Sargassum horneri, a brown macroalga, recently invaded the California coast, including into critical foundational communities such as kelp (Macrocystis pyrifera) forests. Despite its rapid spread, empirical tests that evaluate mechanisms underlying S. horneri's invasion success are lacking. To fill this knowledge gap, we conducted three field experiments on temperate rocky reefs in southern California using growth as a proxy for invasion success. We first tested whether S. horneri success differed with herbivory strength and native diversity by conducting a 2-factor experiment varying site (with different baseline levels of urchin densities and native algal diversity) and urchin access. We found S. horneri growth only differed among urchin treatments and not sites. We then evaluated whether S. horneri could successfully invade established algal canopies as a driver or whether it required open space as a passenger via a 2-factor experiment varying S. horneri size (small, medium, large) and canopy type (S. horneri, kelp, -canopy). We found that all S. horneri sizes grew fastest when canopy was lacking and light was high and slower in both canopy habitats with lower light; overall, small S. horneri grew slowest. Finally, we evaluated whether herbivore consumption for native species could facilitate S. horneri's invasion by conducting a 2-factor experiment varying species (M. pyrifera, S. horneri) and herbivore access. We found uncaged algae were consumed and caged algae grew, but there was no difference between species. Taken together, our results suggest that S. horneri is a “passenger” invader that will take advantage of points in time and space where light is plentiful, such as when M. pyrifera is removed via disturbance. Further, our results suggest that herbivory and native algal diversity are likely not key determining factors of the invasion success of S. horneri.

Sargassum horneri是一种棕色大型藻类，最近入侵了加利福尼亚海岸，包括进入重要的基础群落，如海带（Macrocystis pyrifera）森林。尽管它迅速传播，但缺乏评估S. horneri入侵成功背后机制的经验测试。为了填补这一知识空白，我们使用生长作为入侵成功的代理，在加利福尼亚南部的温带岩礁上进行了三项实地试验。我们首先通过在不同地点（具有不同基线水平的海胆密度和本地藻类多样性）和海胆访问进行 2 因素实验来测试S. horneri 的成功是否因食草强度和本地多样性而异。我们发现了S. horneri生长仅在海胆处理之间而不是地点之间有所不同。然后，我们评估了S. horneri是否可以作为驾驶员成功侵入已建立的藻类树冠，或者它是否需要作为乘客的开放空间，通过一个不同的S. horneri大小（小、中、大）和树冠类型（S. horneri，海带，-树冠）。我们发现，当树冠缺乏且光照较高时，所有S. horneri体型在光照较低的两个树冠栖息地中生长最快；总体而言，小S. horneri增长最慢。最后，我们评估了本地物种的食草动物消费是否可以促进S. horneri通过进行不同物种（M. pyrifera，S. horneri）和食草动物进入的 2 因素实验来入侵。我们发现未笼养的藻类被消耗，笼养的藻类生长，但物种之间没有差异。综上所述，我们的结果表明S. horneri是一种“过客”入侵者，它将利用光线充足的时间和空间点，例如当M. pyrifera通过干扰被移除时。此外，我们的结果表明，食草动物和本地藻类多样性可能不是S. horneri入侵成功的关键决定因素。