The present study estimated the population density of T. atlantica in different landscapes of the archipelago of Fernando de Noronha, providing information that was previously nonexistent. Comparison of the results of the present study with similar studies of the genus Trachylepis on other islands demonstrated that the density of T. atlantica is greater than that of T. adamastor (0.012 individuals/m2), endemic to Tinhosa Grande Island , and T. seychellensis (0.021 individuals/m2), endemic to the Seychelles Islands .
The increased density of T. atlantica in comparison with those of other Trachylepis species may be explained by the fact that Tinhosa Grande Island has a small size and no vegetation, offering few resources for T. adamastor. In contrast, the Seychelles Islands have a larger area than Fernando de Noronha but harbour numerous natural and invasive predators of T. seychellensis, such as snakes and birds of prey . Both the Tinhosa Grande and Seychelles Islands exhibit unfavourable environments preventing their Trachylepis species from reaching the densities observed for T. atlantica.
The phenomenon known as density compensation  indicates the expected abundance of T. atlantica and why it is currently lower than expected. According to Buckley and Jetz , insular species tend to reach population densities above those of their continental conspecifics. An example of this phenomenon is provided by the tegu lizard (Salvator merianae), which presents densities of 6.9*10-4 individuals/m2 in the main island of Fernando de Noronha  and 6.3*10-5 individuals/m2 in continental Brazil (Espirito Santo State) , i.e., at least 10 times higher in insular environment.
The low native species richness of the Fernando de Noronha archipelago and the absence of natural predators prior to human occupation in the sixteenth century are probably the key factors in the high density of T. atlantica on the archipelago . The lack of species sharing the same ecological niche as T. atlantica means that there is low interspecific competition, which, allied with the absence of natural predators, resulted in low predation rates during the early establishment of this species on the archipelago .
However, the actual population density of T. atlantica is not sufficient to maintain the species in the long term. If all the archipelago presented the same density found on the secondary islands, which are free of cats and exhibit low densities of tegu lizards , the abundance of the Noronha skink would be 6,504,540 individuals. Thus, it can be inferred that the actual population of T. atlantica is at least 50.7% below the expected abundance.
The Fernando de Noronha 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), especially on the main island . Cats were probably introduced to the archipelago with the first landing of the European colonizers, during the sixteenth century . These animals are superpredators in insular environments and are responsible for the extinction of birds, mammals and reptiles on several islands worldwide [24,25]. On the Fernando de Noronha archipelago, the number of cats is estimated to be approximately 1,300 individuals on the main island, which is considered one of the highest densities on islands worldwide .
Due to the evolution and speciation of the Noronha skink in the absence of natural predators, behavioural defences are also absent in this species, making it vulnerable in interspecific encounters. Cats, rats and cattle egrets are the main threats and prey upon T. atlantica individuals daily. Dias et al.  described the predation frequency of T. atlantica by cats reported by the resident inhabitants who maintain cats in their households with the aim of controlling the Noronha skink population.
The predation pressure generated by invasive species, especially cats, may directly influence the population densities of the T. atlantica recorded in the present study. This hypothesis is corroborated by Case and Bolger , who noted that cats are considered successful predators of small reptiles in several types of environments. Smith et al.  concluded that the predation of reptiles by invasive species on Christmas Island was the main factor of the decline of its native reptiles.
Through stable isotope analyses and analyses of prey fragments in faecal samples of cats and rats and the stomach contents of tegu lizards, Gaiotto  determined the potential trophic relations of producers and consumers on Fernando de Noronha. The results showed that T. atlantica corresponded to 18.8% of the cats’ diet and 30.3% of the rats’ diet .
No proven presence of cats, a proven low presence or absence of tegu lizards , and a high density of rodents  are reported for the secondary islands of the Fernando de Noronha archipelago. The absence of a single invasive species with a high predation potential, such as cats, may have allowed the higher density of T. atlantica on the secondary islands (0.357 individuals/m²), which was 119% higher than the density observed on the main island (0.167 individuals/m²). Not only is the density higher, but the body size and mass of T. atlantica individuals are also significantly higher on the secondary islands than on the main island, despite the expected body size and mass of males being greater than those of females.
The greater body size and mass of T. atlantica on the secondary islands may also be related to longevity. Classic studies have shown that reptiles grow constantly during their lives and, according to Goss , amphibians and reptiles retain their cartilage epiphyses throughout their lives, allowing constant growth. These findings were corroborated by Andrews , who noted that even if growth is insignificant after reaching asymptotic size, reptiles grow throughout their lives.
According to Olsson and Shine , the constant growth of reptiles is best observed in some lizards with short longevity. This information corroborates the results of the present work. The secondary islands of the Fernando de Noronha archipelago have been subject to fewer anthropic interferences and harbour fewer invasive species, which may increase the longevity of T. atlantica compared to the individuals on the main island.
The effects of the insular conditions on body size are referred to as the “island rule” and may be associated with intrinsic (p.e., climate) or extrinsic (p.e., predation) factors [33,34,35]. According to Russell et al. , when islands exhibit the same biogeographic climatic conditions, extrinsic factors may not explain body size differences. However, the high predation rate of cats on the Noronha skink and competition with other invasive species have probably created selective pressure that determines the observed density and body size differences, resulting in population decline on the main island.
With the arrival of new invasive species in recent decades, such as the tegu lizard (Salvator merianae), which was introduced before 1950 , and the cattle egret (Bubulcus ibis), which arrived naturally in the 1990s , the increases in the mortality rates and population decline of T. atlantica are concerning. According to Gaiotto , 19.6% of the tegu’s diet is composed of T. atlantica. Silva-Jr et al.  also reported predation of cattle egrets upon T. atlantica.