Metabarcoding Reveals Fine Scale Patterns of Trophic Resource Use and Partitioning Along Gradients of Land Use and Deer Density in a Multi-Species Ungulate Community

Across the northern hemisphere, ungulates are expanding in range and abundance, forming novel communities in increasingly human-modified landscapes. These shifts drive new interactions over available food resources, but patterns of resource use and partitioning in Europe's multi-species systems remain poorly understood. This study examined seasonal diets and resource partitioning in diverse cervid communities (moose, roe deer, red deer, and fallow deer) across two Swedish landscapes (coastal-boreal and boreo-nemoral) differing in deer density and land use. Based on their foraging strategies, we expected (Hypothesis 1) diet richness and dietary niche width to be greater in intermediate feeders (red and fallow deer) than in browsers (moose and roe deer), (Hypothesis 2) trophic partitioning between browsers and intermediate feeders to be driven mainly by graminoid use, and (Hypothesis 3) intra- and interspecific overlap to vary with season, deer density, habitat diversity, and proportion of arable land. DNA metabarcoding of 2568 fecal samples showed that deer consumed plants from over 70 families, though diets were typically dominated by fewer than 10. Vaccinium shrubs were key forages year-round, while birch and willow dominated during the growing season. Moose consumed large amounts of pine in spring and winter (> 50% in the boreo-nemoral, 35%-40% in the coastal-boreal landscape), with less during summer-autumn (similar to 15%). Forbs were important for smaller deer, especially in spring and summer-autumn, and more heavily used in winter in the boreo-nemoral landscape, likely due to supplementary feeding with human-provided food like hay or silage. Spruce use was low overall (< 5%), with fallow deer showing the highest intake. Consistent with Hypothesis 1, diet richness and niche width increased from moose to fallow deer. In partial support of Hypothesis 2, principal coordinates analysis (PCoA) revealed that graminoids contributed to trophic partitioning, but the pattern was not a strict browser-intermediate feeder divide. Moose consistently separated from the smaller deer due to avoidance of graminoids and reliance on pine and juniper, while roe deer, although a browser, sometimes overlapped with red and fallow deer through greater use of graminoids. During winter in the coastal-boreal landscape, wavy hairgrass (Avenella flexuosa) contributed to the significant separation between browsing roe deer and intermediate-feeding red deer diets, consistent with Hypothesis 2. Diet overlap among smaller deer varied with season and landscape. Intraspecific overlap was the highest in moose and the lowest in fallow deer, declining during summer-autumn across species. Overlap was influenced by deer density, habitat diversity, and arable land, consistent with Hypothesis 3, but effects were species-specific and explained only limited variation. Our results highlight the dietary plasticity of red and fallow deer, which may intensify resource competition with moose and roe deer in multi-species systems, particularly where supplementary feeding is common. These insights support adaptive, multi-species management of deer in northern ecosystems.