A trophic cascade facilitates native habitat providers within assemblages of multiple invasive marine species.
Anthropogenically influenced habitats are becoming increasingly impacted by multiple invasive species that can regionally control main ecological processes and functions. Management requires predictions of invasive species' effects, but multiple invasions may cause unpredictable outcomes, for example, synergistic impacts or interactions among trophic levels causing trophic cascades. The Baltic Sea is prone to species invasions that increasingly occur at multiple trophic levels. We used a laboratory experiment to test whether multiple invasive predators may cause interactive effects in the Baltic Sea. In aquaria, we simulated a coastal community that has developed dominated by invasive species-the decapod predators Palaemon elegans and Rhithropanopeus harrisii, the amphipod grazer Gammarus tigrinus, and the filter-feeder Dreissena polymorpha. The orthogonal experiment determined separate and interactive effects from predators on the lower trophic level invasive species and native algae including the foundation species Fucus radicans. The main result was that while both predators significantly impacted the community, in no cases were their effects interactive. Another important result was that P. elegans appeared to cause a trophic cascade, as in its presence F. radicans and its epiphytes had the greatest biomass while G. tigrinus grazers were heavily reduced. G. tigrinus were negatively correlated with F. radicans biomass, so variable amphipod grazing may be the link allowing an indirect effect of P. elegans on F. radicans. R. harrisii also reduced G. tigrinus abundance but did not affect F. radicans biomass. Both predators significantly affected the lengths of F. radicans plant stems that were comprised of new vegetative growth-P. elegans reduced the lengths of new vegetative tissue while R. harrisii increased it. Thus, the presence of P. elegans induced shorter and denser F. radicans bushes compared to taller and thinner bushes observed in other treatments. Lastly, R. harrisii caused significant declines in D. polymorpha numbers. Overall, ecological management needs to consider positive effects from predator invasion which may reduce grazing pressure on low trophic level foundation species. A trophic cascade involving two invasive species highlights the importance of multi-species approaches to management of Baltic Sea invasive species and in other regions prone to biological invasions.