide variety of ISPs have been found to date, they all share some similarities such as common moieties and functional groups. These long macromolecules comprise amphiphilic groups. In some cases, alternation of hydrophilic and hydrophobic groups is observed. The conformation of ISPs often matches the structural periodicity of particular crystallographic planes of the ice crystals [START_REF] Leinala | Crystal structure of beta-helical antifreeze protein points to a general ice binding model[END_REF] . A close contact between the surface of the ice crystals and the ISPs is thus possible, enhancing Van der Waals interactions. Various mechanisms have been proposed based on these observations. In particular, it has been proposed that the matching of the protein to the ice surface prevents or slows down the incorporation of water molecules at the surface of the ice crystals 1,[START_REF] Pertaya | Fluorescence microscopy evidence for quasi-permanent attachment of antifreeze proteins to ice surfaces[END_REF][START_REF] Pertaya | Direct visualization of spruce budworm antifreeze protein interacting with ice crystals: basal plane affinity confers hyperactivity[END_REF] in a manner similar to particles in front of a solidifying interface [START_REF] Knight | Adsorption of Biomolecules to Ice and Their Effects upon Ice Growth. 2. A Discussion of the Basic Mechanism of "Antifreeze" Phenomena[END_REF] . Depending on their structure, ISPs can interact with a variety of ice's crystallographic planes. In some cases, growth is favoured along the caxis, while other compounds yield the opposite behaviour. In all cases, the ISPs form an ice-interacting network effectively slowing down crystal growth.

Ice-structuring mechanism of zirconium acetate

Comparisons between ISPs and ZRA can be drawn to understand the ice-structuring properties of ZRA. First, they share some similarities. The moieties of ZRA are similar to those of some ISPs due to the presence of amphiphilic groups. The structure and conformation of the compound is nevertheless critical for the ice-structuring mechanism, as the experimental results obtained with the alternative compounds sharing similar functional groups did not reveal ice structuring properties. It is thus clear, although not surprising, that moieties alone without a periodicity of their location along the compound cannot induce ice structuring. Both ZRA and ISPs require a minimum concentration below which no ice structuring properties are observed. A kinetic effect is also observed where ice-structuring properties disappear when the ice crystal growth velocity exceeds a threshold. Faceted growth is observed in both cases, likely implying control at the atomic level. With unidirectional solidification, the c-axis aligns along the temperature gradient 10 similar to the behaviour of some ISPs where growth along the c-axis is favoured.

ZRA is used in very different applications. In materials processing, besides its influence on the viscosity of the suspension [START_REF] Sakurada | In Situ Direct-Casting of Alumina Slurry Stabilized with Zirconium Acetate Using Enzyme Reaction[END_REF] or its use as a precursor of zirconium [START_REF] Gandıá | Non-aggressive pillaring of clays with zirconium acetate. Comparison with alumina pillared clays[END_REF] in sol-gel or catalysis applications, it is also used as a depletion induced stabilizer of colloidal suspension [START_REF] Sakurada | Depletion stabilization of ceramic suspensions with high solid loading in the presence of zirconium oxy-salts[END_REF] , including carbon nanotubes suspensions [START_REF] Furumi | Aqueous Dispersions of Carbon Nanotubes Stabilized by Zirconium Acetate[END_REF] . Since ZRA is controlling the growth of ice crystals without any other additives or particles 10 , the depletion properties cannot account for its ice-structuring properties. It is also used for waterproofing [START_REF] Mater | Method of waterproofing textiles with zirconyl compounds[END_REF] and flame retardant treatment [START_REF] Forouharshad | Preparation of flame retardant wool using zirconium acetate optimized by CCD[END_REF] , in particular in paper and textiles applications, for cross-linking between textile substrates and films of water-repellent compounds.

Zirconium acetate solution is believed to contain neutral species. We believe that under the conditions where ZRA exhibit ice-structuring properties, it adopts the structure of a hydroxy-bridged polymer [START_REF] Ryu | Influence of cationic additives on the rheological, optical, and printing properties of ink-jet coatings[END_REF][START_REF] Mcalpine | Ammonium zirconium carbonate, an alternative insolubiliser for coating binders[END_REF] , that can be anchored to the surface of the ice crystals via hydrogen bonds (figure 7). Our experimental observations during ice templating can thus be explained by the proposed ice-structuring mechanism. A minimum concentration of ZRA is required to ensure a minimal coverage of the ice surface. The disappearance of ice-structuring properties at high crystal growth velocities could be understood by kinetic considerations. A certain time is required for the adsorption to be effective, and thus cannot occur if the crystals are growing too fast. Two different binding mechanisms can be considered for the hydroxy-bridged polymer structures, depending on whether hydrogen bonds are formed or not.

The relative position of the hydrophilic and hydrophobic side is the outcome of two factors: the strength of hydrogen bonds, and the hydrophobicity of the acetate group. If the hydrophilic side is facing the ice surface (figure 7), hydrogen bonds are created between the hydroxyl groups and the ice surface. Conversely if the hydrophobic side is facing the ice, a more stable situation, hydrogen bonds can also be created between the acetate groups and the ice surface. If the acetate anion was isolated, the dipole would be stronger in acetate than in the hydroxyl group, and thus the hydrogen bonds would be stronger for the acetate groups. However, in the polymer structure presented here, the hydroxyl group is probably more polarized than the acetate, and thus we expect the hydrogen bonding to ice to be more favorable with the hydroxyl group.

The second possible mechanism is a configuration similar to the binding of antifreeze proteins to ice, where the hydrophobic side is facing the ice and the hydrophobic acetate groups order water which are in turn anchored to the polymer structure by hydrogen bonds4. It is nevertheless questionable whether such anchored clathrate binding mechanism is compatible with the structure of zirconium acetate. It would be an extraordinary coincidence that the binding mechanism of ZRA and ISPs are the same, considering how different these compounds are.

As opposed to ISPs, the hydroxy-bridged structure of ZRA exhibits very limited flexibility. The spacing between the hydroxyl groups should thus closely match that of the oxygen at the surface of the ice crystals. A higher concentration is thus required for a similar efficiency. If the anchored clathrate mechanism is acting, the acetate hydrophobic groups must organize the water in a structure that provides an excellent threedimensional match to the ice surface. Depending on the quality of the match, the required concentration for a similar efficiency may vary.

Because it would prevent ZRA from approaching the ice surface, it is essential for this mechanisms that the ice surface does not exhibit any surface charge. Like metal oxides, ice exhibits an isoelectric point at a specific pH, and its surface is charged out of this range. Compounds like ZRA, susceptible to surface interactions, should thus exist in a suitable structure or conformation close the ice isoelectric point.

Figure 7. Proposed ice-structuring mechanisms for the hydroxy-bridged polymer structure of zirconium acetate. Hydroxyl groups can be anchored to the surface of ice crystals via hydrogen bonding, leaving the hydrophobic acetate group exposed to water. The binding effectively slows down the incorporation of water molecules onto the crystal surface. Despite these similarities, the nature of the compounds is fundamentally different: ZRA is a metallic salt, while ISPs are almost systematically long biomacromolecules with a large molecular weight. This opens interesting perspectives for ZRA: its role in ice nucleation, recrystallization and the possible existence of thermal hysteresis, which are all characteristics of antifreeze proteins, has not been investigated so far. The anchoring of zirconium acetate at the ice surface through hydrogen bonds, combined with the non-colligative effects demonstrated previously, may induce a thermal hysteresis during freeze-thaw cycles which should be verified experimentally.

Conclusions

We propose an ice-structuring mechanism for zirconium acetate which can explain its ice-structuring properties similar to those of ice-structuring proteins. The proposed mechanism is the anchoring of the hydroxy-bridged polymer structure onto the surface of ice via hydrogen bonding of the hydroxyl group, leaving the hydrophobic acetate groups exposed at the surface. This mechanism should effectively slow down the incorporation of water molecules onto the crystal surface, resulting in a smooth interface and a facetted growth. The influence of zirconium acetate on ice nucleation, recrystallization, and the possible existence of thermal hysteresis remains to be investigated.

Figure 2 .

 2 Figure 2. Influence of ice crystal growth velocity (a) >20°C min -1 , (b) 15°C min -1 (c) 5°C min -1 (d) 2°C min -1 (e) 0.5°C min -1 (f) 0.2°C min -1 . Scale bar (a-c) 50 μm (d-f) 150 μm. Concentration: 18 g.l -1 ZRA.

Figure 3 .

 3 Figure 3. Structural features below the faceting threshold, for very slow crystal growth rate. The suspension was maintained at -5°C to obtain a very slow growth rate. Features typical of a spiral growth mode, seen from top (a) and from aside (b). Scale bars: (a) 50 μm, (b) 250 μm. Concentration: 18 g.l -1 ZRA.

Figure 4 .

 4 Figure 4. Schematic relationships between crystal growth rate and driving force, and corresponding crystal morphologies for rough (continuous growth) and smooth (hopper and polyhedral) morphologies. After 24 .

Figure 5 .

 5 Figure 5. Investigated compounds. The stars indicate compounds known to have some antifreeze activity.

Figure 6 .

 6 Figure 6. Assessing the role of acetate groups (a) 2 wt.% yttrium acetate (b) 6 wt.% barium acetate (c) 5 wt.% zirconium hydroxyl acetate. (d) 1.5 wt.% acetic acid, (e) 1.8 wt.% propanoic acid, (f) Pluronic F-127; 5 wt.% of (g) D-sorbitol (h) xylitol (i) pentaerythritol (j) dextrin (k) beta-lactose (l) D-quinic acid (m) myo-inositol . Scale bar a-f 50 μm, (g) 250 μm (h-j) 100 μm (k-l) 250 μm (m) 150 μm.

AUTHOR INFORMATION

Corresponding Author * Sylvain Deville, e-mail: sylvain.deville@saint-gobain.com Present Address † EI-CESI, Rue Magellan, Labège, France

Author Contributions

The manuscript was written through contributions of all authors.

ACKNOWLEDGMENT

We acknowledge Jérôme Leloup, Arnaud Villard, Gabriel Llorens and Abishek Tiwari for processing some of the samples.

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