Pine afforestation alters the ground-dwelling arthropod fauna of the Mediterranean heathland

Abstract Treeless habitats have historically been undervalued and extensively afforested. While these afforestation programs aim to deliver economic and ecological benefits, they can negatively impact biodiversity, particularly of ground-dwelling arthropods, due to changes in microhabitat conditions and food resources. This study examines the effects of pine afforestation on the diversity of ground-dwelling arthropods in the Mediterranean heathland (herriza), a unique treeless habitat in the western Mediterranean Basin, subject to extensive afforestation in the second half of the 20th century. We assessed alpha diversity (taxonomic richness, abundance, and inverse Simpson index) and beta diversity (changes in taxonomic composition) of ground-dwelling arthropods (identified at the family level), and the group of spiders (identified at the species level). We also explored changes in functional traits (functional beta diversity) in spiders. Field observations were conducted over five seasonal periods within one year, using pitfall traps in five sites, each with adjacent open and afforested herriza plots. While afforestation had no significant effect on the alpha diversity or abundance of ground-dwelling arthropod families and spider species within individual seasons, pooled analyses revealed distinct trends. Specifically, afforested plots exhibited higher overall arthropod abundance at the family level, yet significantly lower spider species richness and abundance. In contrast, beta diversity analyses showed significant shifts in taxonomic composition between open and afforested herriza plots for both groups. Despite these compositional shifts and high functional beta diversity, overall functional diversity indices for spiders did not significantly change with afforestation. Implications for insect conservation Our results suggest that while afforestation may not collapse existing functional niches, it significantly reshapes the taxonomic assembly of ground-dwelling arthropod fauna, posing a potential threat to the specialized biodiversity of open ecosystems.