Invasion ecology of marine macroalgae: the relevance of stress resistance for the invasion success of Gracilaria vermiculophylla and consequences of its spread

Originating from East Asia, the perennial red macroalga Gracilaria vermiculophylla (Ohmi) Papenfuss has successfully invaded several temperate areas of the Northern hemisphere and continues to spread. In its new range, the seaweed tends to form local mass appearances and to dominate the native community. A high tolerance towards both abiotic and biotic environmental stressors could explain the invasion success of this species. I therefore compared the stress resistance of G. vermiculophylla from six native populations from South Korea and China and eight invasive populations from Europe and NW-Mexico. In short-term experiments G. vermiculophylla individuals were exposed to 1) heat shock, 2) UV-C-radiation and 3) elevated copper concentrations in the water. In a long-term experiment the seaweed had to cope with depletion stress (darkness in combination with low temperature and dryness) for several months. All experiments were carried out twice - one time in the native range in Qingdao, China and one time in the invaded range in Kiel, Germany - to rule out local acclimation effects. In order to compare the resistance against herbivory individuals of native and invasive G. vermiculophylla populations were fed to snails from the native (Littorina brevicula) and the invasive (Littorina littorea) range. In the Baltic Sea, G. vermiculophylla might threaten the habitat-forming native brown alga Fucus vesiculosus through direct competition for resources and by providing a shelter for mesograzers, which prefer to feed on F. vesiculosus. Mesocosm-experiments were conducted over one year in the Kiel Fjord in order to test the direct and indirect effects of G. vermiculophylla on F. vesiculosus