Is there a bottom-up cascade on the assemblages of trees, arboreal insects and spiders in a semiarid Caatinga?

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Predator–prey dynamics regulate the community structure of arthropods and may be affected by changes in their habitat. Bottom-up effects influence the distribution of arboreal insects changing, in turn, the availability of resources and shelter for the spider assemblage. This study examines the effect of habitat structure (tree richness and density) on the assemblages of herbivorous insects and spiders along a gradient of secondary forest succession in a semiarid Caatinga in northeastern Brazil. Insect and spiders were collected by beating the foliage during dry and wet seasons in 2010 and 2011. Plant richness and density varied significantly among stages of succession. In total, 2,590 arboreal insects were collected, corresponding to 14 orders and 151 morphospecies. In addition, we sampled 896 canopy spiders, encompassing 14 families and 53 morphospecies. Insect richness and abundance were negatively related with habitat structure, while spider richness increases with tree richness. We did not find any relationship among insects and spiders but there were differences in species composition of insects among all successional stages, determined by changes in vegetation structure. However, this effect does not extend to the species composition of spiders that remain similar in the different stages of secondary succession. We conclude that habitat structure had a partial bottom-up effect on arboreal insects and spider assemblages, indicating that this factor regulates the richness, abundance and species composition of arboreal insets and species richness of spiders. Thus, habitat structure promotes the maintenance of insects and spider diversity in fragments of Caatinga.