Wildfires and flooding effects on the soil arachnid assemblage in the Cerrado-Pantanal ecotone

Abstract

Natural and recurrent events, such as fires and the flood pulse, play essential roles in shaping biodiversity and in the dynamics of biological communities in different ecosystems. These events, being natural disturbances, are intrinsically intertwined with the dynamics of biological communities, influencing the structuring of populations and fostering complex interactions between the organisms that make up ecological systems. With effects that extend from the regeneration and adaptation of plant species to the determination of the presence and abundance of animals, such events act as shaping forces promoting vital cycles, favoring specific niches and even stimulating the evolution of life forms that inhabit these landscapes in constant transformation. The Cerrado and Pantanal are distinct and biodiverse biomes in Brazil. While the Cerrado is characterized by its savanna vegetation and high species diversity, Pantanal is an extensive wetland that harbors unique aquatic ecosystems. The interaction between fires and flooding is a notable feature of these biomes. In the Cerrado, fires can be a natural and frequent, playing a role in maintaining its biodiversity. In the Pantanal, seasonal floods are essential for vegetation regeneration and diversification. However, changes in fire or flood patterns can significantly impact biological communities and the health of these delicately balanced ecosystems. In short, understanding the intrinsic role of fires and hydrological variations in ecosystem dynamics is crucial for the conservation and sustainable management of these precious natural environments. Through this study, we sought to verify the effects of wildfires and flooding on communities of two orders of soil arachnids, Araneae and Solifugae. This study was divided into three chapters. In the first chapter, we evaluated the effects of specific factors, such as frequency of fires, fire season, flooding and spatial structure, as well as the interaction between these factors on the soil arachnid assemblage in the Kadiwéu Indigenous Reserve. The frequency of fires positively affected spider richness. However, we observed a negative effect of the interaction between non-flooding sites and the frequency of fires on spider richness. There was an influence of flooding on the composition of spiders, which may be related to the fundamental role that hydrological variation plays in shaping the available habitats suitable for spiders. Spatial variables shape the composition of spiders and may reflect environmental heterogeneity, which may influence the distribution and aggregation of spider species in different locations in the study area. In the second chapter, we evaluated the temporal variation in the assemblage of spiders, comparing samples before the fires and different periods after the fires that occurred in 2021. We observed that spider abundance increased in the 3-4 months period after the fires. We also observed an increase in Normalized Difference Vegetation Indices (NDVI) during this period, demonstrating a rapid recovery of vegetation after fire events. Therefore, fire can positively affect the abundance of spiders indirectly, since the increase in NDVI may be related to the regrowth of plants after fires, thereby attracting more herbivorous insects that are potential prey for arachnids. However, there was a reduction in spider diversity after the fires. Considering that different species of the same taxonomic group may have specific habits, fire can have a positive, negative or neutral on certain species. Species replacement was the partition of betadiversity that most explained variation in spider composition. We observed that the number of species that contribute to differences in composition reduces over time. In the third chapter, we evaluated the effects of fire-specific factors (frequency and fire season) on solifuge abundance. Specific factors did not affect the abundance of solifuges. However, the number of individuals is structured over time after the fires, as well as the NDVI, demonstrating the indirect positive effect of fire on the abundance of solifuges. Wildfires and flooding can direct and indirectly affect populations of soil arachnids, promoting impacts on vegetation, influencing the availability of shelter and food. However, the results also indicate that communities are resilient and can recover in a short period.