Biogeochemical niches differ between two species of small mammals, seasons, and with ontogenetic changes in body mass

Metal(loid) contamination is a major threat to ecosystems, as these potentially toxic elements alter the fitness of species and may destabilise food webs. Due to their large abundances and intermediate trophic position between primary producers and apex predators, small mammals are keystone organisms in terrestrial food webs, potentially acting as vectors of contaminants through the food chain. The biogeochemical niche hypothesis (BNH) posits that each species exhibits its own elemental niche and that these niches respond to changes in the abiotic and biotic environment. However, the drivers of seasonal and ontogenetic variation in both essential and potentially toxic elemental concentrations in small mammals are little understood yet, as the BNH has been predominantly tested in vascular plants. Such knowledge can improve our predictions regarding contamination pathways and how the biogeochemical niches of species respond to changes in the environment. Here, we analysed 87 individuals from two distinct omnivorous species (the Yellow-necked mouse, Apodemus flavicollis, and the Bank vole, Clethrionomys glareolus) sampled in the Ore Mountains, Czech Republic. Specifically, we tested whether the concentrations and compositions of essential (e.g., Na, Mg, Ca) and potentially toxic elements (e.g., Pb, Cd, As) change with ontogenetic variability in the body mass of the individuals, and between seasons (spring vs. autumn). In support of the BNH, species differed in their biogeochemical niches, and most elements showed higher concentrations in spring than in autumn, suggesting that seasonal changes in either the bioavailability of elements, resource–consumer, and/ or consumer–consumer interactions can be detected in biogeochemical niches. To finish, we found shifts in elemental composition with species-specific ontogenetic changes in the body mass of the individuals. Our results provide the first insights into a deeper understanding of temporal variation in the biogeochemical niches of animals, that is, an empirical stepping-stone to a framework that can advance population and community ecology in a context of changes in the distribution of Earth elements.