Silver wattle (Acacia dealbata) alters arthropod assemblages in the Sani Pass region of the Drakensberg

Invasive alien plants constitute a significant threat to global biodiversity. Alien invasive plants are currently invading natural ecosystems in South Africa but their impacts on biodiversity are poorly understood, particularly their impact on arthropods. This study investigated the impact of alien silver wattle (A. dealbata) invasion on arthropod assemblages (with an emphasis on Coleoptera) in the Sani Pass region of the Drakensberg, South Africa. Pitfall traps, baited with cattle manure, were used to sample Coleoptera and Diptera assemblages in pristine grassland and also in Acacia dealbata invaded stands. Specimens were identified using the morphospecies concept. Coleoptera species assemblages differed significantly between grassland and invaded sites (Global R = 0.856, p = 0.02). Coleoptera and Diptera species richness and abundances was significantly lower in the invaded stands than in grassland. Tree beating and sweepnetting sampling methods conducted in invaded and grassland sites, respectively, showed lower arthropod abundance in invaded stands than in grassland. Ambient temperature measurements indicated that grassland had a much higher daily maximum temperature than that of invaded sites. The effects of spatial autocorrelation on the sampling regime were also assessed using a newly proposed statistical method. The finding that species richness and especially abundances are greatly reduced in invaded stands should be of great conservation concern as biodiversity is severely altered by this invasive alien plant. This study reiterates that invasive alien plants can constitute a significant threat to arthropod diversity.