Role of top-down and bottom-up forces on the invasibility of intertidal macroalgal assemblages

article i nfo Despite the available information regarding the negative effects of non-indigenous species (NIS) on ecosystem structure and functioning, the mechanisms controlling NIS invasion remain poorly understood. Here, we inves- tigated the relative roles of top-down and bottom-up control on the invasion of intertidal macroalgal assem- blages by the macroalga Sargassum muticum (Yendo) Fensholt. Using a factorial experiment, nutrient availability and intensity of herbivory were manipulated along an intertidal rocky shore. We found that early re- cruitment of S. muticumwas enhanced by low nutrient enrichmentbut noeffectof grazers was observed. In con- trast, atthe end of the experiment (9-months after invasion) top-down control, together with the numberof NIS and the percentage cover of ephemerals, was a significant predictor for the invasion success of S. muticum .I n ad- dition, both top-down and bottom-up forces played a significant role in structuring macroalgal assemblages, which indirectly could have influenced invasion success. Hence, by shaping community structure, main and in- teractive effects of bottom-up and top down forces may indirectly act on invasion. Our study highlights the importance of the recipient community structureon the invasion process and emphasizesthe specifi cr egulation of top-down and bottom-up forces in different stages of S. muticum invasion. © 2012 Published by Elsevier B.V.