Population Density and Spatial-Temporal Niche Use of Ship Rats and Italian Wall Lizards on a Small Adriatic Island

Introduced rats are major drivers of biodiversity loss on islands, yet density-based assessments of their interactions with native reptiles remain scarce. On Mediterranean islands, long-term rat-lizard coexistence is facilitated by behavioral and morphological compensation mechanisms in lizards, but there are no studies on integrative assessments of population density and spatial and temporal niche partitioning. In this study, we quantified population density, spatial distribution, and diel activity patterns of ship rats (Rattus rattus) and Italian wall lizards (Podarcis siculus) on a small uninhabited Adriatic islet. Rat density was estimated using spatially explicit capture-recapture (SECR), while lizard density was assessed using line-transect distance sampling. Camera trapping was used to examine the spatial and temporal niche use of all recorded terrestrial vertebrates. Rat density reached 32.1 individuals ha^-1, and lizard density was estimated at 2345 individuals ha^-1, both falling within the range reported for insular populations. Only three terrestrial vertebrates were detected on the islet: R. rattus, P. siculus, and European rabbits (Oryctolagus cuniculus), highlighting the structural simplicity of this island system. Rats and lizards showed broad spatial overlap across all habitat types, but pronounced temporal segregation, with rats being predominantly nocturnal and lizards strictly diurnal. Despite the presence of invasive rats, lizard density remained relatively high and within ranges reported for other insular systems, suggesting that temporal niche partitioning may facilitate coexistence under long-term sympatry. However, studies across seasons and years are needed to determine whether this pattern is consistent. These results provide quantitative evidence of rat-lizard coexistence in a simplified Mediterranean island system and contribute to the global understanding of invasive species impacts under long-term sympatry.