Enemy release an unlikely explanation for the invasive potential of the brown alga Sargassum muticum: experimental results, literature review and meta-analysis

The enemy release hypothesis predicts that non-indigenous species (NIS) are released from natural enemies (e.g. grazers etc.) in their new environment, thus facilitating invasion. Sargassum muticum is a conspicuous and successful invasive brown alga and several studies have investigated whether native herbivores feed less on S. muticum relative to native algae within the invaded range. Some of these studies have concluded that S. muticum is avoided by native herbivores, but this conclusion may be premature as the results are rather inconclusive and seem to depend partly on the type of algae included in the comparison. We conducted therefore a series of feeding experiments to test if Danish S. muticum is grazed less upon than a range of native algae by the sea urchin Psammechinus miliaris and complemented the experiments with a meta-analysis based on published data. In no-choice trials, P. miliaris consumed S. muticum faster than it consumed slow-growing Fucus vesiculosus and Halidrys siliquosa , whereas Saccharina latissima , Ceramium virgatum and Ulva intestinalis were grazed upon at higher or the same rates as S. muticum . More or less identical patterns were seen in the two- and multiple-choice experiments, where S. muticum was generally consumed faster than F. vesiculosus and H. siliquosa , but slower than S. latissima and the two most fast-growing algal species ( U. intestinalis and C. virgatum ). We screened the literature for comparable data and found 26 experiments with 27 species of algae and 14 species of invertebrate grazers. Meta-analysis on these data showed the same overall trend as observed in our experiments; S. muticum is generally consumed at the same rate or faster than other thick, leathery and canopy-forming algae, which are assumed to constitute the major competitors to S. muticum , but slower than more fast-growing sub-canopy species and more opportunistic algae. We question therefore that enemy release can explain the invasion success of S. muticum .