An invasive appetite: Combining molecular and stable isotope analyses to reveal the diet of introduced house mice (Mus musculus) on a small, subtropical island

House mice (Mus musculus) pose a conservation threat on islands, where they adversely affect native species’ distributions, densities, and persistence. On Sand Island of Kuaihelani, mice recently began to depredate nesting adult mōlī (Laysan Albatross, Phoebastria immutabilis). Efforts are underway to eradicate mice from Sand Island, but knowledge of mouse diet is needed to predict ecosystem response and recovery following mouse removal. We used next-generation sequencing to identify what mice eat on Sand Island, followed by stable isotope analysis to estimate the proportions contributed by taxa to mouse diet. We collected paired fecal and hair samples from 318 mice between April 2018 to May 2019; mice were trapped approximately every eight weeks among four distinct habitat types to provide insight into temporal and spatial variation. Sand Island’s mice mainly consume arthropods, with nearly equal (but substantially smaller) contributions of C3 plants, C4 plants, and mōlī. Although seabird tissue is a small portion of mouse diet, mice consume many detrital-feeding arthropods in and around seabird carcasses, such as isopods, flesh flies, ants, and cockroaches. Additionally, most arthropods and plants eaten by mice are non-native. Mouse diet composition differs among habitat types but changes minimally throughout the year, indicating that mice are not necessarily limited by food source availability or accessibility. Eradication of house mice may benefit seabirds on Sand Island, but it is unclear how arthropod and plant communities may respond and change. Non-native and invasive arthropods and plants previously consumed (and possibly suppressed) by mice may be released post-eradication, which could prevent recovery of native taxa. Comprehensive knowledge of target species’ diet is a critical component of eradication planning. Dietary information should be used both to identify and to monitor which taxa may respond most strongly to invasive species removal and to assess if proactive, pre-eradication management activities are warranted.

Methods
House mouse metadata includes house mouse samples as rows and columns include data related to the date collected, location, and habitat type as well as the sex and age of each mouse specimen. The mouse metadata set contains data from mice captured on Sand Island from April 2018 to May 2019. We used eight existing traplines across Sand Island to capture mice and account for differences in food source composition, availability, and accessibility. Each trapline consisted of five pre-baited traps with peanut butter and oats spaced 10 m apart, for a total of 40 traps (n = 40). We trapped mice across four habitat types: forest, herbland, shrub, and wetland. We conducted seven trapping sessions; each averaged 6.1 (± 0.7 SD) trapping days (or 5.1 ± 0.7 SD trap nights), and intervals between trapping sessions averaged 55.8 (± 15.0 SD) days. For each mouse captured, we determined sex (female, male, unknown), reproductive condition (if female), and age (juvenile, adult, unknown). Mice were humanely euthanized to remove colon contents and hair (which was collected from the lower abdomen area) in a sterilized lab setting. Colon samples were individually stored in vials of 70% isopropyl alcohol, labelled, and placed in a -20°C freezer until shipment to the mainland for further preparation; hairs were kept in dry, individually-sealed vials at room temperature. Mouse metadata includes information for 318 house mouse samples (both colon and hair samples) used for next-generation sequencing (NGS) and stable isotope analysis (SIA).
Molecular data includes house mouse fecal samples as rows and raw read counts of plant and arthropod ASVs as columns. Each column is an ASV, and the assigned taxonomy is included in the first row. Data have not been normalized by total read counts per sample. The ASV tables contain data from mouse fecal samples, from which DNA was extracted, amplified, and sequenced. DNA extraction, library preparation, pooling, and sequencing were performed at the University of Illinois at Chicago Genome Research Core within the Research Resources Center. For plant amplicons, DNA sequencing was performed on an Illumina MiSeq instrument employing V3 chemistry. For arthropod amplicons, DNA sequencing took place on an Illumina NovaSeq instrument owing to high host DNA amplification.

Stable isotope data includes house mouse hair tissue samples as rows and the carbon and nitrogen elemental composition (C:N ratio) and carbon and nitrogen isotopes (δ13C and δ15N) data as columns. The stable isotope tables contain data from mouse hair and diet item samples. All samples were processed in isotope-ratio mass spectrometers (Delta Plus Advantage Mass Spectrometer and Delta Plus XL Mass Spectrometer, Thermo Fisher Scientific, Waltham, USA) at Northern Illinois University and the University of Tennessee – Knoxville to obtain carbon and nitrogen elemental composition (C:N ratio) and carbon and nitrogen isotopes (δ13C and δ15N).