Host Evaluation: Reliance on Symbionts: A new metric for quantifying nutrient exchange in nutritional mutualisms

Abstract Mutualisms between autotrophs and heterotrophs that exchange nutrients are of pivotal importance in the ecology and evolution of life on Earth. However, investigation of holobiont trophic strategy (reliance on autotrophy vs. heterotrophy) is hampered by practical challenges in quantifying their diverse nutritional pathways. One prevalent solution is the use of stable isotope analysis (SIA); in particular, the overlap of the two partners in isotopic space serves as an estimate of autotrophic contribution to the holobiont's diet. Yet, existing methods are inconsistently applied throughout the literature and fail to incorporate the variation of the isotopic signal of both partners. We developed a novel metric called Host Evaluation: Reliance on Symbionts (HERS) that addressed existing limitations by (i) selecting and fitting two optimal sizes of ellipse areas and (ii) combining metrics generated from both heterotroph (host) and autotroph (symbiont) carbon and nitrogen stable isotope values into an index, ranging from zero to one. The accuracy and stability of HERS were tested with nine synthetic datasets representing diverse holobiont trophic strategies and a published empirical dataset composed of seven coral genera. HERS performance was assessed relative to other stable isotope metrics through comparison with existing literature that employed non‐SIA methods. Our results identified ellipses encompassing 40% and 95% of isotope variance as the optimal sizes to include in HERS to accurately estimate trophic strategy. HERS was stable across all tested scenarios and was more consistent with existing literature than all other stable isotope metrics. Variance and accuracy greatly improved with increased sample size, with greater sampling necessary to assess the trophic strategy of holobionts that are strongly heterotrophic or autotrophic. We demonstrated that HERS is a rapid, cost‐effective metric that can be used to assess diverse mixotrophic scenarios. For the first time, this metric incorporates the overlapping proportions of symbiont ellipse areas, resulting in a more relevant biological assessment. Furthermore, HERS employs two differently sized ellipse areas, providing a balanced estimate across different trophic strategies. Finally, based on our analyses, we recommend HERS users to sample ≥20 individuals.