Sources of variation in consumer-diet ?15N enrichment: a meta-analysis

Abstract

Measurements of δ¹⁵N of consumers are usually higher than those of their diet. This general pattern is widely used to make inferences about trophic relationships in ecological studies, although the underlying mechanisms causing the pattern are poorly understood. However, there can be substantial variation in consumer-diet δ¹⁵N enrichment within this general pattern. We conducted an extensive literature review, which yielded 134 estimates from controlled studies of consumer-diet δ¹⁵N enrichment, to test the significance of several potential sources of variation by means of meta-analyses. We found patterns related to processes of nitrogen assimilation and excretion. There was a significant effect of the main biochemical form of nitrogenous waste: ammonotelic organisms show lower δ¹⁵N enrichment than ureotelic or uricotelic organisms. There were no significant differences between animals feeding on plant food, animal food, or manufactured mixtures, but detritivores yielded significantly lower estimates of enrichment. δ¹⁵N enrichment was found to increase significantly with the C:N ratio of the diet, suggesting that a nitrogen-poor diet can have an effect similar to that already documented for fasting organisms. There were also differences among taxonomic classes: molluscs and crustaceans generally yielded lower δ¹⁵N enrichment. The lower δ¹⁵N enrichment might be related to the fact that molluscs and crustaceans excrete mainly ammonia, or to the fact that many were detritivores. Organisms inhabiting marine environments yielded significantly lower estimates of δ¹⁵N enrichment than organisms inhabiting terrestrial or freshwater environments, a pattern that was influenced by the number of marine, ammonotelic, crustaceans and molluscs. Overall, our analyses point to several important sources of variation in δ¹⁵N enrichment and suggest that the most important of them are the main biochemical form of nitrogen excretion and nutritional status. The variance of estimates of δ¹⁵N enrichment, as well as the fact that enrichment may be different in certain groups of organisms should be taken into account in statistical approaches for studying diet and trophic relationships.