Investigation of the responses of soil invertebrate communities to four decades of prescribed fire frequency levels in semi-arid savanna rangelands

Abstract

Fire is a key evolutionary force that influences the form and function of savanna ecosystems. These habitats evolve alongside fire, which is essential for preserving ecological equilibrium. However, fire can also affect biodiversity, including invertebrate populations that are critical to the ecosystem processes. Prescribed fire frequencies are often implemented to reduce the risk of wildfire, but their impacts on biodiversity remain under-explored. The current study assessed the effects of fire frequency on the population dynamics and biodiversity of soil invertebrates using a 41-year-old fire experiment at The University of Fort Hare in Alice town, Eastern Cape Province (South Africa). The experiment included six different burning treatments: no-burn, annual, biannual, triennial, quadrennial, and sexennial burning, arranged in a completely randomised design with two replications. Using ants as an indicator group, the results demonstrated that there were no significant differences (F_{(11, 1)} = 0.676; p = 0.848) of ground-dwelling insect species between different fire regimes. Findings further showed that fire frequencies had no significant effect on the insects’ alpha diversity metrics, including Shannon diversity, species richness, and Pielou's evenness. Earthworm communities also showed no significant differences across treatments (F_{(11, 1)} = 1.737; p = 0.161), as indicated by the dissimilarity matrix analysis. Although most of the ants (5982 in total) were caught across all treatments, the majority of species belonged to the Myrmicinae and Formicinae subfamilies, with lower abundances observed in the Ponerinae and Dolichoderinae subfamilies, except in the quadrennial treatment. The Tetramorium capense, a member of the Myrmicinae subfamily, was the most prevalent species across all fire regimes. Furthermore, cluster and principal component analyses revealed no significant correlations between soil chemical parameters and arthropod assemblages across the different burning treatments. Generally, the findings suggest that prescribed fire frequencies did not have detrimental effects on the studied environment, as indicated by the bioindicator species used.