Invasive alien plants have the potential to alter ecosystem function. While a growing number of studies have focused on the effects of invading plants on native biodiversity and the underlying community dynamics, there is still a lack of studies that detail the impact of invasive plants, such as Parthenium hysterophorus L. (Asteraceae), at higher trophic levels. This study investigated whether P. hysterophorus and its biological control agent, the stem-boring weevil Listronotus setosipennis (Hustache) Coleoptera: Curculionidae, affected ant diversity, assemblages and arthropod activities in the KwaZulu-Natal and Mpumalanga provinces of South Africa. The objectives of this study were to assess the impacts of P. hysterophorus invasion and presence of L. setosipennis on the diversity and assemblage composition of ants, a dominant terrestrial group. Additionally, whether vegetative variables in habitats invaded by P. hysterophorus affected ant assemblages, was examined. Lastly, the study investigated the impacts of the presence and absence of L. setosipennis on other arthropod activities. To study ant diversity and assemblage composition, ants were collected using pitfall traps, over five sampling periods from December 2019 to March 2020, in nine locations around KwaZulu- Natal and Mpumalanga provinces. At each site, three treatments were sampled; viz. P. hysterophorus invaded habitat, P. hysterophorus invaded habitat with L. setosipennis present, and habitat without P. hysterophorus. Species richness and abundance were compared between treatments using ANOVA and the Post-hoc Turkey test. Assemblage composition was analyzed using non-metric multidimensional scaling (NMDS). A Canonical Correspondence Analysis (CCA) was also used to correlate the best environmental variable with ant diversity and assemblage composition. Some 16 463 ant specimens that were collected were identified from four subfamilies, 27 genera, and 55 species. Results indicated that the presence of P. hysterophorus and its biological control agent L. setosipennis did not significantly alter ant diversity, as indicated by species richness and abundance, and assemblage composition, although some differences occurred across locations. The results also showed that vegetative variables (P. hysterophorus height, flowering and cover) did not significantly influence ant assemblages. A separate experiment at six locations in KwaZulu-Natal and Mpumalanga provinces, arthropod activities on P. hysterophorus plants were studied during timed observations at 07h00, 10h00 and 12h00 during monthly sampling from December 2020 to March 2021. Although some arthropod groups were less common visitors to P. hysterophorus than others, this study showed that the presence of L. setosipennis had no significant effect on overall arthropod activities. However, results demonstrated greater activity for some arthropods groups, specifically Hymenopterans, in both treatments, accounting for 60% of all arthropods recorded. Other studies have reported either a positive or negative impact on arthropods by invasive alien plants. These plants may provide a better food resource for native insects and other arthropods, causing them to be attracted to them. However, invasive alien plants have also been linked to a decline in invertebrate species diversity or changes in the composition of populations. This study contributed to growing knowledge on the impacts of invasive alien plants and on terrestrial arthropods, the most prominent group of invertebrates, that are also known to be significant indicators of biological change.

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