Study area
The archipelago of Fernando de Noronha (3°51'13.71"S, 32°25’25.63”W) is composed of 21 islands and islets with a total area of 18.22 km² [22,34]. It is one of the four Brazilian oceanic insular sets, including the Archipelago of Sao Pedro and Sao Paulo, the Rocas Atol, Trindade and Martin Vaz Islands and the Archipelago of Fernando de Noronha.
The archipelago has a wet and dry tropical climate, with the rainy season occurring from March to July and the dry season from August to February [35]. Currently, the predominant vegetation of the archipelago is deciduous seasonal forest, similar to that in the Brazilian northeast [36]. On the south face of the archipelago, which is exposed to equatorial trade winds, the vegetation is arboreal with some patches where grass prevails [37]. The urbanized area is located in the center of main island, outside the Marine National Park (Figure 2).
Fernando de Noronha was discovered in 1503 by Vespucci and was occupied by several different countries, such as the Netherlands, France, Portugal and the United States; in 1942, it became Brazilian federal territory, and in 1988, it started to be administered by the Brazilian State of Pernambuco [36,38]. Currently, only the main island is occupied by humans, and the entire terrestrial area is occupied by two federal protected areas: a Marine National Park, which is an IUCN category II protected area covering 70% of the terrestrial territory, and a Protected Area designated as IUCN category V that is destined for human use, covering 30% of the terrestrial area. The estimated human population was 2.9 thousand inhabitants in 2017, with approximately 7.5 thousand concurrent tourists visiting the archipelago at a given time in 2016 [39].
The archipelago exhibits rich diversity of exotic and invasive species such as rats (Rattus rattus and R. norvegicus), mice (Mus musculus), rock cavies (Kerodon rupestris), domestic dogs (Canis lupus familiaris), domestic cats (Felis silvestris catus), cattle egrets (Bubulcus ibis), tegu lizards (Salvator merianae), and rococo toads (Bufus jimi) [34], especially in the main island. From this list, cats, cattle egrets, synanthropic rodents and tegu lizards were reported as predators of the T. atlantica [19, 22, 26, 40].
Cats were probably introduced to the archipelago with the first landing of the European colonizers, during the sixteenth century [22]. Considered as superpredators in insular environments and responsible for the extinction of birds, mammals and reptiles on several islands worldwide [41,42], the cat population on Fernando de Noronha, is estimated at approximately 1,300 individuals, which is considered one of the highest densities on islands worldwide [22]. The cats are reported to occur only in the main island of the archipelago, at higher densities in the urbanized area of the Protected Area [22].
The synanthropic rodents are present in all islands of the archipelago at high densities [40]. The tegu lizards were introduced before 1918 [43] and the cattle egrets arrived naturally in the 1990’s [43]. Although present in all islands of the archipelago, the tegu lizards and the cattle egrets are observed at higher densities in the urbanized area of the Protected Area and at lower densities or absence on the secondary islands [19, 44].
Study design
Five field campaigns were performed for 20 days each in February and November 2015, February, October and November 2016, and April 2018 to determine the density, distribution, and morphometry of the Noronha skink in different landscapes of the archipelago. This approach assumed no fluctuation of its density and distribution throughout the year.
To determine the density and distribution of the T. atlantica in the archipelago, point transects were chosen in accessible areas of the main island and some secondary islands, generating point abundance values. The points were located not only along trails or in human-altered habitats, but also in conserved landscapes inside the Marine National Park as well. This method was performed in all campaigns but at different sampling sites.
The morphometry parameters were obtained from opportunistic captures of T. atlantica during the field campaigns, but at different sites than the point transects, distributed both in the main island and secondary islands of the archipelago.
The study was approved by the Ethics Committee of the School of Veterinary Medicine of the University of Sao Paulo (CEUA protocol number 1827250515) and by the Brazilian Ministry of Environment (SISBio permit number 41682).
Point transects
The point transects were obtained on the main island and some secondary islands (Rata, Meio, Morro da Viúva, and Chapéu) of the archipelago. At each point transect, a single observer remained still for seven minutes, observing a 2 m radius area covering 12.6 m2. The number of visualized individuals during the observation period was recorded and the observations were performed in all landscape types (forest, shrub, grassland, rocky coast/quarry, and urban), which were sampled during the day when the temperature was approximately 29.8 ± 2.2°C, during the period of increased activity for the species defined by Rocha et al. [12]. These inclusion criteria are essential for the observation of ectothermic animals, which are more active in warm periods of the day.
A database of the point transects which included the T. atlantica cluster size, bin start and end (0-2 m), landscape type (which was used as covariate in the abundance model), and effort (which was the same for all transects, since they were only visited once) was used to generate a detection function and the density/abundance estimations. To achieve that, the Distance package of R was used, considering that the recorded individuals in each point transect were clustered (for example, a colony) and a surveyed radius truncated at 2 m (i.e., we considered binned or grouped distance data), since the distances from the observer and the individuals could not be measured due to the fact that the skinks were elusive. Moreover, the best model (half-normal, hazard-risk and uniform) to fit the detection function was chosen according to its Akaike Information Criterion (AIC). The abundance of T. atlantica both in the main island and the secondary island was calculated using this model.
The distribution map was obtained using the DSsim package of R, using the mean density of the T. atlantica (previously obtained) as constant, adjusted by hotspots and lowspots given by the density observed in each point transect. These adjustments were based on the density of T. atlantica in each transect (recorded individuals divided by 12.6 m2). The point transects were represented in a map.
Morphometric evaluation
To perform morphometric evaluation of T. atlantica, individuals were opportunistically captured on the main and secondary (Rata, Meio, Sao Jose, and Chapeu) islands at distinct points from those used under the density and distribution assessment methods. The individuals were obtained throughout the study period, from different landscape types of the archipelago.
The evaluated parameters were head length (HL), rostrum-cloacal length (RCL), tail length (TL), and total length (TotalL) measured with a rigid ruler (in centimetres) and body mass (in grams) measured with a digital scale with a one-gram precision. Autotomized animals and those with regenerating tails were not considered in the TL, TotalL or body mass analyses.
The morphometric parameters of the individuals were compared between the main and secondary islands and by sex. For this purpose, independent t-tests were used to compare the mean parameters. The variances of the parameters were checked by the Levene test.