In the current study, the data collected from the unburnt, triennially and annually burnt plots did not reach asymptote for individual treatments separately and combined. This postulates that sampling efforts were insufficient at each treatment plot separately and combined, however, the data was analysed to measure the variation in abundance, species richness and diversity of arthropods at the long-term experimental burnt plots of KNP. Similar studies conducted in grasslands, woody forests and savannas landscapes demonstrated clearer impact on the abundance, species richness and diversity of arthropods although the analysed data did not reach asymptote (Kunz & Krell, 2011; Magoba & Samways, 2012; Otieno et al., 2021). The studies demonstrated how prescribed fire can be an ecologically sound approach that balances the abundance, species richness, composition and diversity of arthropods in the forests, grasslands and oak savanna landscapes in the United State of America (Ferrenberg et al., 2006; Harper et al., 2000; Siemann et al., 1997). Likewise, the current study reports the significant impact of prescribed burns (i.e., annually and triennial) on the abundance, species richness, and diversity of sampled arthropods at the long-term experimental burnt plots of KNP, although the data did not reach asymptote.
The abundance, species richness and diversity of multi-taxa significantly differed between the annually burnt and unburnt plots while the triennially burnt did not differ with the unburnt treatment at the long-term EBPs of KNP. This variation may have been elicited by the prescribed burn that occurred a month before sampling in the annually burnt plots resulting in minimal recovery time. The current study showed that burning improves the richness and diversity of species whilst decreasing arthropod abundance at the burnt plots of KNP. The findings corroborate with previous studies that reported differences in abundance, species richness and diversity of arthropods after incidental/ prescribed fires in Europe, United states of America and Africa (Ferrenberg et al., 2006; Siemann et al., 1997; Valkó et al., 2016; Yekwayo et al., 2018). Of the plots sampled at KNP, the abundance, species richness and diversity was significantly different between the annually burnt (i.e., which was burnt approximately a month before the initial sampling) and the unburnt plot. Furthermore, no statistical variations was notable between the triennially burnt (i.e., burnt at least two years before arthropod sampling) and either the annually burnt or unburnt plots. Several studies showed that the abundance, richness and diversity of arthropods sampled at least 6 months (i.e., up to 10 years) after incidental or prescribed fires does not statistically differ with that on the unburnt plot at protected areas (Ferrenberg et al., 2006; Graham et al., 2009; Pryke & Samways, 2012; Valkó et al., 2016; Yekwayo et al., 2018). Since arthropods were sampled a month after burning at the annually burnt plots, we therefore speculate that the notable variation in abundance, species richness and diversity of multi-taxa arthropods might have been influenced by the time between the fire incidence and initial sampling (< 6 months). Given this variation, the results indicate that responses of arthropods are time-since-fire related.
Amongst the most abundant taxa sampled from the experimental burnt plots of KNP were Hymenoptera: Formicidae, Coleopterans and Araneae. This is in agreement with previous studies which reported the three groups of arthropods (i.e., Hymenoptera: Formicidae, Coleopterans and Araneae) as the most abundant arthropods sampled at sites where multi-taxon were used as ecological indicators to measure the impact of prescribed or incidental fires (Ferrenberg et al., 2006; Kaynas, 2016; Pryke & Samways, 2012; Valkó et al., 2016; Yekwayo et al., 2018). The abundance of Hymenoptera: Formicidae was not surprising since they are known to be ubiquitous group of insects contributing to a variety of ecological functions in grasslands and savannas (da Silva et al., 2020; Graham et al., 2009; Underwood & Fisher, 2006; Van Schalkwyk et al., 2019). Of the individual groups of arthropods sampled, only the most dominant taxon (i.e., Hymenoptera: Formicidae) was significantly affected by the frequency of burns at the protected savanna of KNP. This notable difference in the abundance, species richness and diversity reflect the sensitivity of Hymenoptera: Formicidae to prescribed fires. The sensitivity of Hymenoptera: Formicidae to ecological disturbances (i.e. fire) at the EBPs illustrates their renown use as a reliable ecological indicator.
The response (e.g., abundance, species richness and diversity) of the most abundant taxon (i.e., Hymenoptera: Formicidae) emulated that of the multi-taxa at different burning regimes at KNP. Likewise, Siemann et al. (1997) reported that the abundance, species richness and diversity of dominating group of arthropods was similar to that of multi-taxa combined at different burnt plots at the oak savanna of Cedar Creek Natural History Area, Minnesota. Moreover, Yekwayo et al. (2018) demonstrated that the abundance, species richness and diversity of the most abundant group of arthropod (i.e., Hymenoptera: Formicidae) was similar to that of the multi-taxon in a study which measured the impact of fire on arthropods at the Cape Winelands and Kogelberg Biosphere Reserves, Western Cape, South Africa.
The increase in the number of arthropods with the time post-fire and frequency of prescribed burns demonstrates the direct and indirect effect of fire on the ground dwelling arthropods at the KNP. The results showed that shortly after burn, the number of individual arthropods was significantly lower compared to numbers at plots burnt a year before sampling and the unburnt. However, species richness and biodiversity indices contrasted the arthropod abundance data, with statistically high richness and biodiversity at the annually burnt compared to triennially burnt and unburnt plots. These results corroborate with previous studies that reported benefits of fires on the composition of arthropods, although the numbers were drastically affected in the short term (Ferrenberg et al., 2006; Lazarina et al., 2017).
Ground-dwelling invertebrates and immature inhabiting combustible live (e.g., plants) or dead material (e.g., litter) are highly prone to fires and may be burnt during veld fires (Kwok & Eldridge, 2015; Kwok et al., 2016; Vasconcelos et al., 2009). As such, the destruction of various habitats such as plant, litter, dung during burning is speculated to have significantly reduced the numbers of arthropod at the annually burnt plots in the current study. While Yekwayo et al. (2018) reported lower species richness and diversity of arthropods, the current study reported otherwise in annually burnt plots. However, a study by Pryke and Samways (2012) reported that the diversity of multi-taxon is significantly higher at a recently burnt plot (i.e., 3-month post fire) compared to those sampled a year to three years after fires.
Underground nests and animal dung encouraged survival of ants and coprophagous beetles (i.e., Coleopterans) which appeared to be less than 2-folds lower in the annually burnt plot compared to triennially burnt and unburnt plots at KNP. Neither abundance, species richness and diversity of Coleopterans (i.e., dominated by coprophagous beetles) were affected by frequency of burns at KNP. Some studies demonstrated the resilience of coprophagous Coleopterans against incidental and veld fires and this has been associated with the dung and underground nesting behaviour of this specific group of arthropods (Nunes et al., 2019; Palusci et al., 2021). Although statistical variation was notable for Hymenoptera: Formicidae, the underground nests were speculated to have protected the scavenging ants from the direct fire. The insignificant impact on the assemblage of Orthopterans and Araneae is a result of overwintering strategy of some developmental stages below the soil surface and dispersal abilities of these groups (Jing & Kang, 2003; Lipovšek & Novak, 2016; Narimanov et al., 2021).
The current study shed some light on the long-term benefits of late summer prescribed burns on the conservation of arthropod species and biodiversity in the savanna landscape of KNP. Results on the long-term impact of late summer prescribed burns filled a gap outlined by Parr et al. (2004) which emphasized the need to assess the overall response of arthropods (i.e., multi-taxon) at the protected areas where late summer fires have been constantly used as a veld management tool. The current study reported that the impact of the late summer fire is temporal and the abundance of arthropods significantly improves with the time-since-fires. The rate recovery by arthropods emulates that incurred post-burn or incidental fires. Improved species richness and diversity at burnt plots elucidate the benefit of fire at the protected areas of Kruger National Park. In conclusion, burning during late summer season should be encouraged although it has temporal impact on the abundance of arthropods inhabiting savanna landscape of KNP. The intensity of ongoing prescribed burns at the protected reserve of KNP encouraged the conservation of arthropod species for over 67 years, thus, the disruption of functions and ecological services rendered by arthropods is temporal.