Fire has long been considered an essential determinant of vegetation structure in savanna ecosystems (Sankaran & Hanan 2008:17) and is frequently utilised as a management tool by conservationists. Changes in fire management in southern Africa over the past century reflect an increasing level of ecological consideration (Van Wilgen 2009:105), partially through the insight of manipulations such as the experimental burn plots (EBPs) in the Kruger National Park (KNP), South Africa. This fire experiment began in 1954 (Biggs et al. 2003:46) and has facilitated research projects on fire ecology in savanna ecosystems (Govender, Trollope & Van Wilgen 2006:43). Fire extent, frequency and burn shape influence wildlife distributions across landscapes (Cromsigt, Archibald & Owen-Smith 2017:47; Roberts 2008:1). Moreover, spatially patchy fire drives the spatial and temporal distribution of grazers, in particular, to congregate on immediate post-fire patches (McGranahan & Kirkman 2013:176). Burning affects forage quality and quantity as well as animal behaviour and diet (Roberts 2008:1). Fire frequency further influences ungulate distribution through altered predation risk (Burkepile et al. 2013:139). At a landscape scale, fire reduces plant biomass (Hanan et al. 2008:171), resulting in temporary displacement of herbivores. Fire affects nutrient cycling (Van der Vijver, Proot & Prins 1999:173) and soil organic nitrogen and carbon (Holdo et al. 2009:115), which influence ecosystem productivity. It also modifies plant species composition (Anderson et al. 2007:343) and diversity (Belsky 1992:187). On the other hand, herbivores also drive changes in vegetation cover (Holdo et al. 2009:7). Thus, understanding fire’s influence for the management of grazers is essential (Eby et al. 2014:1196). Post-fire stimulation of plant nutrients is thought to benefit grazing mammals (Eby et al. 2014:1198) as plant re-growth is higher in Ca, P, Mg and K and protein (Van der Vijver et al. 1999:173). Eby et al. (2014:1196) argued that herbivores are attracted to burned patches due to forage quality, rather than a decrease in predation risk. Woolfolk et al. (1975:190) and Anderson et al. (2007:343) reported a greater body mass gain in herbivores on post-burn areas. Fire may affect ungulate density and, together with annual rainfall and distance to water drives herbivore densities and distribution in savannas. Artificial surface water provision is widely practised in South African conservation. Access to water constrains herbivore movement and alters their distribution, particularly during drier periods (Bleich, Marshal & Andrew 2010:638; DeLeeuw et al. 2001:297; Smith et al. 2007:85). Reduced rainfall in drier habitats has an impact on the life history characteristics, physiology and composition of plants and animals (Chesson et al. 2004:236). Rainfall not only provides water for herbivores but also drives forage quality and quantity (Marshal, Krausman & Bleich 2005:360).

This study aimed to (1) quantify herbivore (buffalo, impala, kudu, giraffe, wildebeest and zebra) densities on the KNP Satara EBPs relative to surrounding control areas (central KNP and Satara management section); (2) relate fire frequencies to herbivore densities and (3) track species density changes across the EBPs over time and across rainfall (pre-, during and post-drought) to relate herbivore densities to mean distance to water and rainfall (mm).

Fire showed a vast influence on herbivore densities across the EBPs. The higher collective density of herbivores on the EBPs compared to the non-fire manipulated areas of the larger Central KNP and Satara section is because of variances in fire regimes. A decrease in abundance of herbs and small shrubs was found by Morrison et al. (1995:240) on infrequently burned landscapes such as the non-fire manipulated parts of KNP. Fire regimes influence changes in habitat structure, plant species composition and palatability (Lyon et al. 2000:23). A study by Morrison et al. (1995:144) suggests that animals are attracted to an area because of forage quality rather than the fear of predation. Therefore, the EBPs may be dominated by palatable plant species to attract higher herbivore densities compared to their non-fire manipulated surroundings.

The significant differences on herbivore densities across individual burn strings (N’Wanetsi, Satara and Marheya) and their fire treatment plots suggest differences in the temporal nature of each. Selective herbivore use of the EBPs may be because of the differences in vegetation quality and quantity caused by fire frequency and intensity and rainfall (Shackleton & Scholes 2000:81). Therefore, vegetation palatability across these burn strings may differ, causing herbivores to congregate and forage selectively. Selective foraging of herbivores influences the distribution and biomass of plants, in turn affecting fire regime attributes (Smith et al. 2013:72). The annual burn plots of the Satara EBPs have been documented with low grass species richness than the triennial burn plots because of fire frequency and intensity (Shackleton & Scholes 2000:79). Therefore, the continued burning of the annual plots, whilst exposed to higher densities, may have decreased grass species richness even further in the past decade. This causes a shift from annual to triennial plots on overall herbivore densities to occur. An island study conducted by Feeley and Terborgh (2005:116) suggests that higher herbivore densities in annually burned areas cause a shift in the plant communities towards unpalatable plant species over time. Additionally, annually burned plots exposed to grazing because of high herbivore densities reduce the abundance of palatable plant species relative to unpalatable ones (Jourbert et al. 2014:126). Therefore, plots become dominated by fire-tolerant plant species with reduced composition (Boakye et al. 2014:603), in turn affecting herbivore densities. However, burning at a 3-year interval supports both fire-tolerant and fire-intolerant species with high species composition (Boakye et al. 2014:603), further facilitating higher herbivore densities and composition. Research also outlines that fire frequency and grazing intensity have a detrimental effect on plant species diversity, composition and their nutritional levels (Jourbert et al. 2014:126). Palatable grass species like Panicum maximum were reported to increase in infrequently burned areas like triennial burn plots (Smith et al. 2013:72), whereas Urochloa mosambicensis was reported to increase in abundance in intermediate and greater fire intensity (Smith et al. 2013:80). Therefore, higher herbivore densities shifting to triennially burned plots suggest that herbivores follow quality forage and diverse plant species that the annual plot may have lacked. Moreover, a 3-year interval burning results in high-vegetation biomass, improving the proportion of palatable species (Jourbert et al. 2014:128) compared to the annual burn frequency. However, an increase in the relative abundance of unpalatable species because of annual burning results in a decline in quality of leaf litter, causing a decline in nutrient revenues and primary production rates (Pastor & Naiman 1992:690). This motivates selective foraging of herbivores, whilst directly affecting herbivore populations across treatment plots. Although the loss of grass biomass with frequent burning is attributed to reduced plant diversity (Auld & Denham 2006:15; Snyman 1998:645), herbivore densities are also affected.

Water abundance and mean distance to water showed a slight effect on the herbivore densities on the EBPs. However, this does not exclude abundance of water as the cause of high herbivore congregations on the EBPs but may indicate that it is not the only factor. Factors like rainfall over the medium term drive forage quality and quantity in African savanna ecosystems (Peel & Bothma 1995:41; Venter, Scholes & Eckhardt 2003:5). Herbivores respond indirectly to this driver, with 87% of the variance in large ungulate biomass being explained by rainfall and fire (Fritz & Duncan 1994:77). The strong correlation between rainfall and herbivore densities on the EBPs coincides with Fritz and Duncan (1994:77) that rainfall is significantly important to herbivore densities in fire-prone semi-arid savanna ecosystems. Rainfall forms the archetype that determines the moisture and nutrients available to plants, therefore, influencing the way habitats are selectively utilised by herbivore populations. Hence, impacts on the EBP trial cannot be attributed to fire alone but to a combination of fire, past herbivory and rainfall. In events like ‘drought’, where rainfall is scarce, changes in different functional plant responses are experienced (Silver et al. 2013:17), indirectly causing changes in herbivore population densities and distributions. Ecosystems like semi-arid savannas with intermittent low annual rainfall seasonally modify CO₂ and N cycles, resulting in poor water and mineral uptake, hindering growth and causing reduced litter decomposition, alternately altering above ground net primary production (Silver et al. 2013:17), in turn influencing changes in herbivore densities before and after a drought period.

The EBPs are preferred grazing patches for buffalo, giraffe, impala, kudu, zebra and wildebeest, compared to the non-fire manipulated surrounding areas. Our findings suggest that the differences in the burn strings of Satara may not only be because of fire but also because of higher herbivore densities. Higher herbivore densities impose increased grazing pressure and this questions the level of impact caused by the fire and grazing on the EBPs or the impact of both events in succession (Shackleton & Scholes 2000:80). Moreover, the vast differences between the EBPs, Satara section and the central KNP in herbivore densities validate that they are different in the nature of fire regimes and applications, and thus cannot be used to make informed decisions on the broader central KNP or Satara management section. This motivates management to reconsider the EBPs as a fire–herbivory experiment, especially when the density estimate is more than seven times higher in places. Although fire proved to have an influence on the herbivore densities on the EBPs, factors like rainfall may have contributed to some extent, as the correlation between herbivore densities and rainfall was stronger. This further accentuates that fire and rainfall are critical drivers of herbivore densities on the EBPs and fire-prone semi-arid savanna ecosystems. Nonetheless, results from this study may be influenced by the 2015–2016 drought period, as this event can affect an ecosystem to some degree.

Careful interpretation of results is needed to unbundle the effects of fire, herbivory and rainfall, all of which may have a greater impact on the EBPs. The annual treatments have not burned effectively since 1996 compared to the triennials because of inadequate fuel load to sustain a fire. In some cases, the herb layer is too wet and green to facilitate and enable burning. This disrupts the burning schedule for the fire treatment. In addition, burning the annuals in August no longer seems effective, suggesting a revised burning season or a renaming of the treatment as near-annual burns rather than annual burns. The EBPs being focal points for research in KNP, long term rainfall data for individual burn strings should be available to prevent researchers from relying on regional or section data. Putting up rain gauges on the EBPs for long-term data collection is advisable, as regional rainfall has proven to have a greater effect on herbivore densities within the EBPs.