Effects of time-since-fire on ant-plant interactions in southern Brazilian grasslands

Abstract Grasslands and savannas are dynamic ecosystems, strongly regulated by environmental disturbances such as fire. Maintaining disturbance regimes in these ecosystems are of great conservation concern, and require studies with the use of ecological indicators. Biotic interactions are a major component of biodiversity that are particularly sensitive to environmental changes, and therefore should be considered in ecosystem assessment and monitoring. One of the most common interactions between insects and plants is mediated by extrafloral nectaries (EFNs), where ants use food resources from plants in exchange for protection against herbivores. Here, we explored variations on this ant-plant interaction across a post-fire successional gradient (i.e., few months to more than two years since the last fire) in eleven grassland patches in South Brazil. We evaluated time-since-fire effects on (i) the grassland habitat structure, (ii) an EFN-bearing plant population: Chamaecrista repens, (iii) the EFN-visiting ant communities, and (iv) the ant-plant ecological networks. We expected fire-induced habitat changes to benefit this plant population, and to produce positive cascading effects on interacting ants, thus influencing the structure of ant-plant networks. Freshly-burnt sites presented increased C. repens abundance, and larger individuals with more reproductive structures and EFNs in comparison with sites longer without disturbances. Plant abundance and size were inversely related to the cover of dominant grasses along the habitat gradient. The density of EFN-visiting ants, but not their species richness, increased in freshly-burnt sites, stimulated by the greater resource offer and the habitat openness, which probably facilitated ant activity. Moreover, with increasing time-since-fire, ants interacted with fewer plants, and few species formed the network generalist core. These results could further suggest that the defense service against herbivory provided by ants is also diminished in grasslands longer without disturbances. Overall, by using a series of indicators of habitat change from multiple ecological levels, this study highlights the importance of disturbance for grassland biodiversity and their ecological interactions, helping to improve management decisions.