Diversity and distribution of arthropods in native forests of the Azores archipelago

Since 1999, our knowledge of arthropods in native forests of the Azores has improved greatly. Under the BALA project (Biodiversity of Arthropods of Laurisilva of the Azores), an extensive standardised sampling protocol was employed in most of the native forest cover of the Archipelago. Additionally, in 2003 and 2004, more intensive sampling was carried out in several fragments, resulting in nearly a doubling of the number of samples collected. A total of 6,770 samples from 100 sites distributed amongst 18 fragments of seven islands have been collected, resulting in almost 140,000 specimens having been caught. Overall, 452 arthropod species belonging to Araneae, Opilionida, Pseudoscorpionida, Myriapoda and Insecta (excluding Diptera and Hymenoptera) were recorded. Altogether, Coleoptera, Hemiptera, Araneae and Lepidoptera comprised the major proportion of the total diversity (84%) and total abundance (78%) found. Endemic species comprised almost half of the individuals sampled. Most of the taxonomic, colonization, and trophic groups analysed showed a significantly left unimodal distribution of species occurrences, with almost all islands, fragments or sites having exclusive species. Araneae was the only group to show a strong bimodal distribution. Only a third of the species was common to both the canopy and soil, the remaining being equally exclusive to each stratum. Canopy and soil strata showed a strongly distinct species composition, the composition being more similar within the same stratum regardless of the location, than within samples from both strata at the same location. Possible reasons for these findings are explored. The procedures applied in the sampling protocol are also discussed.