Disease shapes community composition by removing species with strong interactions. To test whether the absence of keystone predation due to disease produced changes to the species composition of rocky intertidal communities, we leverage a natural experiment involving mass mortality of the keystone predator Pisaster ochraceus from Sea Star Wasting Syndrome. Over four years, we measured dimensions of mussel beds, sizes of Mytilus californianus, mussel recruitment, and species composition on vertical rock walls at six rocky intertidal sites on the Central California coast. We also assessed the relationship between changes in mussel cover and changes in sea star density across 33 sites along the North American Pacific coast using data from long-term monitoring. After four years, the lower boundary of the Central California mussel beds shifted downward toward the water 18.7 cm ± 15.8 cm (SD) on the rock and 11.7 cm ± 11.0 cm in elevation, while the upper boundary remained unchanged. In Central California, downward expansion and total area of the mussel bed were positively correlated with mussel recruitment but were not correlated with pre-disease sea star density or biomass. At a multi-region scale, changes in mussel percent cover were positively correlated with pre-disease sea star densities but not change in densities. Species composition of primary substrate holders and epibionts below the mussel bed remained similar across years. Extirpation of the community below the bed did not occur. Instead, this community became limited to a smaller spatial extent while the mussel bed expanded.

疾病通过去除相互作用力强的物种来塑造社区组成。为了测试由于疾病引起的梯形捕食性缺失是否会导致潮间带岩石群落的物种组成发生变化，我们利用了一项自然实验，该实验涉及海星浪费综合症中梯形捕食者Pi蝶Pi鱼的大量死亡。在过去的四年中，我们测量了贻贝床的尺寸，Mytilus californianus的尺寸，贻贝的募集和物种组成在加利福尼亚中部海岸的六个岩石潮间带站点的垂直岩壁上。我们还使用来自长期监测的数据评估了北美太平洋沿岸33个站点的贻贝覆盖变化与海星密度变化之间的关系。四年后，加利福尼亚中部贻贝床的下边界朝着岩石上的水18.7 cm±15.8 cm（SD）向下移动，高程为11.7 cm±11.0 cm，而上边界保持不变。在加利福尼亚中部，贻贝床的向下扩展和总面积与贻贝的募集成正相关，但与疾病前的海星密度或生物量却没有相关性。在多区域范围内，贻贝覆盖率的变化与疾病前海星密度呈正相关，但与密度变化无正相关。多年以来，贻贝床以下的主要基质持有者和表生动物的物种组成保持相似。没有发生床底下的群落灭绝。取而代之的是，当贻贝床扩展时，该群落的空间范围受到了限制。