Presumed seed specialists rely on fungi as their primary food source at the sub-Arctic treeline

Range boundaries limit local populations, which may experience pronounced fluctuations in resource availability, particularly at higher latitudes, often seen as resource pulses. In boreal forests, conifers undergo pulses of seed production followed by intervals of low seed production, profoundly affecting consumers dependent on these resources. Red squirrels ( Tamiasciurus hudsonicus ) are considered seed specialists across the boreal forest. We evaluated how annual changes in white spruce ( Picea glauca ) cone production at the sub-Arctic treeline near Churchill, MB, Canada, influenced squirrels’ use of alternative food sources, predicting that low cone production would increase reliance on alternate foods. Cone crops varied from 2020–2023, with a mast year in 2022 of 471 cones per tree, approximately 70–80% lower than mast years elsewhere, and lower crops in other years (6-115 cones per tree). Furthermore, the number of filled seeds (containing an embryo) per cone was low, ranging from 0.6 ± 0.03 (mean ± SE) in 2022 to 3.6 ± 2.6 in 2023. Using stable isotope ratios of hair and Bayesian mixing models, we found that squirrels primarily consumed fungi (~ 70% of diet), even in mast years, with other food sources varying with cone production. The dominance of fungi in squirrel diet even in mast years, highlights the dietary plasticity of red squirrels beyond seed specialization challenging the seed specialization paradigm. Flexible foraging strategies likely allow populations to persist in resource-limited environments and may facilitate range expansion as climate change reshapes habitats.