Devil's tongue weed (Grateloupia turuturu Yamada) in northern Portugal: Passenger or driver of change in native biodiversity?

- Models explaining the invasive potential of Grateloupia turuturu tested experimentally.
- Spread of G. turuturu in rock pools facilitated by the loss of native algal canopies.
- Local assemblages unaffected by G. turuturu on the short-term.
- G. turuturu would act as a 'passenger' rather than a 'driver' of ecological change.

Understanding the mechanisms underlying biological invasions is essential to separate their actual ecological effects from those of other human disturbances. This study examined experimentally whether the non-native red seaweed Grateloupia turuturu is an opportunistic species taking advantage of degraded local conditions (passenger model), or the primary driver of changes in the structure of benthic assemblages (driver model). In a first experiment, traits of G. turuturu likely associated to its invasion success were compared between unmanipulated controls and treatments subjected to the removal of canopy-forming macroalgae to test for the passenger model. In a second experiment, rock pool assemblages where G. turuturu was selectively removed were compared with initially similar control (unmanipulated) assemblages to test for the driver model. Over a period of four months, G. turuturu showed larger cover, higher number of individuals and longer fronds in the canopy-removed compared to the control pools, while no significant differences were detected between assemblages where G. turuturu was continuously removed and those where invasion was allowed to occur. Present findings suggest that G. turuturu would rely on disturbances removing potential native competitors to spread into the recipient habitat, rather than being the main driver of ecological alterations.