Quiterie Duron
email: quiterie.duron@ird.fr

Garcia-Iriarte Oriana

Brescia Eric Fabrice

Vidal

Á O Garcia-Iriarte

Á E Vidal

Comparative effects of native frugivores and introduced rodents on seed germination in New-Caledonian rainforest plants

Keywords: Seed dispersal, Seed destruction, Rattus, Flying foxes, Frugivorous pigeon

Native frugivores play an important role in native plant community dynamics by participating in seed dispersal. Today many island forests are invaded by introduced omnivores, such as rats, but their role in dispersing native plants is still little known. Here, we evaluated whether native seeds from New-Caledonian rainforests can germinate after passing through an invasive rat digestive tract and compared seed germinability and germination time between seeds ingested by invasive rats and native frugivores. We offered native fruits of Ficus racemigera and Freycinetia sulcata to the rats Rattus rattus and R. exulans, three flying foxes Pteropus spp. and the pigeon Ducula goliath. Our results showed that seeds can germinate after passing through an invasive rat digestive tract, and suggest that rats can disperse seeds of both plant species. However, invasive rats may be less efficient than native frugivores, as more seeds were destroyed when passing through rat digestive tracts than through native frugivores, and because germinability was lower and germination time was longer for seeds passing through invasive rats than through native frugivores. The reduced efficiency of rats may result from their generalized diet, the structure of their digestive tract, and/or their feeding behavior. In New-Caledonian rainforests, dispersal services on both plant species are likely well fulfilled by flying foxes and Ducula pigeons, but rats do not seem to be as efficient dispersers. Consequently, management measures to protect native frugivores should help to conserve seed dispersal services.

Introduction

Seed dispersal is a key process in plant community dynamics as it allows species to colonize new areas and avoid competition with siblings, predation, and spread of pathogens [START_REF] Traveset | The ecology of seed dispersal[END_REF]. Frugivorous vertebrates, especially mammals and birds, play an important role in seed dispersal [START_REF] Jordano | Fruits and frugivory[END_REF]. Vertebrates disperse seeds from 70 to 94 % of all woody species in tropical rainforests, and seeds from 65 to 74 % in subtropical humid forest [START_REF] Jordano | Fruits and frugivory[END_REF]. One of the main ways for frugivores to disperse seeds is by endozoochory, i.e. by the ingestion and excretion of seeds. While frugivores benefit by obtaining energy and nutrients from the fruit pulp, endozoochory benefits the plant by facilitating the transport of seeds away from the parent plant location, and can also help germination [START_REF] Traveset | Effect of seed passage through vertebrate frugivores' guts on germination: a review[END_REF][START_REF] Reid | Avian gut-passage effects on seed germination of shrubland species in Mediterranean central Chile[END_REF][START_REF] Jordano | Fruits and frugivory[END_REF]). Passage through a frugivore's digestive tract can enhance germination success by abrasion of the seed coat and the removal of the pulp [START_REF] Barnea | Does ingestion by birds affect seed germination?[END_REF][START_REF] Traveset | Effect of seed passage through vertebrate frugivores' guts on germination: a review[END_REF][START_REF] Jordaan | The role of avian frugivores in germination of seeds of fleshy-fruited invasive alien plants[END_REF]. Endozoochory is crucial for the dispersal of many fleshy fruited plants [START_REF] Traveset | The ecology of seed dispersal[END_REF] and it is recognized that these mutualistic interactions are currently threatened by human disturbance and the drivers of global environmental change, such as invasive species [START_REF] Tylianakis | Global change and species interactions in terrestrial ecosystems[END_REF][START_REF] Markl | Meta-Analysis of the Effects of Human Disturbance on Seed Dispersal by Animals: human Effects on Animal Seed Dispersal[END_REF]. On islands, endozoochory relations are even more particularly prone to be disturbed by invasive species because of the uniqueness and the low diversity of native frugivore assemblages [START_REF] Traveset | Biological invasions as disruptors of plant reproductive mutualisms[END_REF][START_REF] Traveset | The ecology of seed dispersal[END_REF].

Invasive species can affect dispersal processes negatively by disrupting plant-seed disperser interactions [START_REF] Kelly | Relative (un) importance of introduced animals as pollinators and dispersers of native plants[END_REF][START_REF] Traveset | Biological invasions as disruptors of plant reproductive mutualisms[END_REF], or positively by compensating when native seed dispersers are few or have disappeared [START_REF] Kaiser-Bunbury | Conservation and restoration of plant-animal mutualisms on oceanic islands[END_REF][START_REF] Pattemore | Invasive rats and recent colonist birds partially compensate for the loss of endemic New Zealand pollinators[END_REF]. For example, the efficiency of introduced birds as native seed dispersers was already demonstrated in Hawaiian forests [START_REF] Foster | Introduced birds and the fate of Hawaiian rainforests[END_REF], and the capacity of invasive rats to disperse small seeds of both native and invasive plants was studied in New Zealand and Hawaiian forests [START_REF] Williams | Small mammals as potential seed dispersers in New Zealand[END_REF][START_REF] Grant-Hoffman | Herbivory in invasive rats: criteria for food selection[END_REF][START_REF] Shiels | Frugivory by introduced black rats (Rattus rattus) promotes dispersal of invasive plant seeds[END_REF][START_REF] Shiels | Are introduced rats (Rattus rattus) both seed predators and dispersers in Hawaii?[END_REF]. However, invasive species can belong to groups or taxa that did not exist among native species, in particular flightless terrestrial mammals that have historically been absent from most oceanic islands [START_REF] Lever | Naturalized mammals of the world[END_REF][START_REF] Whittaker | Island biogeography. Ecology, evolution, and conservation[END_REF][START_REF] Scheiner | The theory of ecology[END_REF] and did not evolve together with native plant species [START_REF] Nogales | Effect of native and alien vertebrate frugivores on seed viability and germination patterns of Rubia fruticosa (Rubiaceae) in the eastern Canary Islands[END_REF]). These two particularities may result in lower seed dispersal performances of invasive vertebrates versus native vertebrates. The effects of invasive species on seed dispersal can be ambiguous and our objective was to study the relative capacities of invasive rats and native frugivores to facilitate the dispersal process in an island system.

On islands, native vertebrate frugivores include birds, flying foxes and many Squamata but few terrestrial mammals [START_REF] Olesen | Lizards and birds as generalized pollinators and seed dispersers of island plants[END_REF][START_REF] Nogales | Effect of native and alien vertebrate frugivores on seed viability and germination patterns of Rubia fruticosa (Rubiaceae) in the eastern Canary Islands[END_REF]. Nonetheless, most islands have been invaded by exotic terrestrial mammals [START_REF] Courchamp | Mammal invaders on islands: impact, control and control impact[END_REF][START_REF] Blackburn | Avian extinction and mammalian introductions on oceanic islands[END_REF]), some of them being partly frugivorous like pigs, deer, rabbits and rats. These species can strongly affect plant communities and mutualisms directly, through predation and competition [START_REF] Traveset | Biological invasions as disruptors of plant reproductive mutualisms[END_REF][START_REF] Traveset | A review on the effects of alien rodents in the Balearic (Western Mediterranean Sea) and Canary Islands (Eastern Atlantic Ocean)[END_REF]. Invasive rats (Rattus spp.), and particularly the Black rat Rattus rattus, even if their diet is omnivorous and depends on seasonal resource availability [START_REF] Caut | Dietary shift of an invasive predator: rats, seabirds and sea turtles[END_REF][START_REF] Shiels | Biology and impacts of Pacific island invasive species. 11. Rattus rattus the Black Rat (Rodentia: Muridae)[END_REF], feed mainly on plant material (seeds, fruits, vegetative parts; [START_REF] Sugihara | Abundance and diets of rats in two native Hawaiian forests[END_REF][START_REF] Meyer | The impacts of rats on the endangered native flora of French Polynesia (Pacific Islands): drivers of plant extinction or coup de gra ˆce species?[END_REF][START_REF] Grant-Hoffman | Herbivory in invasive rats: criteria for food selection[END_REF][START_REF] Shiels | Dietary niche differentiation among three species of invasive rodents (Rattus rattus, R. exulans, Mus musculus)[END_REF]. Invasive rats are generally considered to be seed predators and are known for their detrimental effects on plants [START_REF] Meyer | The impacts of rats on the endangered native flora of French Polynesia (Pacific Islands): drivers of plant extinction or coup de gra ˆce species?[END_REF][START_REF] Grant-Hoffman | Herbivory in invasive rats: criteria for food selection[END_REF][START_REF] Shiels | Are introduced rats (Rattus rattus) both seed predators and dispersers in Hawaii?[END_REF], but they may also have positive effects by acting as seed dispersers [START_REF] Williams | Small mammals as potential seed dispersers in New Zealand[END_REF][START_REF] Grant-Hoffman | Herbivory in invasive rats: criteria for food selection[END_REF]. When eating fleshy fruits, small seeds may pass intact through the rat digestive tract and may be dispersed by endozoochory [START_REF] Traveset | Effect of seed passage through vertebrate frugivores' guts on germination: a review[END_REF][START_REF] Williams | Small mammals as potential seed dispersers in New Zealand[END_REF][START_REF] Bourgeois | Invasional meltdown potential: facilitation between introduced plants and mammals on French Mediterranean islands[END_REF]; Grant-Hoffman and Barboza 2010; Shiels 2011). The length limit of seeds reported to pass intact through the rat digestive tract was about 2.0-2.5 mm for R. rattus and 0.11 mm for the Pacific rat R. exulans [START_REF] Williams | Small mammals as potential seed dispersers in New Zealand[END_REF][START_REF] Shiels | Frugivory by introduced black rats (Rattus rattus) promotes dispersal of invasive plant seeds[END_REF]). However, our knowledge on the capacity of invasive rats to participate in native seed dispersal is still limited [START_REF] Shiels | Are introduced rats (Rattus rattus) both seed predators and dispersers in Hawaii?[END_REF].

Since most of the world's islands (82 %) are now occupied by invasive rats [START_REF] Atkinson | The spread of commensal species of Rattus to oceanic islands and their effects on avifaunas[END_REF][START_REF] Capizzi | Rating the rat: global patterns and research priorities in impacts and management of rodent pests[END_REF], a better understanding of the role of invasive rats in their host ecosystems is essential before considering their management. In the New Caledonia archipelago, two common invasive rats R. exulans and R. rattus coexist in all natural areas from sea level to 1629 m a.s.l (Beauvais et al. 2006). R. exulans reached the archipelago 3000 years ago with the initial Melanesian settlements, while R. rattus arrived with European colonization about 150 years ago (Beauvais et al. 2006). New Caledonia is listed as one of 35 biodiversity hotspots with exceptional rates of floral endemism that reach 83.2 % in rainforests [START_REF] Mittermeier | Global biodiversity conservation: the critical role of hotspots[END_REF][START_REF] Morat | Le re ´fe ´rentiel taxonomique Florical et les caracte ´ristiques de la flore vasculaire indige `ne de la Nouvelle-Cale ´donie[END_REF]. A study conducted on 123 new-Caledonian rainforest tree species revealed that 72 % have adaptations for dispersal by vertebrates [START_REF] Carpenter | Reproductive traits of tropical rain-forest trees in New Caledonia[END_REF], and pinpointed the importance of conserving native frugivorous species to maintain dispersal processes of endemic plants. Native frugivores on New Caledonia's main island (Grande Terre) include 47 species of Squamata (42 gekkos and 5 skinks) (Herve ´Jourdan, personal communication), 15 species of birds (Barre ánd Dutson 2000;[START_REF] Tassin | Can Ptilinopus greyii (Columbidae) disperse seeds in New Caledonia's dry forests?[END_REF], and four species of flying foxes [START_REF] Flannery | Mammals of the South-West Pacific and Moluccan Islands[END_REF], and may have been more diversified before human and rat colonization [START_REF] Balouet | Fossil birds from late Quaternary deposits in New Caledonia[END_REF][START_REF] Balouet | The fossil vertebrate record of New Caledonia[END_REF].

Few comparisons of the effects of passage through invasive versus native species' digestive tracts on seed germination have been conducted [START_REF] Nogales | Effect of native and alien vertebrate frugivores on seed viability and germination patterns of Rubia fruticosa (Rubiaceae) in the eastern Canary Islands[END_REF]. To the best of our knowledge, only one study conducted in the Cook Islands, on Ficus prolixa, compared the differences in native seed germination after passage through native frugivorous (bird and flying foxes) and invasive rat species digestive tracts [START_REF] Staddon | Dispersal of fig seeds in the Cook Islands: introduced frugivores are no substitutes for natives[END_REF]. Therefore, the presence of both native frugivores (birds and flying foxes) and invasive rats in New-Caledonian rainforest represented a unique opportunity to test their relative effects on seed germination. In this study, we focused on the first steps of dispersal by endozoochory i.e. seed survival and germination after passing through the digestive tract, and compared these in native frugivores and invasive rats. Our aims were to evaluate whether seeds can survive and germinate after passing through the invasive rat digestive tract and to compare seed germination effectiveness (germinability and germination time), between non ingested seeds and seeds ingested by native frugivores or invasive rats.

Methods

Fruits and seeds characteristics

Two native plant species were selected based on morphological, ecological, and phenological criteria and their accessibility: Ficus racemigera (Bureau, Moraceae) and Freycinetia sulcata (Warb., Pandanaceae). Their fruits are known to be eaten by both native frugivorous birds and flying foxes, as well as by invasive rats in New Caledonia (M. Boissenin and H.Vandrot, personal communication). Moreover, species of the genera Ficus and Freycinetia are known to be eaten by flying foxes [START_REF] Parsons | Dietary variation in spectacled flying foxes (Pteropus conspicillatus) of the Australian Wet Tropics[END_REF][START_REF] Nakamoto | The role of Orii's flyingfox (Pteropus dasymallus inopinatus) as a pollinator and a seed disperser on Okinawa-jima Island, the Ryukyu Archipelago, Japan[END_REF][START_REF] Heer | Effects of ingestion by neotropical bats on germination parameters of native freestanding and strangler figs (Ficus sp., Moraceae)[END_REF]) and bats (Burrows 1996) in other parts of the world. Ficus species were also noted as the most common food source for Ducula galeata in the Marquesas archipelago [START_REF] Blanvillain | The biology of the critically endangered Marquesan Imperial-Pigeon (Ducula galeata), Nuku Hiva, Marquesas Archipelago (French Polynesia)[END_REF] and are consumed by Ducula pacifica in Tonga [START_REF] Mcconkey | Seed dispersal by Pacific pigeons (Ducula pacifica) in Tonga, western Polynesia[END_REF]). Ducula goliath was observed feeding on Freycinetia arborea, a species closely related to Freycinetia sulcata, in Mont Panie ´, New Caledonia (personal observation). Seeds of both the genera Ficus and Freycinetia were found in R. rattus and R. exulans stomach and caecum contents for specimens collected in Mont Panie ´rainforest area, and these results oriented our choice to study germination of these two plant species. Ficus and Freycinetia have small seeds (\1.5 mm) which may allow dispersal by endozoochory by both native frugivores and invasive rats [START_REF] Williams | Small mammals as potential seed dispersers in New Zealand[END_REF][START_REF] Shiels | Frugivory by introduced black rats (Rattus rattus) promotes dispersal of invasive plant seeds[END_REF][START_REF] Shiels | Are introduced rats (Rattus rattus) both seed predators and dispersers in Hawaii?[END_REF]. For both species, a sample of fruits and seeds was weighed with a precision balance Mettler AE 240 (precision 0.1 mg), fruits were measured using a digital caliper (precision 0.001 mm) and seeds from binocular photography with ''Image J'' software (Table 1).

Ficus racemigera (hereafter Ficus) is a species of tree endemic to New Caledonia and Vanuatu, found in dense rainforests at various elevations. The fig ''fruit'', called a syconium, is a multiple pseudocarp, which includes several achene fruits. Each achene is made up of a seed surrounded by its calyx and fixed by a pedicel to the syconium fleshy receptacle. Each fig contains more than 300 small brown and rounded seeds (length 1.03 ± 0.89 mm; width 0.72 ± 0.84 mm). Mature fruits, selected for their reddish color and smooth texture, were collected in rainforest areas on Mont Koghi (22°10 0 38 00 S, 166°30 0 30 00 E, 450 m a.s.l.) in February and March 2015. Freycinetia sulcata (hereafter Freycinetia) is a hemiepiphyte native liana. Their fruits, or berries, are gathered in a yellow scented infructescence. Each infructescence contains more than 1000 seeds (length 1.15 ± 0.08 mm; width 0.28 ± 0.03 mm), which are covered by a thick mucilaginous flesh. Seeds are slightly curved with hyaline membrane margins on each side [START_REF] Stone | Pandanaceae. In: Kubitzki PDK (ed) Flowering Plants Á Monocotyledons[END_REF]. Mature fruits, distinguished by their yellow color and their strong scent, were collected on Mont Panie ´(20°37 0 30 00 S, 164°46 0 56 00 E, 650 m a.s.l.) in March and May 2015.

Frugivorous vertebrate species tested

We tested two groups of vertebrate disperser species i.e. native frugivores (three species of flying foxes, Pteropus ornatus, P. tonganus and P. vetulus, and the imperial pigeon, or Notou, Ducula goliath), and introduced omnivorous rodents (two invasive rat species, R. rattus and R. exulans). P. ornatus is endemic to New Caledonia. Adult males can weigh 570 g with body size of 23 cm [START_REF] Flannery | Mammals of the South-West Pacific and Moluccan Islands[END_REF]. P. tonganus is native to New Caledonia and has a large distribution in the Pacific. Adults weigh between 530 and 710 g [START_REF] Flannery | Mammals of the South-West Pacific and Moluccan Islands[END_REF]. P. vetulus is endemic to New Caledonia and it is the smallest species of flying fox of the genus Pteropus in New Caledonia, weighing between 180 and 220 g [START_REF] Flannery | Mammals of the South-West Pacific and Moluccan Islands[END_REF][START_REF] Brescia | Ame ´lioration des connaissances et recommandations pour la sauvegarde des populations de Me ´gachiropte `res (roussettes) en Province Sud de la Nouvelle-Cale ´donie[END_REF]. D. goliath is reputedly the largest arboreal pigeon in the world and its exclusively frugivorous regime confers an essential role as dispersal agent for many fruit species in rainforests [START_REF] Barre | Contribution to the knowledge of the New Caledonian imperial pigeon Ducula goliath (Gray, 1859), with emphasis on sexual dimorphism[END_REF].

The feeding experiments were done on animals held in captivity in a zoological park. For P. ornatus, we worked with a group of five males held in a 3.4 m 9 3.1 m 9 6.2 m aviary; for P. tonganus, we worked with one male and two females held in a 3 m 9 2.5 m 9 4.6 m aviary; and, for P. vetulus we worked with two males and four females held in a 3 m 9 2.5 m 9 7 m aviary. For D. goliath, we did the experiments with two males and three females, each one being held in an individual cage of 4.9 m 9 2 m 9 1.5 m. All the animals were fed daily in the morning by zookeepers, with fresh fruits and water was provided ad libitum.

The two invasive rats, R. rattus and R. exulans, were also considered. In Mont Panie ´area, adult R. rattus usually weigh around 177.4 ± 30.1 g (±SD, min = 120 g, max = 263 g) whereas adult R. exulans weigh around 69.5 ± 12.8 g (±SD, min = 50 g and max = 95 g). Ten adult R. rattus and four R. exulans were live trapped from two locations in forests near Noume ´a (Ouen Toro dry forest, 22°18 0 36 00 S, 166°27 0 16 00 E and Koghi rainforest, 22°10 0 38 00 S, 166°30 0 30 00 E). Each rat was held in an 40 cm 9 33 cm 9 28 cm or 60 cm 9 41 cm 9 37 cm individual metal mesh cage with bedding material and shelter. They were fed daily with mixed seeds and water was provided ad libitum.

Fruit consumption trials

All the feeding trials were performed from February through May 2015 with fresh fruits of Ficus and Freycinetia collected the day before. Before each feeding trial, all the cages were cleaned entirely to remove all remains of old faeces and uneaten food, in order to facilitate faeces collection. For native frugivores, the fruits were offered at the same time as their usual diet during their morning meal for one or 2 days depending on the species. Fruits were offered to five D. goliath, five P. ornatus, three P. tonganus, and six P. vetulus. Rats were deprived of their usual food (mixed seeds) 24 h before feeding trials to ensure that they consumed the tested fruits. The fruits were offered to four R. rattus and four R. exulans by offering fruits in each cage for 2 days. Rats were returned to their regular diet for at least 48 h between [START_REF] Richards | The spectacled flying-fox, Pteropus conspicillatus (Chiroptera: Pteropodidae), in North Queensland. 2. Diet, seed dispersal and feeding ecology[END_REF]). Consequently, we assumed that 100 % of seeds ingested by native frugivores survived. However, flying foxes also spit out fruit pulp and some seeds after squeezing the juice from the fruits [START_REF] Banack | Diet selection and resource use by flying foxes (genus Pteropus)[END_REF][START_REF] Shilton | Old World fruit bats can be long-distance seed dispersers through extended retention of viable seeds in the gut[END_REF]. Consequently a portion of the seeds may be not dispersed as far as the others. For invasive rats, in parallel to the consumption trials described above, a parallel experiment was set up to estimate the percentage of seed survival when seeds were ingested by the two species of rats [START_REF] Williams | Small mammals as potential seed dispersers in New Zealand[END_REF][START_REF] Shiels | Are introduced rats (Rattus rattus) both seed predators and dispersers in Hawaii?[END_REF]. The number of seeds offered to rats was controlled by placing a precise number of seeds (100 seeds for Ficus and 50 seeds for Freycinetia), into a highly palatable bait composed of a mixture of crushed kibbles and peanut butter. This bait was offered individually to six R. rattus and four R. exulans but faeces could only be collected from five R. rattus and two R. exulans. All the droppings were carefully collected and inspected under a binocular microscope Leica MZ75 (49 magnification) in order to determine the percentages of damaged and undamaged seeds passing through the rat digestive tract. Seeds were considered undamaged when there was no visible change in the seed coat after passage through the digestive tract [START_REF] Nogales | Effect of native and alien vertebrate frugivores on seed viability and germination patterns of Rubia fruticosa (Rubiaceae) in the eastern Canary Islands[END_REF]. The removal of the calyx for the Ficus seeds was not considered to be damage.

Germination trials

Undamaged seeds were removed from native and invasive species faeces and were cleaned with tap water in order to avoid the potential effects of nutrients coming from faeces. Then seeds were quickly placed on water-wet cotton (humidity 100 %) in transparent Petri dishes. Four replicates of 25 seeds each (pooled from different individual plants and different individual animals) were tested for each treatment [START_REF] Tassin | Can Ptilinopus greyii (Columbidae) disperse seeds in New Caledonia's dry forests?[END_REF][START_REF] Carvalho-Ricardo | Frugivory and the effects of ingestion by bats on the seed germination of three pioneering plants[END_REF]) except for R. exulans, for which very few seeds were obtained from the faeces (n = 12 for Ficus and n = 9 for Freycinetia). There were six ingested seed treatments: R. rattus, R. exulans, P. ornatus, P. tonganus, P. vetulus, and D. goliath. Non ingested seeds, used as a control treatment, were manipulated in the same manner as ingested seeds. In the case of Ficus, two sets of control treatments were carried out: seeds without calyx (''no calyx'' treatment) and seeds with calyx (''calyx'' treatment). The reason for this was that when non-ingested seeds were hand-extracted, the seeds came with the calyx, though ingested seeds were found without the calyx. Freycinetia seeds do not have an individual calyx, so the second control group was not necessary in this case. The Petri dishes were placed in an incubator at 26.5 °C with a natural photoperiod. Germination, defined as the emergence of 2 mm of the primary root (De Carvalho-Ricardo et al. 2014), was checked daily with a binocular microscope Leica MZ75 (4x magnification). Germination was monitored 15 days after sowing for Ficus and after 49 days for Freycinetia.

Statistical analysis

Seed percent survival was compared between rat and seed species with Chi-square tests for homogeneity with Yates' continuity correction. Concerning germination data analysis, for each Petri dish, the percentage of seeds that had germinated (''germinability'') at the end of germination monitoring was calculated. Germination times were also measured. The ''date of germination start'' was noted as the day when the first seed had germinated in a Petri dish, and ''T50 00 corresponded to the number of days required to reach 50 % germination of the total number of sown seeds.

We assessed whether the germinability varied according to the different treatments using Generalized Linear Models (GLMs, quasibinomial family to account for over-or under-dispersion). We used a Chisquare test to assess the significance of the deviance change between the null model and the model with the ''treatment'' variable. GLMs were performed initially on data when grouped into four categories of treatments: controls (with and without calyx), invasive rats (R. rattus and R. exulans), native pigeons (D. goliath) and native flying foxes (P. ornatus, P. tonganus, and P. vetulus). Secondly, all treatments were compared by pairs using post hoc Tukey's tests, with the Holm correction [START_REF] Aickin | Adjusting for multiple testing when reporting research results: the Bonferroni vs Holm methods[END_REF].

The germination times for each treatment were analyzed using Generalized Additive Models (GAMs) with the proportion of seed germinated as a response variable and quasibinomial errors to account for overor under-dispersion. Seed germination is not a linear function of time so we fitted change in number of seeds germinated with time using a thin plate regression spline [START_REF] Wood | Generalized Additive Models: An Introduction with R[END_REF]. We set the spline basis dimension 'k' to 6 to limit the complexity of the splines and prevent overfitting. GAM model predictions were then used to calculate the T50 of each treatment. All analyzes were performed under R Version 3.1.2 with packages multcomp and mgcv.

Results

Assessment of seed percent survival

Visual inspections of faeces showed that 100 % of seeds were undamaged after passing through native frugivore guts. By contrast, the seed survival results showed that the invasive rats destroyed a large proportion of the seeds that they ingested. For Ficus, the percentage of intact seeds passed through the R. rattus' digestive tract averaged 30.6 ± 13.5 % (±SD, n = 5) and was significantly higher than the percentage of intact seeds that passed through R. exulans: 14 ± 1.4 % (±SD, n = 2, v 2 = 7.02, df = 1, p = 0.008). For Freycinetia, 30 ± 19.0 % (±SD, n = 5) of seeds were found intact in R. rattus faeces and only 16 ± 2.8 % (±SD, n = 2; v 2 = 4.77, df = 1, p = 0.029) in R. exulans faeces. No difference in survival rates was recorded between Ficus and Freycinetia species (v 2 = 0.014, df = 1, p = 0.905).

Germinability

The low number of seeds (n = 12 for Ficus and n = 9 for Freycinetia) found in R. exulans faeces did not allow comparison of germinability and germination time with other treatments, so the R. exulans data were excluded from the statistical analysis of separate species. When analyzes were performed on treatment category, R. exulans data were grouped with R. rattus data in the ''invasive rats'' group.

For Ficus, the Chi-square test conducted in GLM results showed the existence of a significant effect of the treatment category, i.e. a difference in germinability among controls, native pigeons, native flying foxes and invasive rats (deviance explained by the model: 6.69 %, p \ 0.001). Seeds passing through native flying foxes (Pteropus spp.) had a higher germinability than those passing through invasive rats (p = 0.012). However, no difference in germinability was recorded between invasive rats and the control treatments (p = 1) nor with native pigeons (D. goliath, p = 0.140). When all treatments were compared (Table 1a; Fig. 1), control seeds with calyx had lower germinability than control seeds without calyx (p = 0.013). Control seeds with calyx also had lower germinability than seeds ingested by Ducula goliath (p = 0.004), Pteropus ornatus (p = 0.005), P. tonganus (p = 0.005) and P. vetulus (p = 0.005). However, no significant difference was registered between control seeds with calyx and R. rattus (p = 0.355). For R. rattus, if we take into account the seed percent survival after gut passage, only 27 % of ingested seeds are likely able to germinate.

For Freycinetia, the Chi-square test conducted in GLM results also showed the existence of a significant effect of the treatment category on the germinability (deviance explained by the model: 24.8 %, p \ 0.001). Seeds ingested by invasive species (i.e. Rattus spp.) had a 50 % lower germinability than those ingested by native pigeons (p \ 0.001), native flying foxes (p \ 0.001) and the control seeds (p \ 0.001). However, there was no difference between seeds ingested by native pigeons or native flying foxes and the control (p = 0.191 and p = 0.086 respectively). When regarding native species, seeds ingested by D. goliath had a higher germinability followed by P. vetulus and P. tonganus. In contrast, P. ornatus treatment had the lowest germinability among native species (Table 1b; Fig. 1). Seeds ingested by R. rattus and P. ornatus had lower germinability than control seeds (p \ 0.001 and p = 0.006 respectively). Seeds subjected to R. rattus treatment had lower germinability than those subjected to D. goliath (p \ 0.001), P. tonganus (p \ 0.001) and P. vetulus treatments (p \ 0.001). For R. rattus, if we take into account the seed percent survival after gut passage, only 12 % of ingested seeds are likely able to germinate.

Germination time

First germinations of Ficus seeds were registered 2 days after sowing for seeds subjected to passage through D. goliath whereas seeds subjected to R. rattus treatment began to germinate 5 days after sowing (Table 2a; Fig. 2). The last germination was recorded on day 13 for the control without calyx treatment. T50 (number of days required to reach 50 % of the total number of sown seeds) differed for all treatments (Table 2a; Fig. 2). Whereas seeds ingested by native species germinated quickly (4.35 \ T50 \ 5.65 days), seeds ingested by R. rattus reached T50 about 3 days later (T50 = 8.25 days, Table 2a).

First germinations of Freycinetia seeds were registered 15 days after sowing for seeds ingested by D. goliath, P. ornatus and P. vetulus, whereas seeds ingested by R. rattus began to germinate on day 20 (Table 2b; Fig. 3). The last germinations were recorded on day 40 for R. rattus and Pteropus spp. treatments (Table 2b; Fig. 3). For the native species treatments, the smaller T50 s were recorded for P. vetulus (T50 = 19.95 days), followed by D. goliath (T50 = 20.22 days), and the control treatment (T50 = 22.36 days, Table 2b). As only 39.7 % of seeds have germinated with R. rattus treatment, the T50 was not estimated.

Discussion

Our results suggest that both native frugivorous vertebrates (D. goliath and Pteropus spp.) and introduced rodents (Rattus spp.) are potential dispersers of Ficus racemigera and Freycinetia sulcata seeds. However, seed dispersal by invasive rats is likely less efficient than by native frugivores as (1) more seeds were destroyed when passing through rats' digestive tracts than native frugivores', (2) germinability was lower for seeds passing through rat digestive tracts than native frugivores' and (3) T50 was longer when seeds were eaten by rats than by native frugivores. These results are consistent with other studies, which showed that invasive dispersers may be not as effective as native dispersers. For example, in the Canary Islands, invasive vertebrate frugivores defecated more damaged seeds of Rubia fruticosa than natives and fewer seeds germinated [START_REF] Nogales | Effect of native and alien vertebrate frugivores on seed viability and germination patterns of Rubia fruticosa (Rubiaceae) in the eastern Canary Islands[END_REF]. Invasive rats were found to be less effective than native species in dispersing Ficus prolixa in the Cook Islands [START_REF] Staddon | Dispersal of fig seeds in the Cook Islands: introduced frugivores are no substitutes for natives[END_REF]). This may be explained by the fact that, especially on islands, nonnative seed dispersers generally have a more generalist and opportunistic diet than many native species, which are more specialized and so adapted to disperse native plant species [START_REF] Aslan | Effects of native and non-native vertebrate mutualists on plants[END_REF].

Differences in seed percent survival, germinability and germination time, may occur because of frugivore morphology and behavior, or from fruit and seed characteristics [START_REF] Traveset | Effect of seed passage through vertebrate frugivores' guts on germination: a review[END_REF]. First, vertebrate's body size determines the size of seeds that can pass intact through its digestive tract [START_REF] Traveset | Effect of seed passage through vertebrate frugivores' guts on germination: a review[END_REF]Pe ´rez-Me ´ndez et al. 2015). Rats are smaller than flying foxes or imperial pigeons, and this difference may partly explain the lower percent survival for seeds in the rat treatment. The size limit for seeds passing intact through the digestive tract is reported to be 2.0-2.5 mm for R. rattus and 0.11 mm for R. exulans [START_REF] Williams | Small mammals as potential seed dispersers in New Zealand[END_REF][START_REF] Shiels | Frugivory by introduced black rats (Rattus rattus) promotes dispersal of invasive plant seeds[END_REF]) whereas a limit of Table 2 Germinability (%, mean ± SE) for each treatment and significant differences between treatments, day of first germination, and T50, i.e. number of days required to reach 50 % of the total number of sown seeds; for a) Ficus 3.7 mm has been noted for a species of flying fox (Pteropus conspicillatus) [START_REF] Richards | The spectacled flying-fox, Pteropus conspicillatus (Chiroptera: Pteropodidae), in North Queensland. 2. Diet, seed dispersal and feeding ecology[END_REF]). In our case, Ficus and Freycinetia seeds were about 1.0-1.15 mm which might explain the lower survival of seeds from rat treatment, particularly for R. exulans. Second, small ground-dwelling mammals, flying foxes, and birds differ in the structure of their digestive tracts, and seed survival and germination may be affected differently throughout the digestive process, from ingestion to excretion [START_REF] Traveset | Effect of seed passage through vertebrate frugivores' guts on germination: a review[END_REF]. For example, invasive rats can destroy seeds by chewing them with their teeth before swallowing fruit fragments, whereas birds do not have teeth and usually swallow the whole fruit without mastication. Moreover, seed retention time in the digestive tract can influence the seed coat because the greater the retention time, the greater the mechanical and chemical abrasion to the seed coat which can lead to variable germination efficiency [START_REF] Barnea | Does ingestion by birds affect seed germination?[END_REF][START_REF] Traveset | Effect of seed passage through vertebrate frugivores' guts on germination: a review[END_REF]. Seed retention times were noted to range from 12 to 45 min to several hours for some other species of the genus Pteropus [START_REF] Funakoshi | Feeding ecology of the northern Ryukyu fruit bat, Pteropus dasymallus dasymallus, in a warm-temperate region[END_REF][START_REF] Shilton | Old World fruit bats can be long-distance seed dispersers through extended retention of viable seeds in the gut[END_REF][START_REF] Carvalho-Ricardo | Frugivory and the effects of ingestion by bats on the seed germination of three pioneering plants[END_REF]. According to the fruit species ingested seed retention time went from 37 to 181 min in a New Zealand frugivorous pigeon, Hemiphaga novaeseelandiae, [START_REF] Clout | Germination of miro (Prumnopitys ferruginea) seeds after consumption by New Zealand pigeons (Hemiphaga novaeseelandiae)[END_REF][START_REF] Wotton | Seed retention times in the New Zealand pigeon (Hemiphaga novaezeelandiae novaeseelandiae)[END_REF], and would be about 30 min for a New-Caledonian dove, Ptinolopus greyii, when eating Ficus spp. [START_REF] Boissenin | Interactions entre l'avifaune frugivore et la flore ligneuse en fore ˆt se `che de Nouvelle-Cale ´donie[END_REF]. By contrast, it is reported to range from 12 to 48 h for rats [START_REF] Perrin | Preliminary investigations of the digestive processes of the white-tailed rat Mystromys albicaudatus (Smith 1834)[END_REF][START_REF] Sakaguchi | Comparison of fibre digestion and digesta retention time between rabbits, guinea-pigs, rats and hamsters[END_REF]. The longer time passed in the rat digestive tract may lead to an excessive seed chemical abrasion, which may explain the lower seed germinability [START_REF] Traveset | Effect of seed passage through vertebrate frugivores' guts on germination: a review[END_REF]).

In our experimentation, germinability differences between native and invasive treatments were higher for Freycinetia sulcata than for Ficus racemigera. As noted by [START_REF] Traveset | Effect of seed passage through vertebrate frugivores' guts on germination: a review[END_REF], a frugivorous species can have different effects on seed germination depending on fruit and seed traits, such as size, shape, pulp consistency, seed coat thickness, etc. Seeds with high permeability and a thin coat may germinate more rapidly thanks to faster water absorption [START_REF] Traveset | Seed trait changes in dispersers'guts and consequences for germination and seedling growth[END_REF]. Freycinetia seeds have thicker coats than Ficus (personal observation) which may explain why the first germination was on day 15 for Freycinetia and on day 2 for Ficus. For Ficus, a higher germinability was observed when seeds were ingested or when the calyx was removed. In other Ficus species, the effect of seed passage through various vertebrates (primates, bats and birds) has also generally been enhancement of germination [START_REF] Traveset | Effect of seed passage through vertebrate frugivores' guts on germination: a review[END_REF]. Likewise, regarding morphological features of Freycinetia fruits, the hypothesis that seeds need to be released from the mucus sheath through ingestion by frugivores is a possibility, but, to the best of our knowledge, it has never been experimentally demonstrated before.

When compared with non-ingestion of seeds and ingestion by flying foxes, ingestion by the native imperial pigeon (Ducula goliath) seemed to better Fig. 3 Cumulative proportion of seeds (%) germinated of Freycinetia sulcata according to the treatment enhance germination for Freycinetia. By contrast, for Ficus, the native pigeons did not have a greater facilitative effect than when no ingestion or ingestion by flying foxes occurred. The effects of birds on germination are variable according to seed species, sometimes enhancing and other times inhibiting germination, whereas no influence of ingestion by flying foxes on seed germinability has been reported [START_REF] Traveset | Effect of seed passage through vertebrate frugivores' guts on germination: a review[END_REF]. However, flying foxes and frugivorous pigeons are recognized to play an important role in seed dispersal, as they can allow seeds to germinate far from the mother plant [START_REF] Galindo-Gonzalez | Bat-and birdgenerated seed rains at isolated trees in pastures in a tropical rainforest[END_REF][START_REF] Olesen | Lizards and birds as generalized pollinators and seed dispersers of island plants[END_REF][START_REF] Nakamoto | The role of Orii's flyingfox (Pteropus dasymallus inopinatus) as a pollinator and a seed disperser on Okinawa-jima Island, the Ryukyu Archipelago, Japan[END_REF]. Ficus is a key genus for flying foxes; for example, Pteropus dasymallus in Japan consumed nine different species of Ficus [START_REF] Nakamoto | The role of Orii's flyingfox (Pteropus dasymallus inopinatus) as a pollinator and a seed disperser on Okinawa-jima Island, the Ryukyu Archipelago, Japan[END_REF], and Pteropus conspicillatus in Australia ate 13 different species of Ficus [START_REF] Parsons | Dietary variation in spectacled flying foxes (Pteropus conspicillatus) of the Australian Wet Tropics[END_REF]). Ficus species were frequently eaten by Pteropus conspicillatus in a study conducted in the Australian wet tropics [START_REF] Parsons | Dietary variation in spectacled flying foxes (Pteropus conspicillatus) of the Australian Wet Tropics[END_REF]). In the Cook Islands, native pigeons and doves appeared to be the most effective native dispersers of Ficus prolixa [START_REF] Staddon | Dispersal of fig seeds in the Cook Islands: introduced frugivores are no substitutes for natives[END_REF] and Ducula galeata fed principally on fruits of Ficus in the Marquesas archipelago [START_REF] Blanvillain | The biology of the critically endangered Marquesan Imperial-Pigeon (Ducula galeata), Nuku Hiva, Marquesas Archipelago (French Polynesia)[END_REF]. Likewise, Ficus racemigera may be well dispersed by native Pteropus spp. and Ducula goliath in New Caledonia. Almost no information on Freycinetia germination or dispersal is known except that Freycinetia spp. can be dispersed by bats [START_REF] Van Der Pijl | The dispersal of plants by bats (chiropterochory)[END_REF][START_REF] Lord | Pollination and seed dispersal in Freycinetia baueriana, a dioecious liane that has lost its bat pollinator[END_REF]) and that Ducula goliath was observed feeding on Freycinetia arborea in Mont Panie ´(personal observation), a Freycinetia sulcata closely related species.

Seed survival and germination are just two of many required steps for the development of a new plant individual. Another important condition is that animals have to defecate in suitable sites for germination (Calvin ˜o-Cancela 2004). To avoid competition, it is better for seeds to be taken away from the mother tree. Ducula pigeons and flying foxes help fulfill this condition by travelling long distances. In New Caledonia, [START_REF] Boissenin | Movements of the New Caledonian over-hunted flying foxes: implications for management[END_REF] showed that by night P. ornatus and P. tonganus equipped with satellite transmitters can fly distances between 8 to 15 km, from their day roosts to feeding areas. In contrast, rats are territorial and do not travel long distances [START_REF] Bramley | Home ranges and interactions of kiore (Rattus exulans) and Norway rats (R. norvegicus) on Kapiti Island, New Zealand[END_REF][START_REF] Ringler | Invasive rat space use on tropical islands: implications for bait broadcast[END_REF]. On an island in the Indian Ocean, spool-line experiments showed that R. rattus made trips from 63 to 192 m inside the forest [START_REF] Ringler | Invasive rat space use on tropical islands: implications for bait broadcast[END_REF]. In a New-Caledonian rainforest, spatially explicit capture-recapture analyzes showed that R. rattus and R. exulans home ranges vary from 0.41 to 1.34 ha (Duron et al. unpublished results). These results suggest that invasive rats are not able to disperse seeds as far as native species. However, small movements may still help plants to avoid competition with siblings and, spread of pathogens and may allow colonization of treefall gaps [START_REF] Traveset | The ecology of seed dispersal[END_REF].

In view of these elements, invasive rats do not seem to be efficient dispersers when native pigeons and flying foxes are still present. Moreover, some species of passerines and Squamata are frugivorous or partly frugivorous and may play a disperser role in the rainforest, but very little is actually known. Nonetheless, the question of the importance of rats in the dispersal process is crucial as invasive rat populations are more and more subject to management operations i.e. eradication (complete removal) or control (rat population reduction). In New Caledonian rainforest, dispersal services for both species studied seem wellfulfilled by native flying foxes and imperial pigeons. However, flying foxes and imperial pigeons are popular game species in New Caledonia, and their populations are globally considered as decreasing [START_REF] Mickelburgh | Old World fruit bats: An action plan for their conservation[END_REF][START_REF] Barre | Oiseaux de Nouvelle Cale ´donie. Liste commente ´e[END_REF][START_REF] Brescia | Ame ´lioration des connaissances et recommandations pour la sauvegarde des populations de Me ´gachiropte `res (roussettes) en Province Sud de la Nouvelle-Cale ´donie[END_REF]). Low population densities i.e. less competition between individuals can lead to a cessation of dispersal function [START_REF] Mcconkey | Flying foxes cease to function as seed dispersers long before they become rare[END_REF]. If native dispersers' populations become small or disappear entirely, the role of invasive rats in seed dispersal may become more important [START_REF] Staddon | Dispersal of fig seeds in the Cook Islands: introduced frugivores are no substitutes for natives[END_REF].

In New Caledonia, an assessment has been undertaken to determine whether controlling invasive rats would be beneficial for native biodiversity. Studying the trophic interactions and functional roles of invasive rats before making decisions about their management is crucial in order to avoid causing undesirable changes in the ecosystem [START_REF] Zavaleta | Viewing invasive species removal in a whole-ecosystem context[END_REF]. Today, controlling rats in New Caledonia does not seem to pose a threat to essential seed dispersal services. Assuming that fruit resources may be limited, controlling rats may reduce competition with native frugivores. In some studies, a decrease in competition was hypothesized to foster seed dispersal [START_REF] Bourgeois | Invasional meltdown potential: facilitation between introduced plants and mammals on French Mediterranean islands[END_REF][START_REF] Chimera | Patterns of seed dispersal and dispersal failure in a hawaiian dry forest having only introduced birds: introduced birds, invasion, and dispersal failure[END_REF]

Fig. 1

 1 Fig. 1 Germinability (% mean ± 1SD) of Ficus racemigera and Freycinetia sulcata seeds according to the following treatments: controls (i.e. non-ingested seeds: CN control without calyx, and CC control with calyx), introduced omnivorous rodents: RR R. rattus, and native frugivorous species (DG Ducula goliath, PT Pteropus tonganus, PO P. ornatus and PV P. vetulus). Within a plant species, means sharing the same letter are not significantly different (p [ 0.05)

  Fig. 2 Cumulative proportion of seeds (%) germinated of Ficus racemigera according to the treatment. Control CN control without calyx and Control CC Control with Calyx

Table 1

 1 Summary of fruit and seed color, size, weight, and numbers for Ficus racemigera and Freycinetia sulcata. Fruits measures for Freycinetia were made on the whole inflorescence

		Ficus racemigera	Freycinetia sulcata
	Family	MORACEAE	PANDANACEAE
	Life form	Shrub or tree	Secondary hemiepiphyte
	Fruit color	Red	Yellow
	Fruit length (mm)	14.42 ± 1.47 (n = 33)	81.28 ± 11.1 (n = 21)
	Fruit width (mm)	16.09 ± 1.85 (n = 33)	46.57 ± 7.03 (n = 21)
	Fruit fresh mass (mg)	2.00 ± 0.22 (n = 25)	70.45 ± 8.6 (n = 6)
	Number of seeds per fruit	329 ± 15	[1000
	Seed color	Brown	Yellow
	Seed length (mm)	1.03 ± 0.90 (n = 31)	1.15 ± 0.08 (n = 36)
	Seed width (mm)	0.72 ± 0.85 (n = 31)	0.28 ± 0.04 (n = 36)
	Seed mass (mg)	0.21 ± 0.04 (n = 100)	0.31 ± 0.00 (n = 90)
	Seed shape	Reniform	Curved

Acknowledgments This study was funded by the Northern Province of New Caledonia to REFCOR project (Re ´ponses des Ecosyste `mes Forestiers au COntro ˆle des Rongeurs, Convention No 12C240). We are grateful to Me ´lanie Boissenin, Bruno Fogliani, Charly Zongo from IAC (Institut Agronomique Ne ´o-Cale ´donien) and Philippe Marmey from IRD (Institut de Recherche pour le De ´veloppement) for the valuable discussions about the experiment. We thank Almudena Lorenzo, Oriane Lallemand and Marianne Bonzon from Michel Corbasson Zoological and Forest Park of Noumea, ''Direction de l'Environnement'' of the Southern Province of New Caledonia, who allowed us to work with imperial pigeons and flying foxes. We also thank Edouard Bourguet and Fre ´de ´ric Rigault for their help in collecting fruits, Josepho Bahormal for his help in taking care of rats, and Carol Frost for English advice and useful comments on the paper draft.


Martin Thibault
email: thibault@iac.nc

Fabrice Brescia

Herve Jourdan

Eric Vidal

Fabrice Brescia

Hervé Jourdan

Invasive rodents, an overlooked threat for skinks in a tropical island hotspot of biodiversity

Keywords: diet analysis, endemic skinks, island conservation, Rattus exulans, Rattus rattus

come   L'archive ouverte pluridisciplinaire

Introduction

The main causes of biodiversity loss are now well identified: habitat fragmentation and destruction, invasive species and climate change [START_REF] Sala | Global biodiversity scenarios for the year 2100[END_REF][START_REF] Pereira | Global biodiversity change: the bad, the good, and the unknown[END_REF]. [START_REF] Sax | Species invasions and extinction: the future of native biodiversity on islands[END_REF] and [START_REF] Tershy | The importance of islands for the protection of biological and linguistic diversity[END_REF] have shown that, over the last 500 years, the majority of documented plant or vertebrate extinctions have occurred on islands. This pattern is especially true of extinctions caused by exotic species [START_REF] Bellard | Alien species as a driver of recent extinctions[END_REF]. [START_REF] Walsh | Exotic species richness and native species endemism increase the impact of exotic species on islands[END_REF] showed that species richness and endemism increase the impact of exotic species on islands.

Rodents are among the most widespread commensal invasive species. They have been introduced to over 80% of the world's islands, with many harmful impacts [START_REF] Atkinson | The spread of commensal species of Rattus to oceanic islands and their effects on island avifaunas[END_REF]. We know that these opportunistic species are responsible for species depletion and/or extinction worldwide, especially in insular ecosystems [START_REF] Courchamp | Mammal invaders on islands: impact, control and control impact[END_REF]Harper & Bunburry 2015). Invasive rodents can severely affect turtles and seabirds [START_REF] Caut | Dietary shift of an invasive predator: rats, seabirds and sea turtles[END_REF][START_REF] John | Population and habitat of the St Lucia Whiptail Lizard (Cnemidophorus vanzoi) on Praslin Island, St Lucia (West Indies). Report to St Lucia National Trust[END_REF], amphibians [START_REF] Mccallum | Evidence of predation by kiore upon lizards from the Mokohinau Islands[END_REF]) and mammals [START_REF] Hanna | Island mammal extinctions are determined by interactive effects of life history, island biogeography and mesopredator suppression[END_REF][START_REF] Smith | Disease and competition, not just predation, as drivers of impacts of the ship rat (Rattus rattus) on island mammals[END_REF]. Several studies have shown that Rattus rattus is the most damaging rodent for island ecosystems [START_REF] John | Population and habitat of the St Lucia Whiptail Lizard (Cnemidophorus vanzoi) on Praslin Island, St Lucia (West Indies). Report to St Lucia National Trust[END_REF][START_REF] Ruffino | Invasive rats and seabirds after 2,000 years of an unwanted coexistence on Mediterranean islands[END_REF][START_REF] Traveset | A review of the effects of alien rodents in the Balearic (western Mediterranean Sea) and Canary Islands (eastern Atlantic Ocean)[END_REF][START_REF] Banks | A review of the evidence for potential impacts of ship rats (Rattus rattus) on wildlife and humans in Australia[END_REF][START_REF] Shiels | Biology and impacts of Pacific Island invasive species 11. Rattus rattus, the ship rat (Rodentia: Muridae)[END_REF], and introduction of this species is followed by the greatest number of declines or extinctions of native island biota, according to [START_REF] Towns | Have the harmful effects of introduced rats on islands been exaggerated?[END_REF]. [START_REF] Case | The role of introduced species in shaping the distribution and abundance of island reptiles[END_REF] revealed that island reptiles are highly threatened by introduced species, especially on islands that were predator-free before introductions. They found that rodents consume reptile eggs, young, and even adults, depending on prey size. These authors record several island reptile declines (lizards, tuatara) induced by the introduction of R. rattus, R. exulans or R. norvegicus. The blue-tailed skink (Cryptoblepharus egeriae) is assumed to have declined because of rodent predation on Christmas Island [START_REF] Low | Home ranges of introduced rats on Christmas Island: a pilot study[END_REF], which is also the case for the endemic Belize leaf-toed gecko (Phyllodactylus insularis) on Half Moon Caye [START_REF] Meerman | Half Moon Caye, Belize: terrestrial survey results and management implications[END_REF] and the whiptail lizard (Cnemidophorus vanzoi) on St Lucia [START_REF] John | Population and habitat of the St Lucia Whiptail Lizard (Cnemidophorus vanzoi) on Praslin Island, St Lucia (West Indies). Report to St Lucia National Trust[END_REF]). On Rodrigues Island, large gecko species (Phelsuma edwardnewtoni and P. gigas) were devastated after rat introductions [START_REF] Vinson | The saurian fauna of the Mascarene Islands[END_REF]. The only two native lizards of Lord Howe Island (Phyllodactylus guentheri and Leiolopisma lichenigerum) are also known to have declined after the introduction of the black rat R. rattus in 1918 [START_REF] Cogger | The reptiles of Lord Howe Island[END_REF]. Several New Zealand skink and gecko species are now confined to rat-free offshore islands, but bone deposits indicate that they were widespread on the mainland before rats were introduced [START_REF] Worthy | Osteological observations on the larger species of the skink Cyclodina and the subfossil occurrence of these and the gecko Hoplodactylus duvaucelii in the North Island, New Zealand[END_REF]). Furthermore, in New Zealand, skink and gecko species showed habitat plasticity after rat introduction [START_REF] Whitaker | The ecology and control of rodents in New Zealand nature reserves[END_REF][START_REF] Hoare | Avoiding aliens: behavioural plasticity in habitat use enables large, nocturnal geckos to survive Pacific rat invasions[END_REF] or removal [START_REF] Towns | Response of lizard assemblages in the Mercury Islands, New Zealand, to removal of an introduced rodent: the kiore (Rattus exulans)[END_REF][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]. More recently, [START_REF] Harper | Invasive rats on tropical islands: their population biology and impacts on native species[END_REF] reviewed the impacts of invasive rats on islands, pointing out how herpetofauna features in the rodent diet. Even though island reptile records pre-dating rat introduction are often unavailable, the studies mentioned above, together with historical observations, point to rodents and other introduced species being important threats to the maintenance of island reptile communities over time. Tropical regions and oceanic islands have the highest proportion of threatened squamata species, according to [START_REF] Böhm | The conservation status of the world's reptiles[END_REF]. In particular, New Caledonia is considered by these authors as a world centre of threatened reptile species.

New Caledonia is one of the world's 35 biodiversity hotspots [START_REF] Myers | Biodiversity hotspots for conservation priorities[END_REF][START_REF] Meerman | Half Moon Caye, Belize: terrestrial survey results and management implications[END_REF]. Located east of Australia in the Southwest Pacific, this tropical island hosts an outstanding squamata fauna, including 63 scincidae species (61 endemic), 36 diplodactylidae gecko (all endemic) and six geckonidae gecko species. This herpetofauna is of high biodiversity concern, given the exceptional endemism rates (91.6%) and the unfavourable conservation status for most of the species, with 66 taxa considered as endangered (CR, EN or VU) [START_REF] Whitaker | Skinks and geckos from New Caledonia. The IUCN Red List of Threatened Species[END_REF]Bauer et al. 2012a, b;[START_REF] Sadlier | A new species of scincid lizard in the genus Caledoniscincus (Reptilia: Scincidae) from northwest New Caledonia[END_REF][START_REF] Sadlier | A new species of lizard in the genus Caledoniscincus (Reptilia: Scincidae) from southern New Caledonia and a review of Caledoniscincus atropunctatus (Roux)[END_REF]Sadlier et al. , 2014a, b, c, d), b, c, d). This unique herpetofauna is facing high pressure from introduced alien species and human-induced habitat disturbances. Among invasives, four rodent species (R. rattus, R. exulans, R. norvegicus and Mus musculus) introduced into New Caledonia may be responsible for a significant disruption of native herpetofauna [START_REF] Beauvais | Les espèces envahissantes dans l'archipel néo-calédonien: un risque environnemental et économique majeur[END_REF]. The Pacific rat (R. exulans) was introduced into New Caledonia by Melanesian people between 2400 and 3300 years before present [START_REF] Balouet | Paléontologie des vertébrés terrestres de Nouvelle-Calédonie et paléobiogéographie du Pacifique sud ouest. Unpublished Thèse de troisième cycle[END_REF][START_REF] Balouet | Paléontologie des vertébrés terrestres de Nouvelle-Calédonie et paléobiogéographie du Pacifique sud ouest. Unpublished Thèse de troisième cycle[END_REF]. The ship rat (R. rattus), brown rat (R. norvegicus) and house mouse (M. musculus) arrived more recently with European settlers around 1850 [START_REF] Pascal | Les peuplements néo-calédoniens de vertébrés: invasions, disparitions[END_REF]. Rattus rattus and R. exulans are now widely distributed throughout the Caledonian archipelago [START_REF] Rouys | Factors determining the distribution of introduced mammals in nature reserves of the southern province, New Caledonia[END_REF][START_REF] Pascal | Les peuplements néo-calédoniens de vertébrés: invasions, disparitions[END_REF]) and are often sympatric, a situation widespread in Pacific islands but not known outside the region.

A better understanding of introduced rodent impacts on communities of squamata, especially in New Caledonia, as suggested by [START_REF] Bauer | Study of Amphibians and Reptiles in cooperation with the Institut de Recherche pour le Développement[END_REF], would appear to be vital to squamata conservation. In particular, the contribution of small terrestrial reptiles in invasive rodents' diet remains poorly investigated. The lack of experimental studies demonstrating rodent impacts and response to control methods contribute to explaining this lack of information.

The present study investigates the impacts of introduced rodents (ship rat and Pacific rat) on terrestrial reptiles. We explore a more efficient protocol of diet analysis to assess lizard consumption by rats, going beyond traditional analysis of stomach contents alone, as suggested in similar studies [START_REF] Lewis | Conservation implications of small Indian mongoose (Herpestes auropunctatus) predation in a hotspot within a hotspot: the Hellshire Hills, Jamaica[END_REF]. Using our own lab reference collection and an associated determination key of reptile remains, we evaluate reptile consumption occurrences. Finally, we considered management options in line with the conservation status of specific consumed species, from full removal to exclusion fencing.

Materials and methods

Study sites

Two distinct sites were selected for this study, both on ultramafic substrates (Fig. 1). On each site, rodents were sampled in two contrasting vegetation types: dense and open forest. The Goro site is located near Goro village, in the southern province of New Caledonia (22° 10′ 03″ S; 166° 55′ 36″ E). Fifteen skink (14 endemic) and 14 gecko (11 endemic) species are known to be present at this site (Table 1), of which seven and three species respectively are listed as threatened on the IUCN red list. The dense forest we selected is dominated by Arillastrum gummiferum, hosting 90% endemic plant species, with a canopy height of around 20 m [START_REF] Jaffré | Inventaire floristique des unités de végétation de la zone d'implantation des infrastructures minières et industrielles de Goro Nickel[END_REF]). The open forest is dominated by Gymnostoma deplancheanum and Tristaniopsis spp., rising to a height of 6-10 metres [START_REF] Jaffré | Identification, typologie et cartographie des groupements végétaux de basse altitude du Grand Sud Calédonien et de la vallée de la Tontouta[END_REF].

The Tiebaghi site is in the northern province of New Caledonia (20° 33' 43" S, 164° 15' 43" E) on Tiebaghi Mountain. Thirteen skink (all endemic) and eight gecko species (five endemic) are present, of which four and three species respectively are listed as threatened (Table 1). The dense forest is in a talweg and rises to a height of 20-25 m. The open forest habitat hosts few endemic arboreal species, with a canopy around 10 m and several endemic shrub species [START_REF] Dagostini | Inventaire et caractérisation de la flore et des groupements végétaux du massif de la Tiébaghi[END_REF].

Rodent trapping method

As in [START_REF] Cunningham | Guide to the identification and collection of New Zealand rodents[END_REF], rodents were caught using lethal traps ('Trapper' snap traps for rats and 'E' mouse traps for mice, Pest Management Services, NZ). Trapping sessions involved 25 pairs of rat and mouse traps set at 25 m intervals along a 625 m transect in each of the habitats. The traps were baited with cheese (cheddar) and peanut butter. The mouse traps were placed under plastic tunnels to reduce the risk to non-target species.

We trapped rodents twice a year at each site in both forest types. Overall, four trapping sessions were conducted at each site and habitat. Trapping sessions in Goro were held in April and August 2013, and in January and March 2014. Trapping sessions in Tiebaghi were held in February, June and October 2013, and February 2014. Each trapping session lasted 4 days, i.e. 3 consecutive nights. Traps were baited just before nightfall and checked every morning. Following [START_REF] Cunningham | Guide to the identification and collection of New Zealand rodents[END_REF], each rat caught was weighed, sexed and measured for head-body length, tail length and reproductive status.

Rodent diet analysis

The digestive tracts of all the rodents caught were collected in the field, preserved in 70% ethyl alcohol [START_REF] Sugihara | Abundance and diets of rats in two native Hawaiian forests[END_REF] and frozen for further laboratory analysis. The contents were analysed according to the method described in [START_REF] Abbas | Régime alimentaire d'un phytophage introduit, le ragondin (Myocastor coypus, Molina 1782) dans différents types de marais aménagés[END_REF]. Stomach and caecum were opened by incising the convex surface; the interior was scraped and the contents were poured into a 0.5 mm sieve. The contents were rinsed with tap water until the filtrate appeared clear, then placed in a Petri dish with Table 1. List of squamata recorded in each studied site. IUCN status according to [START_REF] Whitaker | Skinks and geckos from New Caledonia. The IUCN Red List of Threatened Species[END_REF]Bauer et al. (2012); [START_REF] Sadlier | A new species of lizard in the genus Caledoniscincus (Reptilia: Scincidae) from southern New Caledonia and a review of Caledoniscincus atropunctatus (Roux)[END_REF]Sadlier et al. ( , 2014[START_REF] Whitaker | Skinks and geckos from New Caledonia. The IUCN Red List of Threatened Species[END_REF]. Habitat: F: forest; MP: forested shrubland; M: Maquis shrubland. Status: E: endemic species; EE: endemic genera; A: native species; I=introduced species. Highlighted species were preyed-upon in this study.

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Scincidae

Status We first analysed the entire digestive tracts of 20 individuals (five individuals of each sex per species) in order to determine which organs provided the most information on skink consumption. Four rats contained skink remains: three scales were found in small intestines, six in colons, 10 in stomachs and more than 60 in caecums. Each time we found scales in a gut, they were present in at least the stomach and the caecum. Therefore we focused our analysis on both stomach and caecum contents (separately) in order to maximize the probability of detecting reptile remains in the most time-efficient way.

Remains found in stomach and caecum were classified into nine categories: plants, insects, crustaceans, gastropods, earthworms, birds, parasites, geckos and skinks. Particular attention was given to the identification of skink remains up to species level, using our laboratory reference collection of skink scales. This collection consists of scales extracted from identified skinks and mounted for observation under microscope. We also used a skink scale determination key developed by our team during this project [START_REF] Jourdan | Seabird recovery and vegetation dynamics after Norway rat eradication at Tromelin Island, western Indian Ocean[END_REF]. Essentially, identifications were based on small groups of well-preserved scales sharing the same morphological patterns.

Data analysis

For each site, we calculated a relative abundance index based on the number of rats trapped during 100 trap nights, following the method described in [START_REF] Nelson | Correction for sprung traps in catch/effort calculations of trapping results[END_REF]. All traps were checked every morning of each trapping session and the 'state' of the trap was recorded. If it was closed without a rat or still open without bait, the trap was considered as active for half a night only. The relative abundance index was then corrected by subtracting half a trap night for each of these traps.

For each item found during the diet analysis, we calculated an occurrence frequency by species and organ (stomach or caecum) as the number of individuals/organs containing the item divided by the total number of individuals analysed. We then related these frequencies to the skink conservation status, in order to determine those skink species potentially more sensitive to predation by introduced rodents.

We used the χ 2 test of independence in R version 3.0.1 (R Development Core Team 2008) to determine whether skink remains occurrence frequencies differed between sites, habitats, rat species, sexes and maturity levels. When there were fewer than five samples, the Fisher exact test was used [START_REF] Fisher | Statistical tables for biological, agricultural and medical research[END_REF].

Results

A total of 135 Pacific rats (R. exulans) and 50 ship rats (R. rattus) were captured during the four trapping sessions at the Goro site, while 35 individuals of R. exulans (Re) and 63 R. rattus (Rr) were trapped at the Tiebaghi site (Table 2). The relative abundance index for Pacific rats was higher at Goro than at Tiebaghi (χ 2 = 12.7, df = 1, p = 3.6 × 10 -4 ), as shown in Figure 2. We recorded 24 Re.100 trap night -1 and 6 Re.100 trap night -1 at the two sites respectively. For ship rats, no significant difference was found between the two sites (11 Rr.100 trap night -1 in Goro; 12 Rr.100 trap night -1 in Tiebaghi). A total of 283 gut samples were taken for diet analysis (one rat was too damaged to be analysable). No mice were trapped at any site during this study.

Table 2. Frequency of occurrence (in %) of skink remains in the digestive tracts (both stomach and caecum) of ship rats (R. rattus) and Pacific rats (R. exulans). Percentages were calculated dividing the number of rat samples that contained skink remains by the corresponding sample size (n). Split percentages were rounded to one decimal place.

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Rattus exulans

Rattus rattus Total Analysis of both stomachs and caecums led to significantly better detection of skink remains than did stomach analysis alone (χ 2 = 9.9; df = 1; p = 0.0017). Combined analysis of these two digestive organs improved our detection capacity twofold compared with the 'classical' approach focused on the stomach alone (Fig. 3). Independent analyses of these two organs also suggest that caecums contained more skink remains than stomachs (χ 2 = 6.04; df = 1; p = 0.014). Full results of the diet analysis are presented in Table 3. Plant items were present in 96.5% and 48.2% of ship rats and Pacific rats, respectively. Insects were found in inverse proportion: 93.5% in Pacific rats and 56.6% in ship rats. Feather remains were found in ship rats only (n=8), four cases at each site.

Squamata remains were found in 19.1% of analysed rats (n=283, Table 3). Of the rats, 15.9% had fed on skinks and 4.6% on geckos; 86% had consumed only one skink species, 13.9% had fed on two species. One Pacific rat contained remains of three species and one ship rat contained remains of seven skink species. No significant differences in skink predation by rats were found between the two sites (χ 2 = 0.02; df = 1; p = 0.89), but skink consumption by the Pacific rat (18.2%; n=170) was slightly more frequent compared with ship rat (11.5%; n=113; χ 2 = 2.72; df = 1; p = 0.099). Rodents' maturity, sex and habitat did not significantly influence skink consumption rate (all analyses in Appendix S1 in Supplementary Material).

We were able to identify 91% of detected preyed skink individuals at species level. In this study, at least 12 skink Table 3. Percent frequency of occurrence of remains present in digestive tracts of ship rats (R. rattus) and Pacific rats (R. exulans) on the two study sites. As some skink species were restricted to one of the two study sites, occurrences were corrected with the number of rodents trapped at the corresponding site. *Conservation status in UICN red list. Sample sizes in brackets. species were found to be preyed upon by rats (Table 3). Of these skink species 11 are endemic to New Caledonia and six are considered threatened, according to the IUCN red list (Table 3). Of the skink species known to be present, 38.5% and 46.6% were consumed at Tiebaghi and Goro sites, respectively. It is noteworthy that Kanakysaurus viviparus (endangered) and Grasciliscincus shonae (vulnerable) are among the three most preyed-upon species by the two rat species. Our improved diet analysis method also revealed predation upon three other vulnerable species at Goro site (Caledoniscincus notialis, Simiscincus aurantiacus and Nannoscincus mariei), although they were not among the most frequently consumed species. The analyses of caecal contents also allowed us to detect the presence of scales from Marmorosphax taom (critically endangered) in one ship rat from the Tiebaghi site.

Discussion

Rodent diet analysis

Analysis of caecal contents enabled us to determine rat diet over several food intakes more precisely than previous studies that focused on stomach analysis alone. Due to an estimated mean transit time of 12 hours in rats (Platel & Crinivasan 2001), stomachs only contain remains from very few food intakes, whereas the caecum, as a digestive dead-end, accumulates remains from multiple food intakes, mainly remains difficult to digest, like fibres, cuticle fragments or scales. Combined analysis of stomach and caecum allowed us to improve our skink remains detection capacity twofold. Therefore we recommend including the analysis of this digestive organ in future rodent diet analysis. We found no skink head bones or jaws in any rat digestive contents. This observation may indicate that rats do not eat skink heads, as other skeletal parts were found in some samples. It also confirms the importance of developing a morphological identification tool based on body scales instead of bones to measure skink predation by rodents. DNA-based methods of diet analysis are increasingly popular and may significantly improve prey detection and identification in the rodent diet [START_REF] Egeter | Detecting frogs as prey in the diets of introduced mammals: a comparison between morphological and DNA-based diet analyses[END_REF][START_REF] Zarzoso-Lacoste | Improving morphological diet studies with molecular ecology: an application for invasive mammal predation on island birds[END_REF]). But such methods require access to prey DNA, which is not possible in our context because of the rarity and patrimonial status associated with most Caledonian skink species. However, we were able to identify about 91% of detected preyed skink individuals to species level due to morphological analysis of scales, as recommended in [START_REF] Fuchs | The reptile skin: a key-feature in the identification of lizards and snakes[END_REF]. This approach is already used to control the reptile skin trade [START_REF] Fuchs | The reptile skin: a key-feature in the identification of lizards and snakes[END_REF] and we believe that the development of scale identification tools represents a major methodological improvement that will lead to better assessment of invasive rodent impacts on island endemic reptiles.

Introduced rats' impact on native skinks

Our results show frequencies of occurrence of skink remains of about 18.2% for R. exulans and 11.5% for R. rattus samples. These frequencies of occurrence are higher than most previously reported in the literature on island rats' diet. In their work on Surprise Islet, located northwest of New Caledonia, [START_REF] Caut | Dietary shift of an invasive predator: rats, seabirds and sea turtles[END_REF] found remains of one skink species (C. haplorhinus; [START_REF] Bauer | Caledoniscincus austrocaledonicus (Reptilia: Scincidae) from Ile Surprise, D'Entrecasteaux Reefs, New Caledonia[END_REF][START_REF] Sadlier | The scincid lizard genus Caledoniscincus (Reptilia: Scincidae) from New Caledonia in the southwest Pacific: a review of Caledoniscincus austrocaledonicus (Bavay) and description of six new species from Province Nord[END_REF] in 13% of the digestive tracts (stomach and droppings) of ship rats they caught and analysed. In Tasmania, [START_REF] Norman | Food preferences of an insular population of Rattus rattus[END_REF] found remains of skinks in a single ship rat's stomach out of the 254 samples analysed. In 1990, [START_REF] Newman | Seasonal fluctuations of numbers, breeding, and food of kiore (Rattus exulans) on Lady Alice Island (Hen and Chickens Group), with a consideration of kiore/tuatara (Sphenodon punctatus) relationships in New Zealand[END_REF] analysed the stomachs of 134 Pacific rats on offshore islands in New Zealand: only three of them contained skink remains. More recently in Hawaiian forests, a diet study on R. rattus, R. exulans and M. musculus conducted by [START_REF] Shiels | Dietary niche differentiation among three species of invasive rodents (Rattus rattus, R. exulans, Mus musculus)[END_REF] revealed the complete absence of any skink remains in the stomachs analysed (n = 12-95 individuals per species). Our result can be explained by the high skink species richness and abundance in the archipelago [START_REF] Bauer | Study of Amphibians and Reptiles in cooperation with the Institut de Recherche pour le Développement[END_REF] combined with the likely improved estimation of predation rates in our study due to caecum content analysis.

For the two sites considered in this study, at least 12 different skink species among the 23 known to be present (c. 52%) were identified in the gut contents of rats. Six of these prey species are under threatened conservation status according to the IUCN Red List criteria [START_REF] Whitaker | Skinks and geckos from New Caledonia. The IUCN Red List of Threatened Species[END_REF][START_REF] Sadlier | A new species of lizard in the genus Caledoniscincus (Reptilia: Scincidae) from southern New Caledonia and a review of Caledoniscincus atropunctatus (Roux)[END_REF]. Two worth mentioning are Marmorosphax taom and Kanakysaurus viviparus, endemic and restricted to a few mountains in the north of the mainland. In addition, four vulnerable species are consumed by rats. These observations, based on only two sites over small areas in New Caledonia, underline the importance of taking introduced rat predation into consideration in plans for the conservation of native skink communities on islands.

We found no remains of the arboreal skink species Epibator nigrofasciolatus (which is a very common species in every habitat in New Caledonia) in either ship or Pacific rats. Moreover, geckonid (arboreal) remains were found as frequently in Pacific rats (n=7) as in ship rats (n=6). This lack of arboreal prey species is unexpected, because the 'arboreal' lifestyle of R. rattus [START_REF] Harper | Invasive rats on tropical islands: their population biology and impacts on native species[END_REF] could have been expected to lead to greater consumption of arboreal squamata prey by this species. Although the relative abundance of the two rodent species is not identical at our two study sites, their overall skink predation appears to be almost identical. We believe the small differences observed might be explained simply by the foraging behaviour of each rat species. The ship rat is arboreal with a diet mostly made of fruits and plant items [START_REF] Shiels | Biology and impacts of Pacific Island invasive species 11. Rattus rattus, the ship rat (Rodentia: Muridae)[END_REF], on the other hand, the Pacific rat is a ground forager that consumes more invertebrates and fewer plants than the ship rat [START_REF] Shiels | Dietary niche differentiation among three species of invasive rodents (Rattus rattus, R. exulans, Mus musculus)[END_REF]. Our dietary analysis reveals the same pattern and accurately describes the differences in the diet of the two species; for example, feather remains were found in ship rats only, whereas earthworms were more abundant in Pacific rat guts.

We found slightly more squamata remains in R. exulans than in R. rattus in this study, as Pacific rats are smaller, ground foragers. However, more samples from other habitat types are needed to confirm this difference. Prey size does not seem to vary with the size of the rodent predator. The preyed-upon skinks in our study (42-102 mm) were in the middle of the size range of the species present at these sites (25-200 mm), and the biggest of these species was consumed equally by both rat species. The skink species most frequently consumed are nocturnal, which corresponds to the rodent activity period and could explain the high rates of occurrence of these species in rat diets. Prey species active at night could therefore be more sensitive to rat predation risk than diurnal species, as observed by [START_REF] Towns | Response of lizard assemblages in the Mercury Islands, New Zealand, to removal of an introduced rodent: the kiore (Rattus exulans)[END_REF].

Cumulative impacts have been suggested for sympatric introduced rodents [START_REF] Montgomery | Invasion meltdown': evidence for unexpected consequences and cumulative impacts of multispecies invasions[END_REF]). Here, our observations reveal that the rodent pressure is not homogeneous across the different skink species. We found Kanakysaurus viviparus and Graciliscincus shonae to be the most consumed skinks (≥6% of rats) globally and per rodent species, while the others skinks were found in less than 3% of rats. An associated study explored the abundance of each skink species at the two sites using sticky-trap grids [START_REF] Jourdan | Seabird recovery and vegetation dynamics after Norway rat eradication at Tromelin Island, western Indian Ocean[END_REF]). This study revealed that K. viviparus was the fourth most abundant species at Tiebaghi and G. shonae, a cryptic species living in the litter, has never been caught on sticky traps. This observation may indicate that some skink species are preferentially preyed upon by rodents. This pattern being shared by the two rodents means there could be a cumulative predation pressure upon some skink species. Unfortunately, two of the three most consumed species have threatened conservation status, which reinforces the stakes of this cumulative negative impact on skinks.

Our results confirm predation by introduced rodents to be a potentially important issue for skink conservation in New Caledonia, as some threatened species appear to be regularly preyed upon. However, this study examined only two sites in forests on ultramafic soil and it would be interesting to investigate introduced rodents' diet in other habitats like primary rainforest, dry forests, littoral habitats and some of the multiple islets surrounding New Caledonia. All these habitats are known to host endemic squamata, among the richest and most endemic of the archipelago's terrestrial fauna [START_REF] Pascal | Mining and other threats to the New Caledonia biodiversity hotspot[END_REF]. Moreover, the lack of ecological information on most of the New Caledonian skink species highlights the need to conduct research in order to better understand the pressures acting upon them. Particular attention needs to be paid to micro-endemic species (short range endemism) with unfavourable conservation status, especially because the three main types of pressures on biodiversity [START_REF] Pereira | Global biodiversity change: the bad, the good, and the unknown[END_REF] coexist in New Caledonia: (1) habitat destruction and fragmentation induced by mining activities, wildfires and urbanisation; (2) impact of multiple invasive species (rodents, feral cats, ants, etc.); and (3) effects of climate change. As elsewhere in the world, and particularly in tropical islands, addressing the second category of threats is perhaps the most feasible task in the short term. Control of new introductions is essential at points of entry (e.g. airports, harbours), but action can be taken against the species already present, especially in the areas most sensitive to biological invasions. According to [START_REF] Blackburn | A proposed unified framework for biological invasions[END_REF]'s proposals, rodent impacts could be addressed either by eradication or by mitigation. Rodent eradications have been implemented on several islands, sometimes leading to the recovery of seabirds, invertebrates, plants, forest birds and even lizards [START_REF] Towns | Response of lizard assemblages in the Mercury Islands, New Zealand, to removal of an introduced rodent: the kiore (Rattus exulans)[END_REF][START_REF] Smith | Recovery of native species following rat eradication on Mokoli'i Island, O'ahu, Hawai'i[END_REF][START_REF] Towns | Have the harmful effects of introduced rats on islands been exaggerated?[END_REF]Le Corre et al. 2015;[START_REF] Jones | Invasive mammal eradication on islands results in substantial conservation gains[END_REF]. However rodent eradication appears to fail more often in tropical areas and eradications on human-inhabited islands remain a challenge [START_REF] Russell | Tropical island conservation: rat eradication for species recovery[END_REF]. Therefore, for the New Caledonian mainland (18 575 km 2 ), a complete removal is almost impossible given current rodent control methods. However, control and monitoring of rat populations at particularly sensitive sites can be implemented, so as to limit both population sizes and impacts (Q. Duron pers. comm.). At the Tiebaghi site for example, the rat impact upon K. viviparus (En) and M. taom (CR) is alarming and may warrant an urgent control program. According to [START_REF] Norbury | Pest fencing or pest trapping: a bio-economic analysis of costeffectiveness[END_REF]'s observations on native skink species recovery in New Zealand, exclusion fences were the most cost-effective option while considering small patches below 1 ha. Considering medium-sized patches from 1 to 219 ha, the same authors highlighted leaky fences as the 'best' solution, whereas lethal trapping appeared most efficient for areas larger than 219 ha, but recent control methods such as A24 self-resetting traps are promising for multi-species control on islands [START_REF] Carter | Controlling sympatric pest mammal populations in New Zealand with self-resetting, toxicant-free traps: a promising tool for invasive species management[END_REF]. As suggested in Doherty and Ritchie (2016), managers must consider the whole range of management approaches, methods, contexts and objectives in order to make control actions the most efficient. For example, in a tropical island hotspot of biodiversity threatened by mining activities, rodent control programs could also be implemented in a biodiversity offsetting perspective as suggested in [START_REF] Holmes | The potential for biodiversity offsetting to fund invasive species eradications on islands[END_REF]. While the support of the population is absolutely essential in human-inhabited contexts [START_REF] Glen | Eradicating multiple invasive species on inhabited islands: the next big step in island restoration?[END_REF], we believe that it is also fundamental to the conservation of native biodiversity on all islands. As underlined by [START_REF] Pascal | Mining and other threats to the New Caledonia biodiversity hotspot[END_REF], joint involvement is required in New Caledonia if we are to preserve a unique reptilian fauna from growing threats.

Figure 1 .

 1 Figure 1. Location of New Caledonia and our two study sites on the New Caledonian mainland.

Figure 2 .

 2 Figure 2. Results of trapping sessions, observed relative abundance indexes. ***: p < 0.01.

Figure 3 .

 3 Figure 3. Frequency of occurrence of skink remains calculated for stomach and caecum contents analysed separately and together; **: 0.01 < p < 0.05, ***: p < 0.01.

Acknowledgments

This study was part of the RMines project funded by 'CNRT Nickel et son environnement'. We express our gratitude to SLN and Vale mining companies for access to the study sites. We want to thank Ross Sadlier and the Australian Museum of Sydney for their help and access to the reptile collection. Thanks to Leo Debar and Helene De Meringo for their contribution to the analysis of scale morphology and to Frederic Rigault, Marc Manceau, Jean Claude Hurlin, Adrien Zubillaga, Wegune Wainebengo, Gilbert Shouene, Virginie Ujicas and Annie Portal for their help in the field. We finally thank Des Smith, Rod Hitchmough and one anonymous reviewer for their useful criticism and suggestions for improvement.