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Bulk tissue versus amino acid carbon stable isotopes to reveal the diet and basal resource use of an invasive rodent

Holthuijzen, Wieteke1; Simberloff, Daniel1

Published Oct 23, 2025 on Dryad. https://doi.org/10.5061/dryad.4f4qrfjqg

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Abstract

Stable isotope analysis is widely used to reconstruct food webs, characterize trophic relationships, and estimate dietary composition. However, bulk tissue stable isotope analysis (BSIA) can yield diffuse or imprecise estimates when dietary sources are isotopically similar. We compared carbon stable isotope analysis of amino acids (δ13C-EAA values) with BSIA (δ13C and δ15N values) to estimate diet composition and basal resource use of non-native, invasive house mice (Mus musculus) on Sand Island, part of a remote atoll in the north Pacific Ocean. We aimed to assess whether δ13C-EAA values could improve source differentiation and yield more precise diet estimates and to evaluate whether δ13C-EAA fingerprints could provide insight into the origin and use of basal resources by mice.

We applied a Bayesian mixing model using δ13C-EAA values from a subset of 10 mice and associated diet items, with sample selection guided by existing BSIA and next-generation sequencing datasets. Diet estimates from the δ13C-EAA model closely resembled those from the BSIA-based model (n = 90): arthropods dominated the mouse diet, with smaller contributions from seabirds and plants. Moreover, estimates from the δ13C-EAA model were more consistent and precise than those from a BSIA model using the same 10 individuals, underscoring the value of informed sample selection. Although the use of δ13C-EAA values did not substantially improve separation among isotopically similar sources, it reinforced our previous findings.

We also used δ13C-EAA fingerprinting to reconstruct basal resource use. Mice obtained most of their essential amino acids from aquatic basal sources—likely through direct consumption of seabird tissues or indirect consumption of scavenging arthropods—whereas terrestrial resources contributed far less. These findings highlight the role of marine-derived nutrients in this food web and suggest that mice may reroute energy and nutrients in the Sand Island system by exploiting marine subsidies.

Collectively, our results suggest that δ13C-EAA values are most effective when paired with pre-existing ecological knowledge, especially for omnivores in complex food webs. Applied judiciously, this method may strengthen ecological inferences and provide additional insight into impacts of invasive species—particularly in vulnerable island ecosystems.