Crossing the line: Mutualism between invasive species at the terrestrial–aquatic interface

Abstract Positive interactions such as mutualism and facilitation between invasive species can magnify their impact on invaded ecosystems. Yet, few studies have explicitly evaluated symbioses between invasive species that span the terrestrial–aquatic interface. We conducted two experiments to investigate the fitness outcomes (growth and germination) of frugivory and ichthyochory involving invasive channel catfish ( Ictalurus punctatus ) that consume fruits and pass seeds of invasive Russian olive ( Elaeagnus angustifolia ). Our results indicate that catfish frugivory of olive fruit provides an energy source that allows the catfish to maintain body mass and seeds passed by catfish were four times more likely to germinate than untreated seeds, suggesting catfish are viable olive seed dispersers. As such, we concluded that the interaction of frugivory of invasive olive fruits by invasive catfish and ichthyochory resulting from passed olive seeds constitute a facultative mutualism spanning the aquatic–terrestrial interface. The impact of the olive–catfish mutualism on the abundance, persistence and spread of both invasive species requires further study. Researchers and managers should consider the potential of positive interactions across ecotones to facilitate invasions