Interactive effects between vegetation structure and soil fertility on tropical ground-dwelling arthropod assemblages

Abstract

The development of forests undergoing restoration determines the structure of the vegetation, influencing ground-dwelling arthropod composition. Soil fertility can interact with vegetation structure, influencing the diversity and composition of arthropod assemblages through bottom-up effects on developing forests. We evaluated if soil fertility (here defined by the attributes directly related to macronutrient availability) and the vegetation structure in three adjacent areas from different land-use/cover categories (abandoned pasture, riparian forest undergoing restoration, and remnant riparian forest) influences ground-dwelling arthropod assemblages. These areas were subject to a gradient of soil fertility due to the presence of a corral that resulted in a local increase of soil nutrients in the three areas, enabling to evaluate the interaction between soil fertility and vegetation structure. We hypothesized that ground-dwelling arthropod assemblages have lower diversity in pasture, intermediate in forest undergoing restoration and higher diversity in the remnant forest. Furthermore, we expected that increasing soil fertility would have a positive effect in these assemblages through bottom-up effects. Environmental conditions were characterized by vegetation structure and soil chemical attributes variables, whereas arthropods were sampled with pitfalls and identified to morphospecies. Diversity indicators responded differentially to the studied effects. Abundances decreased with soil fertility, and were higher in the forest undergoing restoration, followed by pasture and remnant forest. Species richness also varied among categories, following the same pattern as abundances. On the other hand, diversity indices indicated an interaction between soil fertility and land use category, with larger effects of soil fertility on diversity in the more simplified habitats (pasture and forest undergoing restoration), whereas no bottom-up effects were recorded in the remnant forest; species evenness increased with soil fertility in all studied areas. A redundancy analysis (RDA) indicated that both vegetation structure and soil attributes significantly influenced arthropod composition. Arthropod assemblages in areas undergoing restoration with higher soil fertility and low grass cover were more similar to assemblages in remnant forest when compared to areas with low soil fertility, independently of the vegetation structure, indicating an interaction between these two effects. Therefore, ground-dwelling arthropod assemblages responded both to increasing vegetation complexity resulting from the different land use/cover categories, and to increasing soil fertility, so that they are effective indicators to evaluate the recovery of ecosystems in forest restoration projects and to determine the importance of soil fertility in this process.