Ants as Flower Visitors: Diversity and Network Structure Assessed for the First Time in the Atlantic Forest of Southern Brazil.

Ants have long been recognized as key plant partners, given the high number of mutualisms historically documented between both groups. While their ecological roles are well-documented, ants as flower visitors remain understudied in the Atlantic Forest, a world's biodiversity hotspot. Here, we present firsthand records of ant-flower interactions, diversity, and network structure, and characterize the plant community visited by ants in two Atlantic Forest remnants. Fieldwork was conducted from April 2019 through May 2022 in two different vegetation physiognomies, the Mixed and the Dense Ombrophilous Forest, located respectively in the municipalities of Colombo and Morretes, in Paraná state. Using focal-plant methodology, we documented 32 ant species visiting flowers of 71 plant species. The most frequent flower visitors from each forest type were Camponotus crassus Mayr, 1862 (Mixed Forest) and Brachymyrmex heeri Forel, 1874 (Dense Forest) and most of the plant species visited by ants were native trees or shrubs with small flowers. Network analysis revealed similar network topologies across both vegetation types, with significantly low nestedness and intermediate modularity, as opposed to the expected predominantly nested architecture of mutualistic systems. Despite remarkable sampling effort and richness of observed plant species (n = 352), sampling coverage did not surpass 35% in both forests, indicating numerous undiscovered interactions. Future studies could increase sampling and conduct pollen experiments to gain deeper insight into ant-flower interactions. Given tropical ecosystem degradation and pollinator decline, our findings underscore the need for further research on ants as flower visitors, highlighting their role in ant-plant mutualisms in humid tropical forests.