Fire is a frequent and widespread disturbance in many biomes (Pausas & Keeley 2009) and shapes landscapes by modifying vegetation structure and composition (Bond & Keeley 2005). The recent increase in large uncontrolled fire incidences (e.g. in the Western-Mediterranean basin; Pausas & Fernández-Muñoz 2012; and in the North American boreal region; Kasischke & Turetsky 2006) and predictions indicating that the frequency of high-intensity fires will increase in the future as a result of climate change (Pechony & Shindell 2010) have triggered growing concerns about the impacts of changes in long-term fire regimes. Fire management has become increasingly important as both human population densities and pressure to manage fuels for asset protection increase (Gill & Stephens 2009). In parallel, fire has also become one of the primary tools for biodiversity conservation (Andersen et al. 1998; Parr & Chown 2003). Yet, numerous knowledge gaps are still currently impeding informed decision-making in fire management (Driscoll et al. 2010).

Fire management practices for biodiversity conservation are often based on limited information, usually focussing on the response of plant communities in short-term studies or natural experiments by using space-for-time substitution (Driscoll et al. 2010). However, data on the long-term ecological response of multiple taxa to fire regimes are crucial to design appropriate fire management strategies for biodiversity conservation (Andersen et al. 1998). Despite the fact that invertebrates represent the largest component of global biodiversity and ecosystem functioning (Lavelle et al. 2006; Losey & Vaughan 2006), they are relatively poorly studied in fire ecology research (New et al. 2010; Parr & Chown 2003).

In savannas, one of the world’s most fire-prone biomes, arthropods are considered to be generally well adapted to frequent fires with several recent studies across taxonomic groups highlighting their resilience (Andersen & Müller 2000; Andersen et al. 2014; Barrow, Parr & Kohen 2007; Davies et al. 2012; Parr et al. 2004). Most studies have observed a rapid recovery of communities after fire, between 2 and 13 months after fire, depending on the insect order and savanna type (Diniz, Higgins & Morais 2011; Parr et al. 2004). The few existing studies on long-term savanna fire regimes observed only a weak overall effect of fire regime on insect diversity and composition, for example, for ants (Maravalhas & Vasconcelos 2014; Parr et al. 2004) and for termites (Davies et al. 2012). Studies to date therefore suggest that little pyrodiversity (i.e. variability or diversity of fire regimes, Parr & Andersen 2006) may be required to conserve savanna insect diversity (but see Maravalhas & Vasconcelos 2014).

However, Beale et al. (2018) recently showed that the effects of pyrodiversity on birds and mammals across African savannas are rainfall contingent, with positive effects on these taxa particularly pronounced in wet savannas (> 650 mm/year). Beale et al.’s (2018) findings support studies suggesting that fire regime may have a greater effect on ants and termites in mesic savannas (> 650 mm/year; Davies et al. 2012; Parr et al. 2004). It therefore seems likely that invertebrates may also be more responsive to pyrodiversity in wet savannas. Furthermore, the role of fire regime may also depend on species traits, in particular specific habitat and dietary requirements. As a result, although most studies highlight the high resilience of invertebrate groups to single fire events and the weak effect of fire regime, this may not be true in wet savannas for invertebrate groups that have more specific habitat and dietary requirements.

Butterflies (Lepidoptera) are herbivorous species, commonly used as indicator species to reveal impacts of environmental change on biodiversity (e.g. Devictor et al. 2012). Butterflies might be greatly influenced by changes in vegetation structure and composition caused by changes in fire frequency (Pryke & Samways 2012) because of their food specialisation at the larval (Gilbert & Singer 1975) and adult (Mevi-Schütz & Erhardt 2003) stages. Indeed, their distributions and population dynamics could be structured by the various plant species composition and structure shaped by different fire regimes (Smit et al. 2010; Smith et al. 2012). Because of their ecological requirements, butterfly species can be positively affected by frequent fires (Gardiner & Terblanche 2010) or conversely negatively impacted by them (De Andrade et al. 2017). As a result, we expect savanna butterflies to be significantly affected by habitat differences that can develop as a result of long-term fire regimes in wetter savannas, from closed to open savanna depending of the fire frequency (Smit et al. 2010).

We studied butterfly communities associated with different fire frequencies by using a long-term fire experiment in a wet savanna (> 650 mm/year) of Kruger National Park, South Africa. Fire regimes can be characterised by five components (fuel consumption and spread patterns, fire intensity, severity, frequency and seasonality; Bond & Keeley 2005), which are often correlated (Díaz-Delgado, Lloret & Pons 2003; Govender, Trollope & Van Wilgen 2006). We here focussed on the effect of fire frequency on butterfly communities. We sampled plots burnt at different frequencies from annual fires with open savanna to quasi-fire exclusion with closed savanna (Smit et al. 2010). Firstly, based on results observed for vertebrates (Beale et al. 2018), we expected butterfly species richness to be promoted with more frequent fires. Secondly, because changes in fire frequency induce changes in plant diversity and composition as well as vegetation structure (as already shown in the same study area by Smit et al. 2010; Smith et al. 2012), we expected fire frequency to influence both butterfly diversity and composition. Furthermore, we also expected butterfly species to show some degree of specialisation to specific fire frequencies. These last two hypotheses are related to the pyrodiversity–biodiversity hypothesis, which predicts that a spatial diversity of fire regimes should increase biodiversity at the landscape level (Parr & Andersen 2006). Finally, because butterfly communities are likely to show dynamic patterns with time since fire, we sampled butterfly communities across three sampling periods. We expected butterfly diversity to increase with time since fire, in line with the recovery of butterfly communities, particularly in plots burnt at intermediate and high frequencies.

This study represents a first assessment on the effect of differences in long-term fire frequencies on butterfly communities in wet African savannas. We observed significant differences in species abundance and species composition between fire frequencies.

The high abundance of butterflies observed in high fire frequency plots lends support to the idea that most savanna butterfly species are resilient to, or favoured by, some level of disturbance compared with no disturbance (Wilkerson, Roche & Young 2013). Butterflies are sensitive to changes in plant diversity, which may influence their distribution at both larval and adult stage (Gilbert & Singer 1975; Mevi-Schütz & Erhardt 2003). The fact that herbaceous species diversity is greater in plots burnt at high frequency than in plots burnt at medium frequency in the Kruger National Park (Burkepile et al. 2016) may contribute to increased butterfly species abundances.

The results partially confirm the specialisation hypothesis that predicts that different fire frequencies hosting different plant species composition will be associated with different butterfly communities (Gilbert & Singer 1975). Significant differences across fire frequencies (Figures 3a and b) is in accord with the fact that differences in fire frequency result in differences in plant species composition, as already shown in the same study area (Smith et al. 2012). In this long-term experiment, herbaceous vegetation has been shown to be heavily influenced by fire frequency, with lower herbaceous diversity in plots burnt at low frequency than in plots burnt at high or medium frequencies (Smith et al. 2012). This was true even it is accidental fire occurred in the low fire frequency plots, where the differences in vegetation structure and composition compared with frequently burnt plots remain very high. Such differences in host plants for the larval stage or flowers for the adult stage are likely to explain differences in butterfly community composition observed among EBPs (Swengel 2001). Accurate monitoring of the diversity of host plants and data on butterfly functional traits would, however, be necessary to confirm this preliminary finding.

We identified indicator species for each fire frequency, suggesting that butterfly species respond differently to variations in fire frequency. Literature suggests that butterfly forest specialists are generally sensitive to fire and can be negatively affected by frequent burning (De Andrade et al. 2017). Accordingly, we found that low fire frequency benefits the species that prefer shaded areas in mixed woodland and forest, like the Zulu shadefly (Online Appendix 1 Figure S3). Conversely, medium and high fire frequencies had positive effects on some open savanna species, such as the broad-bordered grass yellow and the cupreous blue (Online Appendix 1 Figure S3), which are resilient to fire and promoted by frequent burning (Adamidis et al. 2019).

This study furthermore suggests that butterfly diversity may be able to recover quickly after a fire. Indeed, butterfly species richness and abundance were similar 7 months and 29 months after the last fire in plots burnt triennially. This high speed of recovery has also been observed with ants (Parr et al. 2004) – likely linked to rapid recovery of vegetation. The recovery of butterfly communities could be attributed to an immigration effect from the surrounding landscape (Moranz, Fuhlendorf & Engle 2014; Robinson et al. 2013). The small size of experimental plots means that recovery time may be longer in natural landscapes where bigger fires occur. Further research is necessary to address variations in recovery time between species (Vogel, Koford & Debinski 2010).

This study was conducted in one of the largest and oldest fire experiments globally. Despite its many advantages, it is important to acknowledge the limitations with this long-term experiment, especially for mobile invertebrates such as butterflies. In their adult stage, butterflies can cover vast distances (hundreds of metres) (Pollard 1977) and are therefore able to move from one 7 ha plot to another. Although we did not include individual butterflies along the border of plots (20 m), it is possible that some of the individuals recorded during our surveys came from outside the plots. The presence of non-local vagrant butterflies may have influenced our ability to accurately assess species diversity (Avuletey & Niba 2014). This may explain the lack of difference in community composition based on species occurrence between fire frequencies in our study. More research should be conducted on transient individuals, notably on their ability to cross different vegetation structures, to assess differences in community composition and species richness between EBPs with higher certainty levels (Wood & Samways 1991). Additionally, even long-term experiment may be affected by inter-annual variations, such as, for instance, different climatic variations between years impacting either fire intensity or vegetation recovery after fire. For example, some species were very abundant 5 months after fire compared with 4 or 7 months after fire, in the high frequently burnt plot (Figure 2). Indeed, 4 and 7 months after fire correspond to the summer 2009–2010, although 5 months after fire correspond to 2012. Lastly, although we appreciate the sample size is relatively low, the size and duration of this experiment are valuable. Moreover, nevertheless, we were able to detect differences among the treatments suggesting that in this habitat, fire regimes can have a significant impact on butterfly communities.

This preliminary assessment focusses on fire frequency, an easy parameter for conservation managers to measure (Biggs et al. 2003). In the Pretoriuskop area of Kruger National Park, no part of the landscape currently has a fire return period of 2 years or less (SANPARKS, unpublished) and only 6% of the area burns infrequently (fire return period of > 5 years; SANPARKS, unpublished). This suggests that managers may need to promote a greater diversity of fire frequencies by (1) increasing fire frequency in some areas to provide habitat for species requiring high fire frequency; and (2) decreasing fire frequency in a larger proportion of the area. Our study highlights that insect responses should be given more consideration to determine whether the current fire strategy in Kruger National Park is adequate in the long term for biodiversity conservation. Further studies should be conducted to explore the role of other components of fire regime on wet savanna biodiversity (Beale et al. 2018; Hempson et al. 2017) in relation with butterfly dispersal capacities and the ability of larvae to escape fire, to explicitly address colonisation and extinction processes after fire.