Diversidade e composição de espécies de besouros escarabeídeos em paisagens agrícolas da Caatinga

Abstract

Developing sustainable agricultural landscapes to meet present needs without compromising the ability of future generations to meet their own needs is undoubtedly a pressing global priority, particularly due to the ongoing conversion of vast areas of tropical forests into agricultural lands. This transformation negatively impacts biodiversity by altering soil characteristics, reducing and fragmenting species habitats, and creating mosaics of different land uses and covers. This study focuses on scarab beetles, essential elements for ecosystem functioning but understudied in agricultural environments. Fifteen rural properties in Cariri Paraibano, the driest semi-arid region of Brazil, were investigated to assess how the diversity and species composition of beetles respond to changes in soil (physical and chemical), local ecosystem (crop vs. forest), and landscape structure (composition, configuration, and matrix quality). Sampling was conducted during the rainy seasons of 2022 and 2023, when beetles emerge in the Caatinga. On each property, ten pitfall traps were set up in a cultivated area and ten in the nearest well-preserved forest remnant, with an average distance of 20 meters between each point. Half of the pitfalls were baited with human feces and half with rotting bovine spleen. After 48 hours, all captured animals were collected and identified. To describe the soil's chemical and physical characteristics, analyses of 15 soil descriptors, such as pH, potassium, clay, and sand concentration, were performed. To describe the landscape structure, orthomosaics from aerial photographs taken with a drone were constructed, and land use and cover were classified into exposed soil, cultivation, pasture, regenerating shrub areas, arboreal Caatinga, and Caatinga forest. From the centroid of the sampled crop, the percentages of conserved ecosystems (i.e., arboreal Caatinga + Caatinga forest), the density of conserved ecosystem fragments, and the level of human impact on the matrix were estimated within radii of 100 m, 300 m, and 500 m. A total of 3501 scarab beetles from 13 genera and 33 species were collected. About 36% of the species (s = 12) were exclusive to the forest. The two most dominant species (Dichotomius geminatus and Deltochilum verruciferum), which accounted for 56.2% of the sampled individuals, have a wide distribution within the Brazilian semi-arid region, high phenotypic plasticity, and the ability to colonize even urban environments. Soil conditions influenced species composition but not their diversity. Similarly, the crop vs. forest comparison revealed that the species composition in crops is a subset of the forest species composition. Landscape structure descriptors, i.e., composition, configuration, and the level of human impact on the matrix, did not significantly impact the diversity and composition metrics of the communities. Overall, the results suggest that the conversion of forests into subsistence agricultural fields can alter the species composition of scarab beetles, but species diversity is maintained in the crops, possibly due to the high 12 phenotypic plasticity of the group. They also suggest that modifications in landscape structure are less relevant and, if they occur, are likely at spatial scales smaller than 100 m. It is concluded that forest conversion into agricultural fields can modify the species composition of scarab beetles, but diversity remains in the crops, likely due to high phenotypic plasticity, while modifications in landscape structure appear to have a limited impact.