Acta Herpetologica 14(2): 89-100, 2019 ISSN 1827-9635 (print) © Firenze University Press ISSN 1827-9643 (online) www.fupress.com/ah DOI: 10.13128/a_h-7746 Coping with aliens: how a native gecko manages to persist on Mediterranean islands despite the Black rat? Michel-Jean Delaugerre1,*, Roberto Sacchi2, Marta Biaggini3, Pietro Lo Cascio4, Ridha Ouni5, Claudia Corti3 1 Conservatoire du littoral, Résidence St Marc, 2, rue Juge Falcone F-20200 Bastia, France. *Corresponding author. Email: m.delaugerre@conservatoire-du-littoral.fr 2 Dipartimento di Scienze della Terra e dell’Ambiente, Università degli Studi di Pavia, via Taramelli 24, I-27100, Pavia, Italia 3 Sistema Museale di Ateneo - Museo di Storia Naturale dell’Università di Firenze, Sede “La Specola”, Via Romana 17, I-50125 Firenze, Italia 4 Associazione Nesos, via Vittorio Emanuele 24, I-98055 Lipari (ME), Italia 5 Tunisia Wildlife Conservation Society, Faculté des Sciences de Tunis, Université Tunis El Manar II Tu-2092, Tunisia Submitted on: 2019, 14th April; revised on: 2019, 16th June; accepted on: 2019, 24th June Editor: Uwe Fritz Abstract. How a native gecko manages to coexist with an alien rodent in the Mediterranean since thousands of years? What kind of eco-ethological adaptations or evolutionary adjustments enables this gecko to persist? The present study explores the interaction between the endemic European Leaf-toed gecko (Euleptes europaea) and the alien Black rat (Rattus rattus). In the last 30 years, we compared 26 populations inhabiting “rat” and “rat-free” islands and islets in Tunisia, Sardinia, Corsica and Southern France. Geckos’ populations can persist despite the occurrence of rats. In the presence of rats: 1) geckos’ average body size tends to decrease towards medium-sized individuals; 2) geckos shift their spatial behaviour avoiding to forage “in the open”; 3) geckos’ body condition is not affected by the presence of rats. Moreover, shortly after rats’ eradication, geckos’ population structure seems to change and larger sized geckos prevail while the spatial behaviour is much more conservative. The mechanisms driving the interactions between the two spe- cies still need to be explained. Rats could represent a stressor for geckos, compete for space, be pest vectors and even predators. Coexistence of natives and aliens requires adaptive plasticity and evolutionary adjustments. In contexts where the risk of reinvasion is high, eradication programs need to be carefully evaluated, since the arrival of “new rats” on an island could have much more damaging effects on the insular biota than those caused by the eradicated population. Keywords. Behavioural shift, disturbance, ecological plasticity, evolutionary processes, predation, rat eradication, Euleptes europea, Rattus rattus. In memory of Michel Pascal (1947-2013) INTRODUCTION The long term dynamics of insular biota reflect the interplay between recurrent immigration and extinction events (Brown and Lomolino, 2000). Mainly because of human expansion on global scale, the natural phenom- enon of sea dispersal has been greatly amplified favouring introductions and even biological invasions (Simberloff et al., 2013). In the last centuries, with a speeding up in the last decades human-induced dispersal of species has implied: a) huge acceleration of the introductions’ rate; b) great number of species involved; c) remote origins of some propagules; d) possible genetic modifications originated from captive breeding and farming activities. Insularity-associated features, such as high sensitivity to climate and sea level changes, high rate of speciation, immigration and extinction processes, naiveté of native species, make insular biota particularly fragile (Moser et al., 2018). 90 Michel-Jean Delaugerre et alii Over the last decades, conservation biology has emphasized the detrimental effects of introduced species (mostly mammals) on insular ecosystems, but little atten- tion has been paid to the understanding of the eco-etho- logical adaptations that enable a native species to persist despite the presence of aliens (Martin et al., 2000; Hoare et al., 2007; Ruffino et al., 2009). In systems with novel combination of species (native and introduced ones) that do not show co-adaptation, rapid evolutionary changes may occur (Berthon, 2015; Mooney and Cleland, 2001; Strauss et al., 2006; Sax et al., 2007; Stuart et al., 2014). On islands, in particular, morphological, ecological and behavioural shift are more easily detectable than in more complex systems (Simberloff, 1974). Within the Mediterranean, reptiles, notably lizards, are good tools to investigate insular evolution. These her- petological communities, which are currently found on continental islands and on many land-bridge islets, origin from the neighbouring continents or large islands before their isolation, dating back to the sea level raise of the last interglacials. Trans-marine dispersal is less frequent in the Mediterranean arid islands (Foufopoulos and Ives, 1999). Most of the endemic herpetofauna still remains on some big islands (e.g., Corsica, Sardinia) and relative sat- ellite islands, while on other major Mediterranean islands the endemic herpetofauna has been completely replaced by human-introduced species, and endemic species per- sist only on some of their satellite islands (Corti et al., 1999; Silva-Rocha et al., 2018, 2019). In the present work we investigated the interaction between the native European Leaf-toed gecko Euleptes europaea (Gené, 1839) and the alien Black rat, Rattus rat- tus (Linnaeus, 1758) on some small islands of the Western Mediterranean. The European Leaf-toed gecko, a West- ern Mediterranean endemic species listed in Appendix II of the European Habitat Directive, is a very old inhabit- ant of this region, presumably since several million years ago (Müller, 2001). This species underwent a process of historic extinction or steep demographic decline along the North-Western (Provence, France) and Southern (Tuni- sia) edges of its range (Delaugerre et al., 2011). The Black rat colonized the Western Mediterranean approximately 2000-2400 years ago (Thibault et al., 1987; Vigne and Val- ladas, 1996; Ruffino and Vidal, 2010). Thus, the coexist- ence between the two species might last since hundreds of geckos’ generations and thousands of rats’ generations, since Euleptes lives longer than rats (Salvidio et al., 2010 and ref. therein). According to Ruffino et al. (2009), in the Western Mediterranean, 74% of the islands between 1 and 5 ha (and 99% of islands larger than 30 ha) are colo- nized by Black rats. Among the human introduced spe- cies, rats can dramatically impact on island biodiversity (Recher and Clark, 1974; Atkinson, 1985; Courchamp et al., 2003; Bennett et al., 2005) and Black rats in particu- lar, have caused rapid extinctions on islands, as reported for New Zealand and the Hawaiian Archipelago (Towns et al., 2006; Drake and Hunt, 2009). Alien rats can predate and compete with native species and modify islands’ food chains (Traveset and Richardson, 2006). The worldwide success of the Black rat as colonizer is due to its ability to exploit a large range of habitats and resources (Jones et al., 2008), as well as to shift to seasonal resources (Caut et al., 2008). This plasticity is crucial to survive in poor insular ecosystems, characterized by strong resource variation. On small islets E. europaea and R. rattus are often the only sedentary vertebrates. They are both nocturnal, good climbers on rocky substrates, and they both forage on rocky outcrops and on low vegetation. Rat eradication programs are often focused on one or few emblematic species (Capizzi et al., 2010), without a comprehensive understanding of insular assemblages. Investigating the possible interactions between the Euro- pean Leaf-toed gecko and the Black rat the present work could provide useful data for long-term insular conserva- tion plans. In particular, we explored the effects of the Black rat on the Leaf-toad gecko population structure (analysed on the basis of the relative proportion of size classes), body condition and habitat use, comparing populations liv- ing on islands colonized by rats and islands free of rats. Because we expect rats to interfere with geckos’ habitat use, feeding habits and thermoregulation, geckos living on islands on which also rats live should show: 1) pop- ulation structure with smaller-sized individuals result- ing from a shorter lifespan; 2) poor body conditions; 3) modification of spatial behaviour by minimizing or even avoiding foraging in open spaces. MATERIAL AND METHODS Ecological or evolutionary responses of native species to aliens cannot be easily distinguished from the pre-existing eco- logical differences (Strauss et al., 2006). However, even if each micro insular population differs from the others (e.g., due to age of isolation, presence of competitors and/or predators, etc.), any detectable trend can provide strong correlative evidences. Study species Euleptes europaea is a small gecko (average snout-vent length, hereafter SVL, 38 mm; average adult weight 1.2 g; hatch- ling weight 0.25 g) endemic to the Western Mediterranean, mostly found on islands in rocky habitats. It is strictly noctur- 91Native gecko and Black rat on Mediterranean islands nal, spending daytime in narrow rocky crevices (opening 2-4 mm wide) where dorsal and ventral parts of the body are in contact with the rock. These crevices are also used for egg lay- ing (Salvidio et al., 2010). Achieving its sexual maturity at the age of 3 years (Salvidio and Delaugerre, 2003), it might live 6 to 8 years in the wild and its maximum longevity in captivity attains 21 years (F. Molle in Mertens, 1970). It feeds on flying and ground dwelling invertebrates. On islets, densities are higher than on the mainland and popula- tion size ranges from several hundreds to only few dozen adults. This gecko is able to live on tiny islets (hundreds of square meters) characterized by the presence of few vascular plants, where it represents the only sedentary vertebrate (Delaugerre and Cheylan, 1992). Body size greatly varies on islets with trends towards gigantism, dwarfism and variation of the sexual size difference (Delaugerre and Cheylan, 1992; Salvidio et al., 2010). Rattus rattus (average weight 170 g), is a good climber, both on rocks and trees, but not a long lasting swimmer and therefore considered with poor marine dispersal capacities (≤ 500 m) (Cheylan, 1988; Ruffino et al., 2009). In the Mediterra- nean, rats feed mainly on plants, avoiding or rarely eating halo- philous and nitrophilous ones (Cheylan, 1988; Cassaing et al., 2007). Black rats and Euleptes europaea are both nocturnal and share the same habitat. Study sites and data collection We focused on islands characterized by simple ecosystems, where strong interactions between the target species are more likely to occur, namely small islands characterised by seasonal shortage of food availability for rats, and higher probability of encounters between the two species. We selected 26 islands and islets (Table 1) throughout most of the Leaf-toed gecko range (Fig. 1). Islands were classified as “rat” vs “rat-free”, depending on the presence or absence of rats; islands where rats were erad- icated were considered as “rat-free”. The presence of rats was detected thanks to at least one of the following evidences: direct sightings, fresh and/or old faeces, remains of chewed olive seeds or plants, rats’ nests, rats’ urine scent. Past presence/absence of rats on islands was investigated through literature and observa- tions made by locals. Nine islands (the largest ones) were inhab- ited by rats probably since their early colonization (Abdelkrim et al, 2009), whereas 13 islands (the smallest and most remote ones, without edible plants) were presumably never inhabited by rats. On Lavezzu Island (Corsica) we collected data before and after rat eradication (performed in 2000): the island was considered as “rat” before the eradication and as “rat-free” after eradication (Table 1). Five islands with “transient” rat popula- tions (i.e., small islets close to a colonization source but lacking resources to support a permanent rat population) were treated as “rat”, because rat frequency on these islands is unknown. Sampling methods Observations were carried out in spring, summer and autumn from July 1983 until August 2016. Sampling sessions (n = 47) consisted of 1 up to 6 nights per island (Table 1). Active geckos were searched using battery-powered lamps starting one or two hours after dusk and lasted until dawn. Total sampling effort achieved more than 380 hours (Table 1). Geckos behav- iour was classified as follow: “in the open” when found on bare rocks, on the ground or on plants; “under cover” when found hidden by the vegetation at the base of rocks (Fig. 2). Distance from the ground (height of the first sight) was also measured. Geckos were carefully caught by hand and temporarily stored in bags; sex and adulthood was determined according to Delaugerre and Dubois (1985). Snout-vent length (SVL) of 1795 individuals was measured to the nearest 0.01 mm using a digi- tal calliper, weight (W) of 424 adults was recorded to the near- est 0.01g using a digital scale. Geckos were released in the area of original sighting. Body condition index (BCI) was calculated as from Bonnet and Naulleau (1994). Spatial behaviour of 1012 geckos (i.e., “in the open” or “under cover”), recorded for 24 sampling sessions carried out on 17 islets, was assessed for one hour “catch per-unit-effort” (CPUE). Statistical analyses Population structure. In order to assess whether the popu- lations structure of E. europaea was affected by the presence of rats, we compared gecko’s size between islands with and with- out rats. Since Leaf-toed geckos’ size varies among islands inde- pendently of rats, we firstly normalized body sizes by dividing each measure by the greatest observed value in order to have all values in the 0-1 range (Legendre and Legendre, 2012). Each individual was classified depending on its inclusion in the quar- tile of the distribution of the normalized body size as “small” (1st and 2nd quartiles), “medium” (3rd quartile), and “large” (4th quartile). Geckos’ population structure was assessed by com- puting the proportion of individuals per size class. Different population structures were computed for spring and autumn. Sample size included 1871 individuals from 24 islets (Mean val- ues ± SE: 78 ± 16); islands with less than 10 individuals meas- ured (San Bainzu, Piana and Porro) were excluded. Data were analysed using permutational multivariate analysis of variance, PERMANOVA (Anderson, 2001; McArdle and Anderson, 2001) on the basis of Euclidean distances among islands’ population structures. The PERMANOVA allows the multivariate informa- tion to be partitioned according to the full experimental design without any a priori assumption regarding the distributions of the original variables. The predictors were rat occurrence (yes/ no), season (spring/autumn), number of reptile species, number of sampling sessions (to account for repeated measures within an island), and island size. P-values were obtained by permuta- tion, and the number of permutations was set to 9999. Body condition. The effect of rat occurrence on the body condition of geckos was assessed using a linear mixed model in which the BCI was the dependent variable; rat occurrence, sex, and season were the fixed effects, whereas the island was the random factor accounting for repeated measuring. Spatial behaviour. Linear mixed models were also used to investigate the effect of rat occurrence on the habitat use by geckos. In a first analysis we checked if Leaf-toed geckos avoided 92 Michel-Jean Delaugerre et alii Table 1. Map identification numbers (see Figure 1) of the study islands, geographical region, surface, presence (“rat”) or absence (“rat-free”) of the Black rat, and number of reptile species. For each island we also indicate sampling dates (and number of sampling nights), sampling effort (in minutes), number of geckos measured (n Biometry), number of geckos for which activity data were recorded (n Activity). n° Islet/island Region Surface (ha) Rat status N sp reptiles Year (month): n nights Sampling effort (min) n Biometry n Activity 1 Gallo N_Tunisia 8.9 rat 2 2008 (05):3 1320 49 / rat 2010 (07):1 270 / 30 2 Toro Sardinia 13.22 rat free 3 2015 (06):1 195 34 45 3 Carpa Sardinia 0.4 rat 2 2011 (09):1 430 50 67 4 Porco Sardinia 4.8 rat 4 2012 (05):1 285 44 55 5 Spargiotto Sardinia 11.13 rat free 3 2014 (05):1 360 66 79 6* Lavezzu Corsica 62.73 rat 3 1986 (08):1 390 50 / rat free 2010 (06):3 550 40 41 rat free 2011 (06):3 427 67 41 rat free 2012 (06):2 720 66 / 7 Porraggia Grande Corsica 1.25 rat free 2 1985 (08):1 600 50 / 8 Porraggia piccola Corsica 0.61 rat free 2 1986 (08):1 410 59 / 9 Sperduto grande Corsica 0.92 rat free 1 1984 (10):2 785 40 / rat free 1986 (08):1 330 81 / rat free 2011 (06):1 435 43 43 10 Toro grande Corsica 1.62 rat free 2 1986 (08):1 360 59 / rat free 2005 (04):1 200 50 / rat free 2012 (07):1 140 / 52 rat free 2014 (07):1 250 48 59 11 1st islet NE of Toro piccolo Corsica 0.11 rat free 1 1986 (08):1 240 15 / 12* Vacca Corsica 0.65 rat free 2 1985 (08):3 1395 95 / rat free 2012 (07):1 165 8 15 13 Roscana Corsica 0.2 rat free 1 1986 (08):1 720 94 / rat free 2008 (10):3 2258 125 / rat free 2012 (09):2 824 122 / 14 Locca Corsica 0.79 rat 2 2010 (04):1 90 / 10 15 Mezzumare Corsica 35.66 rat 3 2012 (08):1 213 15 18 16 A Botte Corsica 0.52 rat free 1 2010 (09):3 630 35 43 rat free 2011 (06):6 990 / 62 17 Gargalu Corsica 20.06 rat 4 1985 (04):2 1060 50 / rat 1990 (07):2 505 19 / 18 Palazzu Corsica 0.47 transient 1 1986 (08):1 225 26 / 19 Palazzinu Corsica 0.1 transient 1 1985 (07):2 490 34 / 20 Porri Corsica 0.27 rat free 1 1983 (07):2 590 32 / rat free 1986 (07):1 335 69 / 21 Brocciu Corsica 1.11 transient 1 2012 (06):1 132 30 60 22 Giraglia Corsica 10.35 rat free 4 2000 (09):1 360 38 / rat free 2012 (08):2 406 33 33 rat free 2012 (10):1 230 / 50 rat free 2014 (07):1 230 / 19 rat free 2014 (10):1 190 36 38 rat free 2015 (08):1 220 / 32 23 Saint Ferreol Provence 0.45 rat 2 2016 (05):1 700 23 40 24 La Tradelière Provence 1.05 rat 2 2016 (05):1 460 23 37 25 Rascas Provence 0.76 transient 2 1985 (09):1 290 50 / 26* Gabinière Provence 3.42 transient 2 2003 (10):1 345 69 / transient 2016 (05):1 187 37 43 * On Lavezzu Island (n. 6) rats were eradicated in 2000; on the Toro islets (n. 10 and 11) in 1992, after 2-4 years of presence on the islets; on Vacca (n. 12) in January 2011, after having been detected in July 2010; on Gabinière (n. 26) rats were present in 1937, eradicated in 1966, rats re-invaded the island in 2010 and were eradicated again in 2014. The islets San Bainzu, Piana and Porro, all inhabited by rats, were escluded from the analysis because less than 10 geckoes were measured. 93Native gecko and Black rat on Mediterranean islands “in the open” microhabitats on islands with rats: the catch per unit effort (CPUE) was the dependent variable, while rat occur- rence, habitat type (“in the open” vs “under cover”), island size and the number of reptile species were the fixed predictors. We also added the rat occurrence × habitat type interaction to account for differential habitat use between “rat” and “rat-free” islands. We could not include in the model the season as all but three islands were sampled in spring, and the island entered the model as random effect to control for double measuring within island (i.e., CPUE in open and close microhabitat). Sample size included 16 islands for which we collected more than 10 individ- uals (n = 1001, Mean values ± SE: 62 ± 10) for computing CPUE indexes for both “in the open” and “under cover” microhabitats. In a second analysis we checked if the height above the ground of the first sighting differed in islands with or without rats, using rat occurrence, season, number of reptile species, island size, and the rat occurrence × season as fixed predictors. As in the previ- ous analysis, the islands entered the model as random effect, and geckos observed “under cover” were excluded. The sample for this last analysis included 469 geckos from 14 islands. Analyses were performed using the package lme4 (Bates et al., 2014) in R ver. 3.2.4 (R Core Team, 2018), and otherwise stated, data reported are means ± standard error. RESULTS Population structure The PERMANOVA showed that the population struc- ture significantly varied in response to rat occurrence, season, number of reptile species living on the island, but was invariant in respect to the sampling effort (Table 2). In particular, a) the structure of geckos’ populations liv- ing on the islands with rats showed a larger proportion of medium-sized individuals and a lower proportion of large-sized geckos compared to populations occurring on “rat-free” islands (Fig. 3a); b) irrespective of rat occur- rence, smaller geckos were frequent in spring, while larger ones prevailed in autumn, and the relative abundance of medium-sized individuals did not vary between seasons (Fig. 3b); c) the greater was the number of reptile spe- cies on the island, the smaller was the frequency of larger geckos and the higher the number of medium-sized indi- viduals (Fig. 3c). Moreover, large geckos were more fre- quent on islets rather than on islands (Fig. 3d). Body condition Body condition of the Leaf-toed geckos did not dif- fer between “rat” and “rat-free” islands (0.0292 ± 0.0005 Fig. 2. Vegetation (mostly Lotus cytisoides) covering the base of the granite boulders inhabited by Euleptes europaea on Spargiotto Islet (Maddalena Archipelago). May 2012. Fig. 1. Geographic range of Euleptes europaea and studied popula- tions. Numbers refers to Table 1. Table 2. Results of permutational ANOVA for the variability of the population structure of the Leaf-toed geckos in response to rat occurrence, season and number of reptile species after having been controlled for island size and number of sampling years. P-values are computed with 9999 permutations. Variable df F R2 P Rats 1,24 3.916 0.08 0.0374 Islet size 1,24 3.775 0.08 0.0374 Season 1,24 10.69 0.23 0.0010 N. reptiles 1,24 3.975 0.08 0.0336 N. replicates 1,24 0.0033 <0.01 0.9958 94 Michel-Jean Delaugerre et alii and 0.0285 ± 0.0002 respectively, F1,5 = 0.0053, P = 0.94), either between males and females (0.0282 ± 0.0003 and 0.0292 ± 0.0004 respectively, F1,413 = 2.545, P = 0.11), or season (autumn: 0.0269 ± 0.0004; spring: 0.0295 ± 0.0003, F1,5 = 0.608, P = 0.46) (Table 3). By contrast, the random effect (island) was highly significant (L-ratio χ² = 24.0, d.f. = 1, P < 0.001), and the effect (σ = 0.0021) accounted for 32% of the total variance, suggesting that body condition is highly dependent on the island features. Spatial behaviour Leaf-toed geckos were more active on “rat-free” islands than on “rat” islands (values of CPUE being 7.85 ± 2.35 and 6.33 ± 2.30, respectively), but this difference was significantly depending on the habitat (rat occurrence × habitat type interaction: F1,24 = 11.99, P = 0.0019, Table 4). Geckos on “rat-free” islands were active “in the open” rather than “under cover” (P = 0.057), while the opposite occurred on “rat” islands (P = 0.0091, Fig. 4), suggesting that the Leaf-toed geckos actually avoid insecure micro- habitats in presence of rats. All other predictors were not significant (see Table 4 for details). The random effect of islands (L-ratio χ² = 2.92, d.f. = 1, P = 0.09) was not sig- nificant, suggesting that CPUE was not affected by island features other than rat occurrence and habitat. By contrast, the height from the ground where the geckoes were observed “in the open” was not affected by the rat occurrence (“rat-free” islands: 47.6 ± 2.6; “rat” islands: 32.6 ± 6.5), either by season (autumn: 51.7 ± 5.0; spring: 44.3 ± 2.7) or by their interaction (Table 4). Simi- larly, the effects of both island size and the number of rep- tile species were also negligible (see Table 4 for statistics). We report some additional observation on the spa- tial behaviour. Before rat eradication, on Lavezzu Island in 1982, on 112 sightings ≈85% of geckos were active under plant cover; in 1986 (n = 50) the proportion was ≈80-85% (MD pers. obs.). Ten years after rat eradication, the percentage of geckos active under plant cover was lower but still high: 67% in 2010 (n = 41), 68% in 2011 (n = 41). On the remote Vacca Islet, rats were detected for the first time in 2010 and eradicated by the Bouches de Bonifacio Natural Reserve in less than one year (O. Bonnenfant pers. com.). We do not know the activity pat- tern before eradication, but in July 2012, 87% of geckos were observed in the open. Analogously, few miles away, Fig. 3. Variation of the population structure of the European Leaf- toed gecko on 22 Mediterranean islands in response to (a) rat occurrence (present in yellow, absent in green), (b) season (Spring in yellow, Autumn in green), (c) number of reptile species living on the islands (1 sp. in red, 2 spp. in yellow, 3 spp. in green, 4 spp. in blue), and (d) island size (islet: < 10,000 m² in yellow, island: > 10,000 m² in green). Bars = 95% confidence interval. Table 3. Results of the linear mixed model for the variability of the body condition index of male and female Leaf-toed geckos in response to rat occurrence. Only fixed effects are reported. Predictor df F P Rat occurrence 1,6 0.0053 0.94 Season 1,6 0.608 0.46 Sex 1,5 2.545 0.11 Table 4. Results of the linear mixed model for the variability of the activity of Leaf-toed geckos in response to rat occurrence (df have been calculated using the Satterthwaite approximation). Only fixed effects are reported. Predictor df F P Open vs close habitats Rat occurrence 1,12.7 0.781 0.39 Habitat type 1,24.9 1.344 0.26 N. reptile species 1,8.5 0.629 0.45 Island size 1,8.3 0.802 0.39 Rat occurrence × Habitat type 1,24.9 11.99 0.0019 Height above ground Rat occurrence 1,14.9 1.068 0.32 Season 1,22.3 0.659 0.42 N. reptile species 1,2 0.142 0.74 Island size 1,4.4 2.168 0.21 Rat occurrence × Season 1,19.2 2.446 0.13 95Native gecko and Black rat on Mediterranean islands rats invaded the Toro Islands where they have stayed for about 4 years, until 1992, when eradication occurred (Thibault, 1992). In April 2005 (n = 50) ≈90%, in July 2012 (n = 52) 96% and in July 2014 (n = 59) 93% geckos were observed in the open. DISCUSSION Two out of the three predictions on how the Black rat may influence the activity and demography of Euleptes europaea were confirmed by our results: a) in the pres- ence of rats smaller sized geckos prevail, and b) geckos are less active “in the open”; c) by contrast, no significant effect on body condition was found. Transition to populations characterized by smaller sized individuals could be caused by selective preda- tion on larger geckos. On Ohınau Islet in New Zealand, juveniles of the giant gecko Hoplodactylus duvaucelii are more vulnerable to predation by Rattus exulans (Hoare et al., 2007), likely because adult geckos are similar in size to the Pacific rat. Conversely, a 170 g Black rat could eas- ily predate European Leaf-toed geckos of any size (from 0.3 to 2 g). If this would be the case, a random predation of any size class could ultimately also result in a shortage of the larger sized geckos. In the Mediterranean, despite some detailed studies on this topic (Cassaing et al., 2005; Pérez-Mellado et al., 2008; Petralia et al., 2010; Ruffino et al., 2011), direct predation of Rattus rattus on Sauria was never observed, although predation on lizards has been supposed to occur in other regions (Caut et al., 2008; Gasc et al., 2010); ascertained (Harper and Bunbury, 2015; Thibault et al., 2016; Clapperton et al. 2019) or observed for other rat species (Towns et al., 2006). Abun- dance of lizards (and Tuataras) has been related to the presence of rats but the mechanisms still deserve to be explained and likely could involve competitive processes less obvious than predation (Towns et al., 2006). Other Vertebrates or invertebrates living on the studied islands could interact with the Leaf-toed gecko as possible competitors and/or predators such as: the ant Crematogaster scutellaris, competing for rock crev- ices and predating hatching geckos (Delaugerre, 1981); introduced cows, grazing on vegetation used by geckos foraging “under cover”, e.g., on Lavezzu (Delaugerre and Brunstein, 1987); the bird Monticola solitarius and the ant Tapinoma erraticum predating E. europaea respectively on Lavezzu (Delaugerre and Cheylan, 1992) and Porrag- gia Grande islands (Delaugerre and Brunstein, 1987) and perhaps the Western Whip snake Hierophis viridiflavus, that on Giraglia Island has been observed to have noctur- nal habits (Delaugerre, 2013). However 73% of the stud- ied islands, host just one or two reptile species (Table 1), the second species being typically a Podarcis (diurnal) liz- ard, that is unlikely a predator of the Leaf-toed gecko. On the remaining 27% islands, the presence of three or four (just on three islands) reptile species is equally distribut- ed between rat and rat-free islands. Alternatively, rats may affect gecko’s population structure by interfering with their growth process. Sur- vival of geckos can be reduced by increased physiological stress, which ultimately may cause physiological shifts in life history strategy and demographic fitness components (Rödl et al., 2007; Trompeter and Langkilde, 2011; Naray- an et al., 2013). Rats could act as stressors for geckos just roaming around. Moreover, geckos could be subjected to infections carried by black rats (Prenter et al., 2004), often infested by the adult nematode Mastophorus muris (Cassaing et al., 2005). According to Lafferty et al. (2010), native geckos of the central Pacific islands might serve as paratenic hosts of M. muris; this stage being the most incline in affecting host fitness (Kuris, 2003). Rats might also interact with geckos in a more indirect way, for instance inducing vegetation changes. Lotus cytisoides, for example, is a plant that provides high quality shelters for geckos and for a lot of inverte- brates (geckos’ preys) and it is consumed by rats (Cas- saing et al., 2005; Ruffino et al., 2011). Behavioural shift in microhabitat use might be costly for geckos, since they would be forced to forage under plant cover with consequently limited access to bare rock surfaces more favourable for nocturnal thermoregulation (thigmother- my). Fig. 4. Variation of the foraging mode of the European Leaf-toed geckos, “in the open” (open) and “under cover” (close) in response to the presence of rats. The relative activity of geckoes was obtained from sightings of one hour of “catch per-unit-effort” (CPUE) on 15 islets. Bars = 95% confidence interval. 96 Michel-Jean Delaugerre et alii Information gathered on Lavezzu before and after rat eradication seems to confirm an influence of the presence of rats on geckos’ population structure. Before rat remov- al, as observed for other islands with rats, geckos’ size class structure was skewed towards medium-sized indi- viduals. A decade after eradication (three geckos genera- tions) we observed an upward trend towards larger sized individuals, even if the sample size of some size classes is too small to perform a statistical test. Rats seem to change geckos’ population structure without affecting body condition of individuals. Indeed, geckos’ body condition was not influenced by the pres- ence of rats, sex or season. However, we found a strong significant random effect of islands, suggesting that body condition in both males and females strongly depends on some island features rather than on the presence of rats. Activity, assessed by the catch per unit effort, did not vary just according to rat occurrence, but also depending on the microhabitat. In the presence of rats, geckos avoid- ed to forage “in the open” and were more active “under cover”, probably to avoid disturbance and/or predation risk induced by the ground-dwelling rodents (Whitaker, 1973; Hoare et al., 2007). On Lavezzu Island, the “under cover” was the most common mode both before and after rat eradication, even if the percentages of geckos active “under cover” decreased ten years after eradication. This pattern (even if related to one island) may suggest a certain persis- tence of an avoidance behaviour likely resulted from a pro- longed coexistence with rodents. Though in geckos as well as in most reptiles, young individuals cannot learn through parental care or imitation, the acquisition of a novel behav- iour might take long time. Unlike how observed for the New Zealand Hoplodactylus duvaucelii who recovered its arboreal habitat six months after rat removal (Hoare et al., 2007), Lavezzu Euleptes gecko did not show a similar behavioural plasticity. Dealing with antipredator behav- iours on islands Blumstein (2002) reported that «experi- ence dependant behaviours change rapidly following iso- lation» (and the loss of predator), «whereas more hard wired behaviours may persist for many generations.» Does Euleptes geckos have acquired an avoidance behaviour over the last 2000 years while coexisting with Black rats? Or, does this behaviour date back to the Pleistocene when geckos coexisted with mammals nowadays extinct (e.g., the Tyrrhenian field rat, Rhagamys orthodon, M. Masseti and G. Cheylan (pers. comm.)? However, geckos’ behavioural changes due to the presence of rats seem to be very vari- able (Hoare et al., 2007; Krebs et al., 2015). As suggested by our results, geckos may benefit from rat eradication in most contexts but a lot of cau- tion should be paid when undertaking rat eradications on islands that likely can be reinvaded (Harris et al., 2011; Savidge et al., 2012). Once settled on an island, a rat population may repel new invading rats through aggres- sive interactions, as observed by Granjon and Cheylan (1989) and Abdelkrim et al. (2009) who found that the Lavezzu population was founded by a single coloniza- tion event without further gene flow. Hence, eradication might entice new invaders to settle in a competition-free ecosystem. Moreover, island “old-resident” rat populations are adapted to exploit seasonal resources adjusting home range, demography and intraspecific interactions (Chey- lan, 1988; Clark, 1980; Cassaing et al., 2007; Ruffino et al., 2011; Pisanu et al., 2011). New invading rats (e.g., main- land rats), not familiar with the island conditions, might severely affect the local biota. Observations carried out on the Corsican Toro islands 2-4 years after rat invasion, besides the well-known detrimental effects on the Cory’s shearwater and on the Pallid swift, revealed that rats preyed almost to extinction Silene vellutina (Caryophyl- laceae) (Thibault, 1992), a rare endemic plant that else- where was not (or only marginally) consumed by “old- resident” rats. Since several eradication actions have ‘failed’, and reinvasions occurred (Cheylan and Granjon, 1987; How- ald et al., 2007; Russell et al., 2010; Savidge et al., 2012; Sposimo et al., 2012; Ragionieri et al., 2013), the urge to learn from failures led to the proposal to adopt the meta- populational (Russell et al., 2008) or the eradication units approach (Robertson and Gemmell, 2004) by considering first islands’ assemblages and relative reinvasion sources in order to properly guide eradication programs. Native island species must cope with an increasing number of biological invasions; understanding the mechanisms of invasions, as well as the interaction between native spe- cies and long coexisting aliens, may be crucial to adopt proper conservation actions. ACKNOWLEDGMENTS We would like to thank: Joanne Monks (born Hoare), who kindly granted us to borrow part of the title and structure of the paper (Hoare et al., 2007); Initiative Petites Iles de Méditerranée (PIM), APAL (Tunisia), CEN Corse, Réserves naturelles des Bouches de Bonifacio, de Scandola et des Iles de la Pointe du Cap Corse, Parc naturel régional de la Corse, Parco Nazionale dell’ Arci- pelago de La Maddalena, Parc National de Port-Cros; the colleagues and friends who helped during field work: A. Abiadh, Ch.-H. Bianconi, V. Bosc, D. Brunstein, M.H. Casalonga, G. Deso, Y. Donno, A. Gaio, O. Gerriet, F. Grita, N. Nègre-Santucci, O. Patrimonio, J. 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