Deterministic and stochastic effects underlying marine benthic communities

The role of colonization history and subsequent biotic interactions in determining the species composition in communities has long been the subject of debate in ecology. While one narrative has emphasized deterministic assembly rules, another has emphasized historical contingency. One problem lies in approach: community studies are typically either manipulative but somewhat unnatural, or observational but lacking manipulation. Furthermore, while most ecologists now recognize that both historical and biotic factors shape communities, too few studies have moved beyond qualitative descriptions of their roles. Here we use a manipulative approach that leverages natural variation to provide quantitative estimates of the relative contributions of colonization history and the subsequent biotic interactions. 384 communities were developed on artificial substrata in a homogeneous environment before undergoing reciprocal transplantation. We then compare community structure before and after transplantation as proxies for colonization history and biotic interactions. We found that the importance of history and the ensuing biotic environment differed at different times in community development. Early transplantations resulted in the local environment modifying community history faster compared to postponed transplantations. With a four-week difference in age, colonization history explained 20% more of the variation in older communities than in younger communities. Biotic interactions were able to modify colonization history at the age of 16 weeks, but older communities showed more resistance to the changing biotic environment. Our method provides a manipulative and quantitative approach to understanding the relative contributions of colonization history and biotic interactions to assembly in natural systems.