Entrapped vertebrates in discarded bottles in a small Mediterranean island: low density suggests an 'insular effect'

Among macro-litter, glass bottles are one of the most important categories of litter discarded in the terrestrial environments. Discarded glass bottles pose multiple ecological risks, including habitat disruption and entrapment of biota (invertebrates and small vertebrates) therefore acting as ecological traps. Particularly, the entrapment of biota in bottles has been investigated in the mainland and on islands. On this latter, the presence of these anthropogenic traps can lead to strong impacts on the populations of some species. Being islands an important site of occurrence of species of biogeographic interest, here, we aimed to investigate the entrapment of small vertebrates by discarded glass bottles recorded in a small Mediterranean island (circum-Sardinia). We collected glass bottles on 15 transects and calculated a density index (Abundance Kilometric Index; AKI, in bottle/km). On 38,810 m sampled, 86 glass bottles were collected (AKI = 2.22): 15 bottles along paved roads (AKI = 1.57) and 71 along pedestrian paths (29,230 m; AKI = 2.43). Differences between frequencies of bottles with organisms between paved roads and terrestrial paths were not significant either on the total of bottles and for large and small ones. In bottles, we recorded both invertebrates (terrestrial Gastropoda mollusks and Insecta Coleoptera indet.) and remnants of small vertebrates (one Mus musculus, one Chalcides ocellatus). Our study found a low density of discarded glass bottles (< 3/km) when compared to an analogous research carried out in neighbouring Sardinia. This low density suggests an 'insular effect' acting on animals entrapped by debris, i.e. a reducted rate of entrapment of bottles on small vertebrates. This effect may be driven by both low human presence and low species richness, which limits the risk of small vertebrates being trapped in bottles.