LB 3-22 - Niche partitioning among populations of small mammals in a stochastic dryland environment

Climate change alters dryland precipitation patterns, resulting in drier and more stochastic ecosystems. Niche partitioning among consumers is a crucial process in stochastic environments where resource availability fluctuates unpredictably. Species inhabiting such ecosystems can reduce competitive pressures by diversifying their diets, enabling coexistence through resource partitioning, particularly during resource scarcity. We studied trophic niche dynamics in a small mammal community comprised of four core species—Octodon degus (herbivore), omnivorous Phyllotis darwini and Abrothrix olivacea; Thylamys elegans (insectivore)––inhabiting Bosque Fray Jorge National Park in central Chile. This site receives ~90% of annual rainfall (~95 mm) in winter, and ephemeral plant cover ranges from 0% in dry years to ~60–80% in wet years. Through a monthly live-trapping program coupled with carbon (d13C) and nitrogen (d15N) stable isotope analysis of blood plasma, we estimated Bayesian standard ellipse areas (SEAB) as a proxy for population isotopic (trophic) niches. We compared the niche trophic dynamics of two contrasting years about a ~4X change in annual precipitation, 2018 (81.6mm) vs. 2021 (17.7mm). Since rainfall enhances plant productivity at this site, we expected differences in foraging responses between years, specifically more significant niche partitioning among rodent species in the wetter year. Our results show that mean d13C differed among species during the wet year (2018) when A. olivaceus and T. elegans had the highest but similar d13C values, followed by P. darwini and O. degus. Mean d15N values were significantly different among species in both years. All species had similar d15N values during the wet year except for insectivorous T. elegans, which had higher values than the others, as expected. There was more trophic structure in the dry year (2021) when T. elegans had the highest d15N, followed by O. degus and A. olivacea with similar mean d15N mean, and P. darwini with the lowest d15N values. In addition, there was a complete overlap in isotopic niches among species during the wet year, whereas, in the dry year, there was no isotopic niche overlap among species. Our results suggest that rainfall significantly impacts this small mammal community's foraging behavior and niche partitioning. Moreover, niche partitioning during the wet year may be driven by increased ecological opportunity associated with increased productivity of ephemeral plant species. This adaptative foraging behavior probably promotes food web resilience allowing species to adjust their diets according to rainfall-driven variation in resource availability in this stochastic environment.Funded by ANID/FONDECYT N°3210814, ANID/FB210006, LTERB-NSF 2025816