A Test of Top-down and Bottom-up Control in a Detritus-based Food Web

In food webs based on primary production, biomass of organisms within trophic levels can be simultaneously controlled by resources (bottom-up) and consumers (top-down). In contrast, very little is known about top-down and bottom-up control in detritus-based food webs. Here, we tested whether exclusion of macroconsumers (fishes and shrimps) and/or phosphorus (P) addition affected insect detritivore biomass and decay rate and quality of leaf detritus in a tropical stream. Four treatments were established in a third-order stream in Costa Rica: (1) macroconsumers present, ambient P; (2) macroconsumers excluded, ambient P; (3) macroconsumers present, P added; (4) macroconsumers excluded, P added. Biomass of insect larvae inhabiting leaf packs and mass loss of leaves were measured after 7 and 10 d in situ. After 10 d, biomass and density of insect larvae increased as a result of both P addition and exclusion of macroconsumers. Chironomids (Diptera, Chironominae) were the dominant detritivores in leaf packs, comprising 51-80% of total invertebrate biomass and were responsible for the observed treatment effects. Mass loss rates of leaf packs were accelerated by the presence of macroconsumers but not by P addition. Positive effects of P on insects presumably occurred through increased microbial carbon relative to leaf carbon. However, percentage nitrogen (N), C:N, and fungal biomass of leaves were not affected by either experimental treatment. Laboratory growth studies corroborated positive effects of P on chironomids: growth rates were higher in high-P treatments (high-P stream water and low-P stream water with P added) vs. low-P stream water. However, no differences in the in situ growth rates were observed between a highand a low-P stream. The relative importance of top-down and bottom-up effects was evaluated using several indices. Loss rates of organic matter were affected more by top-down effects of macroconsumers than by bottom-up effects of P. Macroconsumers had negative effects on two trophic levels, contrary to food-web theory predicting alternating negative and positive effects. Positive bottom-up effects of P on insect biomass were greater than negative topdown effects of macroconsumers. In addition, P effects on invertebrates were similar in direction but greater in magnitude than previously published effects of nutrients on consumers in food webs based on primary producers. These results suggest that the impacts of nutrient enrichment on detritivores may be as great or greater than those previously observed on herbivores.