Availability of a Keystone Plant Resource Reduces Niche Overlap and Drives Distinct Species-Specific Changes in Niche Breadth Among Small Rodents.

Abstract

The araucaria pine (Araucaria angustifolia) is a putative keystone species in the Araucaria Forest of southern South America. Its large seeds (pinhões) constitute an important food resource for small rodents, whose abundance and trophic dynamics may vary with seed availability. We investigated the dietary importance of Araucaria angustifolia seeds for three native omnivorous sigmodontine rodents-Akodon paranaensis, Oligoryzomys flavescens, and Oligoryzomys nigripes-and evaluated whether their responses to seed availability support a single functional-group assumption. If rodents act as a single functional group, increased pinhão consumption should reduce niche breadth, increase interspecific niche overlap, and raise rodent abundance during seed production. We assessed rodent abundance through trapping over ten months spanning seed production and non-production periods and analyzed stomach contents to characterize diet composition. Food items were grouped into five categories, with arthropods, pinhões, and other seeds or fruits accounting for more than ninety-five percent of all recorded items. Arthropods were consumed more frequently during the non-production period, particularly by Akodon paranaensis. In contrast, Oligoryzomys species consumed more pinhões and reached peak abundance during the production period. Trophic niche overlap was greater than expected by chance only during the non-production period. During seed production, Oligoryzomys species reduced their niche breadth, whereas Akodon paranaensis broadened its diet, being less dependent on Araucaria angustifolia seeds. Overall, species exhibited distinct responses to pinhão availability, with Oligoryzomys spp.-especially O. nigripes-showing stronger functional and numerical responses, underscoring their key role in Araucaria seed-rodent interactions.