Nectar robbing as a mechanism of biotic resistance against an invasive plant in the Galápagos Islands.

Biological invasions are strongly shaped by biotic interactions in recipient communities, which can either facilitate or constrain the establishment and spread of alien species. Among these interactions, antagonistic relationships with native consumers may act as mechanisms of biotic resistance, yet their role in plant invasions remains insufficiently explored. In this study, we analyzed the effect of nectar robbing (measured both as the intensity and frequency of robbing) by native floral visitors on the fruit set of the introduced plant, Kalanchoe pinnata, in the Galápagos Islands. The results showed that two generalist native species, robbed K. pinnata nectar, and significantly decreased fruit production. In addition, nectar robbing was more frequent in flowers with longer corolla tubes and in plants with larger floral displays, suggesting that pervasive robbing may impose selective pressures opposing floral trait patterns typically associated with legitimate pollination. These findings document a previously undescribed antagonistic interaction between native animals and an introduced plant, highlighting nectar robbing as a mechanism of biotic resistance that can constrain the sexual component of reproduction and potentially influence the evolutionary trajectory of invasive plants on oceanic islands.