Flower visitation does not necessarily mean pollination. In this sense, floral visitors can either act as mutualists (pollinators) or antagonists (floral robbers/thieves), indicating that these interactions are part of a continuum and that a visitor species can present multiple behaviours. We included both mutualistic and antagonistic interactions between plants and floral visitors in a multilayer network to explore the consequences (at the community level) of the dual roles played by flower visitors. The multilayer network of interactions was formed by herbaceous plants (12 species) and insects that visited their flowers (21 species) in an area of Atlantic Forest in Brazil from Jul 2015 to May 2016. The two layers presented similar structures, with high overlap between them. Similar to what was expected, the antagonistic layer was more modular and specialized than the mutualistic layer. Some visitor species exhibited highly central, dual roles, acting as both antagonists and mutualists. Most behaved consistently as mutualists in all their visits, especially bees, which formed a predominantly mutualistic group. Butterflies represented a mixed group in relation to their visits and flies made more antagonistic visits. This research represents an important step towards understanding the role of mutualisms and antagonisms in the structure of interaction networks between herbaceous plants and floral visitors in tropical environments.

We collect floral visitors from herbaceous plants. Weekly visits were made from July 2015 to May 2016 to two sites on a ~300 m trail of Atlantic Forest border vegetation located in the “Estação Ecológica de Caetés” (ESEC) conservation unit.  Plant species were identified using a list published for the location, through comparisons with specimens in the PEUFR Herbarium (UFRPE) and by consulting specialists. Observations of frequency and behaviour of floral visitors were carried out by observing groups of individual plant species that presented flowering intensities between levels 3 and 4, according to the Fournier scale. For the mutualistic species, we included interactions where floral visitors contacted both anthers and stigma, indicating potential pollination. Hereafter, these legitimate visitors will be referred to as pollinators, although we did not evaluate their role in subsequent fruit production of visited plant species. For the antagonistic layer, we included all interactions where the floral visitor collected resources but did not contact floral reproductive structures. Hereafter, such illegitimate visitors will be referred to as antagonistic (floral robbers/thieves).