Dependence of diverse consumers on detritus in a tropical rain forest food web as revealed by radiocarbon analysis

Energy and material flows from dead organic matter, or detritus, to generalist predators have a potential impact on the food web dynamics. However, little is known about how commonly generalist predators depend on detritivorous prey, or the detritus on which the detritivores have fed in terrestrial food webs. To examine this, we measured the diet ages of terrestrial invertebrate and vertebrate consumers (>30 species) at multiple trophic levels in a tropical rain forest, with a particular focus on ants and termites by using radiocarbon (C-14). Here, we defined diet age as the lag time between the primary production and the utilization by consumer organisms. The diet ages varied from 0 to >50years and corresponded to known feeding habits of the consumers. Herbivores such as bees, butterflies, a frugivorous bird and bat, and nectar-feeding ants had young diet ages (0-3years). Meanwhile, detritivores such as termites had old diet ages, which increased according to the food resources in the order of litter (6years), soil (10years) and wood (19years). The diet ages of predators such as wolf spiders, hunting wasps, army ants, tree shrews and an insectivorous bat were intermediate (2-8years), indicating the dependence of many predators on detritivores. Because known dietary components of the predators include herbivores and detritivores, the intermediate ages likely indicate the coupling of energy and material flows between plant-based and detritus-based food webs. Diet ages of soil-feeding termite and army ant differed significantly, although a previous study reported that their nitrogen isotope ratios were indistinguishable despite the differing feeding habits. This indicates that radiocarbon can distinguish the two factors, trophic enrichment and the below-ground processes (humification), both of which could influence the nitrogen isotopic signatures of the terrestrial consumers. Our results show that radiocarbon would provide insights into structures of terrestrial food webs as well as time frame of energy and material flows through the webs.