Dietary plasticity of black rats (Rattus rattus) and its implications for competition with small native mammals in the Andean temperate rainforest of Chile

We investigated the diet of the invasive black rat ( Rattus rattus ) and its overlap with co-occurring small native mammals in protected areas of the temperate forests of southern Chile. Our study was conducted during three consecutive winters between 2022 and 2025. We collected 165 fecal samples that were pooled together by location into 26 pools to describe the diet breadth of the black rat. For metabarcoding analysis, we analyzed pooled extracts aggregated by locality × sampling period (up to six 6 pellets per pool; 21 black rat pools, four small native-mammal pools and one for the only marsupial species present in the study area, Dromiciops gliroides ), using a multi-marker strategy (trnL, COI, 16S). Results are interpreted as pool-level trophic overlap and potential interference rather than direct evidence of individual-level competition. Dietary overlap was quantified using Pianka’s index and Jaccard similarity. Rattus rattus exhibited a broad omnivorous diet spanning 37 plant families, 9 arthropod orders, and eight vertebrate families, including native rodents and D. gliroides . Small native mammals displayed narrower niches, with dietary overlap being high for arthropods (Pianka = 0.835), moderate for plants at higher taxonomic level (family level) (Pianka = 0.40), and low for plant species (Jaccard = 0.11). Substantial inter-individual variation indicated that some black rats relied heavily on anthropogenic subsidies (processed foods, exotic plants, human DNA), whereas other individual black rats overlapped directly with native taxa. Black rats demonstrated the ability to exploit both native and anthropogenic resources, resulting in trophic interference with native small mammals. This overlap increases the risk of competitive pressure and predation of small native mammals in globally significant southern temperate rainforests.