Parasites and stable isotopes: a comparative analysis of isotopic discrimination in parasitic trophic interactions

Stable isotopes are widely used to identify trophic interactions and to determine trophic positions of organisms in food webs. Comparative studies have provided general insights into the variation in isotopic composition between consumers and their diet (discrimination factors) in predator-prey and herbivore-plant relationships while other major components of food webs such as host-parasite interactions have been largely overlooked. In this study, we conducted a literature-based comparative analysis using phylogenetically-controlled mixed effects models, accounting for both parasite and host phylogenies, to investigate patterns and potential drivers in Delta C-13 and Delta N-15 discrimination factors in metazoan parasitic trophic interactions. Our analysis of 101 parasite-host pairs revealed a large range in Delta C-13 (-8.2 to 6.5) and Delta N-15 (-6.7 to 9.0) among parasite species, with no significant overall depletion or enrichment of C-13 and N-15 in parasites. As previously found in other trophic interactions, we identified a scaling relationship between the host isotopic value and both discrimination factors with Delta C-13 and Delta N-15 decreasing with increasing host delta C-13 and delta N-15, respectively. Furthermore, parasite phylogenetic history explained a large fraction (>60%) of the observed variation in the Delta N-15 discrimination factor. Our findings suggest that the traditional isotope ecology framework (using an average Delta N-15 of 3.4 parts per thousand) applies poorly to parasitic trophic interactions. They further indicate the need for a scaled rather than a fixed trophic discrimination factor framework along gradients of host delta N-15. We also identified several conceptual and methodological issues which should to be considered in future research to help integrate parasitic interactions into a holistic isotope ecology framework across diverse trophic interactions.