Grassland ecosystems, covering approximately 40% of the Earth's surface, are characterized by high biodiversity and endemism. These ecosystems are intrinsically linked to disturbance regimes, primarily grazing and fire. Fire, in particular, removes senescent plant tissue, recycles nutrients, and increases plant species richness compared to unmanaged grasslands. The frequency of fires varies with climate and management practices. While the impact of fire on grassland fauna tends to be species-specific, most responses are linked to habitat heterogeneity, resource availability, and interspecific interactions. Many plant and animal species inhabiting fire-prone environments have evolved adaptations to these disturbances. Fire is a key driver of grassland dynamics, promoting and maintaining diversity by breaking species dominance and reducing aboveground biomass of large tussock-forming species. The absence of disturbances leads to homogeneous structure dominated by tall grasses and shrubs, resulting in biodiversity loss. This study investigated the effect of fire frequency (measured as time since the last fire event) on habitat structure and bird communities in South Brazilian Highland Grasslands. It was hypothesized that sites with longer time since fire would exhibit higher vegetation height, lower habitat heterogeneity, lower plant and bird diversity, and different bird food guild composition compared to recently-burnt sites.

The role of disturbances as drivers of biodiversity patterns has been a central theme in ecology. Diversity-disturbance relationships (DDR) can be monotonic, U-shaped, or peaked (hump-shaped). Peaked relationships are often linked to the intermediate disturbance hypothesis (IDH), although the mechanisms and underlying premises of the IDH have been challenged. Peaked responses to grazing disturbances are common in highly productive grasslands. However, the effects of fire on grassland diversity can differ from those of grazing. Although the effect of fire on plant diversity is extensively studied, fire-diversity relationships are often tested with monotonic models, neglecting potential peaked responses. The effect of fire disturbance on bird communities is less understood. Studies suggest that overall avian diversity and abundance decline as fire-suppressed grasslands become denser, making foraging difficult for ground-feeding birds. In contrast, recently-burnt areas exhibit higher bird richness and diversity due to increased arthropod abundance. Raptors are attracted to newly-burned areas for increased prey availability. However, very frequent or infrequent fires can negatively affect seed availability for granivores. While some evidence suggests that South Brazilian Highland Grassland bird communities are influenced by fire disturbance, a comprehensive community-wide study was lacking. Certain species thrive in frequently-burnt areas with shorter vegetation, while others prefer areas with higher vegetation and lower fire frequency. Fire can also negatively impact bird reproductive success if it occurs during the breeding season.

Seven sites in South Brazilian Highland Grasslands, representing a chronosequence of fire disturbance histories (time since fire ranging from 3 to 180 months), were selected. All sites were under low and similar cattle grazing pressure. Birds were sampled during the breeding season (December 2017-February 2018) using 10-minute point counts with an 80m radius. Vegetation sampling was conducted in February 2018 at each point count location, classifying plant species into three life forms (shrubs, forbs, and graminoids). Plant species richness, abundance, vegetation height, and the cover of bare soil, water, and rock were recorded. The abundance of *Eryngium*, an important resource for birds, was also counted. Data were summarized at the site level. Principal Component Analyses (PCA) were used to extract ordination axis scores for habitat structure, bird taxonomic structure, and bird functional structure (based on six food guilds). Linear Mixed-Effect Models (LME) were employed to test the influence of time since fire on plant and bird community descriptors. A multilevel method (MLM3) was used to analyze the relationship between habitat structure and bird functional descriptors. Canonical Correspondence Analyses (CCA) were conducted to explore the relationship between habitat variables and bird communities (taxonomic and functional).

A total of 862 individuals from 70 bird species were recorded, belonging to six food guilds. Insectivores and omnivores were dominant. Four globally threatened and three near-threatened species were identified. Plant community responses to time since fire were mostly quadratic (peaked), except for vegetation height, which increased linearly. Sites with intermediate time since fire showed the highest habitat heterogeneity, plant species richness, and graminoid and forb species richness. Shrub abundance increased with time since fire. Bird taxonomic diversity and abundance were higher in recently-burnt sites and decreased linearly with time since fire. Bird species composition and functional composition (food guilds) were also influenced by time since fire. Granivore birds were more abundant in sites with longer time since fire. CCA revealed significant relationships between habitat variables and bird communities (both taxonomic and functional). Species such as *Emberizoides ypiranganus* and *Sporophila melanogaster* were associated with taller vegetation in sites with longer time since fire, while *Cinclodes pabsti* and *Anthus nattereri* were associated with shorter vegetation and forbs in recently burnt sites. Frugivores were associated with forbs and *Eryngium*, while granivores were associated with taller vegetation.

The findings corroborate the hypothesis that grassland habitat structure and heterogeneity vary with time since fire. Grasslands without disturbance show increased biomass and homogenization, leading to species exclusion and diversity loss. The peaked DDR for most plant community descriptors suggests that intermediate disturbance frequencies are optimal for plant diversity. Vegetation height and shrub abundance increased with time since fire, potentially due to tussock grass dominance and shrub encroachment. The linear relationship between time since fire and bird community descriptors contrasts with the plant responses. While longer time since fire led to lower bird diversity and abundance, it also supported granivore species. The CCA results highlight the strong influence of habitat structure on bird community structure, mediated by bottom-up effects. Habitat heterogeneity, shaped by fire, is crucial for supporting birds with diverse habitat requirements. The optimal fire regime should consider seasonal timing to avoid impacts on bird breeding success.

This study demonstrates that fire disturbance significantly shapes habitat structure and bird communities in South Brazilian Highland Grasslands. A mosaic of areas with different fire frequencies creates diverse grassland habitats, maximizing conservation outcomes. Prescribed patch burning is a valuable management tool, but optimal fire frequency and intensity require further investigation. The relatively small number of spatial replicates limits the generalizability of the findings, emphasizing the need for further research across various grassland systems. Long-term studies are essential for developing effective and sustainable fire management policies that integrate biodiversity conservation and financial sustainability.

The relatively small number of spatial replicates (seven sites) might limit the generalizability of the findings beyond the specific study area. While grazing pressure was assumed to be similar across sites, small variations could have influenced the results. The study focused on the effect of fire, but other factors, such as soil characteristics and microclimate, could also contribute to the observed patterns. Further research is needed to fully explore the interactive effects of grazing and fire on grassland biodiversity.