Gabor Pozsgai , Pedro Cardoso, Simone Fattorini, Francois Rigal, Ana M.C. Santos, Robert J. Whittaker, Isabel R. Amorim, Mário Boieiro, Luís Borda-de-Água, Ricardo Costa, Luís Carlos Crespo, Maria Teresa Ferreira, Abrão Leite, Sébastien Lhoumeau, Thomas J. Matthews, Jagoba Malumbres-Olarte, Catarina Melo, Guilherme Oyarzabal, Fernando Pereira, José A. Quartau, Carla Rego, Sérvio Ribeiro, Alejandra Ros-Prieto, Artur R.M. Serrano, Kostas A. Triantis, Rosalina Gabriel, Paulo A. V. Borges

Insect declines have been reported globally but whilst island ecosystems are potentially facing exacerbated challenges, no long-term studies (LTER) have confirmed this trend. This study utilises the first available LTER data on island invertebrates, targeting epigeal and canopy arthropods from the Azores, and covering over 20 years in three distinct sampling events from 30 standard sites. We investigate changes in abundance, biomass, and species richness within these arthropod communities, focusing on the proportions of endemic and introduced species, and trends among single-island endemics and strict forest-dependent endemics. We reveal no significant declines in overall abundance, biomass, or richness of endemic or non-endemic native arthropod populations, although we observe considerable variability between sampling events in both epigeal and canopy communities. Of the species analysed, 26% experienced declines, 15% increased in abundance, and 59% showed no change. We found minimal evidence for the rapid spread of introduced species or their increasing influence in the communities, nor any exotic-driven extirpation of endemic species. Interestingly, forest-dependent endemic species declined at a lower rate than anticipated, suggesting that the extinction debt for these species may be less severe than previously thought. The three-decade-long conservation of Azorean native forests may contribute to these unexpectedly stable populations.