No bottom-up effects of food addition on predators in a tropical forest

Several studies in temperate forests have demonstrated effects of litter addition on decomposers and predators. However, adding litter does not allow separating the effects of food availability and habitat space. We investigated the response of decomposers and predators to increased food resources and space in forests of the southern Mata Atlântica of Brazil. In two forest ecosystems representing an early successional stage of secondary forests and old-growth forest, we added nutrient-rich organic material, artificial litter of no nutritional value, or a combination of both to the soil surface of 120 plots to separate the effects of habitat space and food on soil food webs. We sampled litter- and soil-dwelling arthropods after three months using pitfall traps, soil sample extraction, and sticky traps just above the soil. Adding artificial litter had no positive effect on any of the 17 analyzed arthropod groups. Combining all sampled arthropods the effect was even significantly negative. Adding food had a positive effect on the abundance of decomposers, but not predators. We found no interactions between added artificial litter and added organic material. Our results suggest that the soil fauna in tropical forests is food limited. The lack of a bottom-up effect on predators suggests that they are not predominantly regulated by the abundance of epigeic prey but rather by competition or predation.