Encroaching shrub species alter arthropod community composition and trophic structure in southern African rangeland ecosystems

Bush encroachment is rapidly transforming savannah and grassland ecosystems in southern Africa, altering vegetation structure and ecosystem functioning, while reducing rangeland productivity. Despite these shifts, its effects on local biodiversity, particularly arthropod communities remain understudied. This study evaluated the arthropod communities within areas dominated by three major encroaching shrub species in southern Africa: Dichrostachys cinerea, Senegalia mellifera and Terminalia sericea. Arthropods were sampled in northcentral Namibia using pitfall traps (ground-dwelling), pan traps (flying), and shrub beating (canopy-dwelling) during both wet and dry seasons. Arthropod community composition differed significantly among encroaching species, primarily driven by canopy-dwelling arthropods. Senegalia mellifera supported distinct communities distinguished by the exclusive presence of woodlice (Isopoda), while, D. cinerea and T. sericea plots hosted distinct taxa, including Caelifera, Mantodea and Strepsiptera. Also the trophic composition differed among shrubs, with omnivores dominating S. mellifera and phytophagous arthropods dominating D. cinerea plots. Formicidae and Collembola were numerically dominant across all plots. Brachycera, Curculionidae and Collembola were highly sensitive to local environmental variation, indicating their potential as bio-indicators for monitoring shrub encroachment and effectiveness of management interventions. Overall arthropod abundance positively correlated with ground cover, while taxonomic diversity showed a negative relationship. Therefore, conservation efforts in bush encroached areas should promote vegetation management strategies that enhance structural heterogeneity to support greater arthropod diversity. Our findings provide a key baseline on how encroaching shrub species affect arthropods, highlighting the need to consider invertebrate biodiversity in de-bushing efforts in southern Africa, which have traditionally focused on rangeland productivity over ecological impacts.