Food webs are composed of networks of interconnected food chains, each originating from a unique basal resource (e.g. phytoplankton, seagrass, detritus). These energy channels are coupled by higher-order consumers, with channels becoming increasingly coupled at progressively higher trophic levels. Although unlikely, it is not clear whether energy channels contribute equally to support biomass at upper trophic levels, nor what processes may drive an asymmetrical pattern of energy flux. 

Results/Conclusions

Using stable isotope (δ¹³C, δ¹⁵N) data from food webs with three or more basal resources, we show that herbivores consume resources in proportion to primary production. Next, for several marine coastal food webs we determine the fraction of each consumer’s diet that is ultimately derived from each of multiple basal resources, and identify the attendant energy channels. We demonstrate that energy flux (mean consumption rate) is greater in channels that are based on resources which are more productive and which have higher biomass turnover rates (production:biomass ratios). Moreover, we show that the indirect dietary contributions of basal resources to consumers at uppermost trophic levels are also a linear function of basal resource production and biomass turnover rate. Our results suggest that higher-order consumers do not rely equally on energy channels. Rather, we identify a suite of food web structural traits associated with the energetic characteristics of basal resources, and suggest that these traits collectively mediate the relative importance of energy channels in supporting biomass at upper trophic levels.