Increased grazing intensity in pastures reduces the abundance and richness of ground spiders in an integrated crop-livestock system

Abstract

Ground spiders can reflect the impacts of land-use intensification. In an integrated annual crop-livestock system, the grazing intensification can negatively influence the abundance and richness of ground spiders. We evaluated the difference in abundance, family and species richness, and species composition of ground spiders in the soybean and pasture environments, and the effect of grazing intensity on the spider community under a soybean-pasture rotation. We hypothesized that pastures would have higher spider species richness and a distinct species composition and that the increase in grazing intensity would reduce spider abundance and species richness. We conducted an experiment in an integrated annual crop-livestock system under a soybean-pasture rotation in southern Brazil that was managed for 14 years by alternating Glycine max for summer grain production and Avena strigosa + Lolium multiflorum for beef cattle grazing during the winter. We sampled four times over 2 years: twice after the grazing cycles and twice after the soybean harvests. Grazing intensification was measured as grazing heights of 10, 20, 30, and 40 cm. Pitfall traps were installed to capture ground spiders. A total of 2589 spiders were collected and classified into 23 families and 43 species. The abundance of spiders was seven times higher, and species richness was 35% higher in the postgrazing compared with the postsoybean environment. Linear mixed effects models showed that spider abundance and richness were strongly influenced by grazing height, particularly postgrazing, but the influence of grazing height on male spider abundance could still be detected in the postsoybean environment. We have shown for the first time that the management of grazing height affects the abundance and richness of spiders in an integrated annual crop-livestock system. To introduce a grazing cycle to a completely crop-centric production system and regulating grazing intensity can help maintain a more diverse spider community.