Response of grassland insect diversity and assemblages to the invasive Gleditsia triacanthos in South Africa

Abstract

<title>Abstract</title> Invasive alien tree species alter open grassland ecosystems by introducing shading and vertical structure, which can modify habitat conditions and influence insect community composition and dynamics. <italic>Gleditsia triacanthos</italic> , a rapidly spreading invasive alien tree, is transforming grasslands in South Africa. This study assessed the impact of <italic>G</italic> . <italic>triacanthos</italic> on terrestrial insect groups, focusing on Hymenoptera (ants), Coleoptera (beetles), Orthoptera (locusts and grasshoppers) and Blattodea (cockroaches and termites). Insects were collected using pitfall traps in five paired (invaded and uninvaded) grassland locations in the Free State province, over four seasons. A total of 19,629 individual insects from four orders, belonging to 34 families and 147 species or morphospecies, were identified across all sites and seasons. Hymenoptera exhibited the highest abundance, whereas Coleoptera had the highest species richness. Insect community composition was significantly affected by invasion and season. Uninvaded sites had higher species abundances and lower dominance, with peaks occurring in summer and declines in winter. However, Blattodea and some ant genera ( <italic>Pheidole</italic> , <italic>Anoplolepis</italic> , and <italic>Camponotus</italic> ) were more abundant under <italic>G. triacanthos</italic> . These shifts in insect communities may disrupt processes such as herbivory, soil aeration and decomposition performed by the studied insect groups, and this potentially triggers cascading effects on higher trophic groups and, ultimately, grassland productivity and function through modification of species interactions. Given the rapid spread of <italic>G. triacanthos</italic> , our findings emphasise the need for continued monitoring of <italic>G. triacanthos</italic> spread and its ecological consequences on grassland ecosystems in South Africa.