Consequences of inferring diet from feeding guilds when estimating and interpreting consumer-resource stoichiometry

SUMMARY 1. Imbalances between the supply of elements from resources and their demand from consumers may constrain key ecological processes, such as growth and production. Most previous studies have estimated such stoichiometric imbalances between consumers and resources by inferring the diet of the former from functional classifications rather than by direct assessments of the diet. However, this does not allow for potentially plastic responses of consumers to a restricted supply of elemental resources. 2. Here, for three streams of very contrasting nutrient availability, we calculated elemental imbalances between consumers and resources using diets derived from empirical gut contents analysis and compared them with those inferred for the functional feeding guilds of the species concerned. 3. In almost every case, elemental imbalances (C:P and N:P) based on the realised diet differed significantly from those expected from the inferred diet, the former revealing greater alignment between the elemental composition of consumers and their resources, particularly for P. 4. Simply inferring the diet, as is commonly done, results in erroneous estimates of elemental imbalances and misleading conclusions about stoichiometric constraints on consumers.