This study compares the vertical stratification of arthropod assemblages in native and exotic forests on Terceira Island (Azores), focusing on how assemblage similarity decays with increasing vertical distance between strata. We tested three hypotheses: (H1) the overall arthropod assemblage similarity decays with vertical distance; (H2) exotic forests exhibit lower decay rates due to structural simplification; and (H3) the contribution of both indigenous and non-indigenous arthropod assemblages to vertical distance decay differs. Arthropods were sampled across four strata in ten native and ten exotic forest plots using pitfall and SLAM traps. We computed the beta diversity between each strata, partitioning turnover versus richness differences. We found a significant decay in the overall arthropod assemblage similarity with increasing vertical distance, driven primarily by species turnover rather than richness differences. Contrary to predictions, the rate of vertical decay did not differ significantly between native and exotic forests, suggesting that structural simplification in exotic forests does not necessarily reduce vertical stratification. However, the species origin of the arthropods had a significant effect on the vertical distribution patterns, with non indigenous species showing a restricted vertical ranges compared to indigenous species. These findings highlight the significance of incorporating vertical dimensions in biodiversity assessments and conservation strategies, emphasising the pivotal role of species biogeographical origin in shaping vertical community structure in island forest ecosystems.

Human activities drive ecological transformation, impacting island ecosystems from species diversity to ecological traits, mainly through habitat degradation and invasive species. Using two unique long-term datasets we aim to evaluate whether species traits (body size, trophic level, dispersal capacity and habitat occupancy) can predict temporal variations in the abundance of endemic, native non-endemic and exotic arthropods in the Azores Islands. We found that body size is crucial to predict arthropods abundance trends. Small-bodied herbivorous arthropods showed a decrease in abundance, while large-bodied indigenous arthropods increased in abundance, mainly in well-preserved areas. Also, large-bodied exotic arthropods increased in abundance across the entire archipelago. Moreover, endemic canopy dwellers increased in abundance, while endemic ground-dwellers decreased in abundance. Simultaneously, exotic arthropods showed the opposite result, increasing abundance in the ground while decreasing abundance in the canopy. Finally, habitat influenced both endemic and exotic spider abundance trends. Endemic spiders that occupy solely natural habitats experienced a decline in abundance, while exotic spiders in the same habitats increased in abundance. Our study underscores the significance of arthropod species traits in predicting abundance changes in island ecosystems over time, as well as the importance of monitoring species communities. Conservation efforts must extend beyond endangered species to protect non-threatened ones, given the increased extinction risk faced by even common species on islands. Monitoring and restoration programs are essential for preserving island ecosystems and safeguarding endemic arthropod populations.