Extrafloral nectar as a resource mediating multispecies interactions

Extrafloral (EF) nectar resources can affect the dynamics of species interactions at the community scale. Furthermore, selection acting on EF nectary traits may extend beyond simple mutualisms between plants and the enemies of herbivores to involve other community members that use EF nectar. We examine how EF nectaries influence and are influenced by interactions with multiple species, highlighting our review with original data from the association between ants and wild cotton (Gossypium, thurberi). Our survey of the literature suggested that EF nectar attracts a diverse assemblage of arthropods and may enhance the diversity and abundance of arthropod assemblages. However, experimental evidence on the importance of EF nectar to terrestrial food webs was equivocal. Exploring potential avenues for selection, we uncovered several costs and benefits of EF nectary traits that have received little empirical attention. These include a constraint faced by plants when attracting both pollinators and protectors via nectar, as well as an ecological cost of nectar when herbivores consume EF nectar as adults. Finally, we discuss how geographic variation in multispecies interactions may affect selection on EF nectary traits. In wild cotton, variation among populations in EF nectar composition was consistent with the benefits afforded by attracting ants; however, non-ant species may also mediate spatially variable selection on EF nectaries. Our synthesis underscores a need for studies that manipulate EF resources at the community level and investigate selection on EF nectaries in a multispecies and geographic context.