New method of isotopic analysis: baseline‐standardized isotope vector analysis shows trophic partitioning in loricariids

Stable isotope analyses have refined the study of trophic niche diversity within an ecosystem, yet traditional trophic partitioning methods may not be appropriate to identify variation among groups with similar dietary requirements. By building on vector‐based analyses, we introduce a baseline‐standardized isotopic vector analysis (BaSIVA) to visualize dietary variation while accounting for isotopic discrepancies between locations. To test the effectiveness of our new method, we collected muscle samples from eleven species of Loricarioidea in five assemblages in Northern Peru. Loricarioidea is a large, ecomorphologically diverse superfamily of scraping‐feeding fishes. Most feed on an indistinguishable mix of detritus and algae, but some lineages have specialized diets of wood, seeds, and macroinvertebrates, making them an excellent group to study trophic variation. Isotopic data were collected using mass spectrometric isotope analyses, and communities were standardized by calculating a mean baseline (algae and periphyton) for each location. The entire community was shifted by subtracting the baseline of ¹⁵N and ¹³C from the consumers at each location, which allowed for comparison between assemblages. Incremental differences of ¹⁵N and ¹³C from the baseline were found via vector analysis, and the azimuth and module of each consumer were calculated. Standardization resulted in a significant shift of assemblages within the isotopic biplot, and vector analysis shows three trophic groups primarily described by differences in carbon assimilation. Isotopic variation between species may account for some diversification in jaw shape within the Loricarioidea, but BaSIVA suggests several instances of trophic overlap in different jaw morphologies. Moreover, results from BaSIVA are better able to delineate trophic groups than traditional trophic positioning methods while accounting for variation in basal resources. We suggest a baseline‐standardized vector analysis should be the standard for vector‐based stable isotope analysis in riverine environments with similar baseline resources.