Landscape composition, patch size, and local characteristics shape ant and beetle assemblages in naturally patchy indigenous forests

Indigenous forests are important reservoirs of terrestrial arthropod biodiversity, but naturally patchy forests are understudied compared to human-fragmented forests. We evaluated the relative influence of landscape composition, patch size, and local characteristics on ants (Formicidae) and beetles (Coleoptera) in Southern Mistbelt Forests in South Africa. These forests exist as small patches within a fire-prone grassland matrix, with the grasslands undergoing modification for forestry, agriculture and rural development. Both taxa responded strongly to predictors at different spatial scales, especially through changes in species composition. Responses to local variables diverged between taxa, with ant assemblages being structured mostly by variation in vegetation openness and beetle assemblages responding to a wider variety of variables relating to ground cover, vegetation structure and woody debris. Species richness and evenness of both taxa, but only beetle assemblage composition, was influenced by the surrounding landscape, whereas patch size influenced ant and beetle assemblages. Assemblage changes were driven mostly by gradual species replacement along all environmental gradients, rather than species attrition. Surprisingly, no differences were detected between forest edges and interiors in biodiversity metrics or responsiveness to predictors, likely due to microclimatic buffering of structurally closed, natural forest edges. Our findings demonstrate that natural variation in local characteristics, patch size, and landscape context among Afromontane forest patches correlate with regional arthropod diversity in these landscapes. Conserving networks of forest patches that encompass the widest possible range of natural forest heterogeneity is critical for arthropod conservation, underscoring the importance of complementarity approaches that prioritize unique forest patches alongside larger, core forests.