Seaweed community traits reveal transient shifts and a stable attractor on tropical reefs

Abstract As environmental conditions continue shifting under accelerating anthropogenic pressure, there is an urgent need to better understand and predict changes in the ecological function of emergent, often novel communities. This motivated the rapid development of trait-based ecological approaches that quantify functional traits of individuals to make inferences about ecological function. Long-term monitoring programs often collect community abundance data annually; yet, to our knowledge, these very different types of datasets have rarely been combined to hindcast shifts in community structure and function. Here, we merge the Moorea Coral Reef Long-Term Ecological Research dataset on benthic communities with a contemporary tropical seaweed trait database to quantify shifts in community function over nearly 20 years and through three distinctly different environmental disturbances. This research was motivated by global shifts on reefs to seaweed domination with unknown shifts in ecosystem functions. Using ordination, we visualized dramatic shifts in community trait space occupancy over time. When comparing mean trait value over time for each trait in each reef type, we describe rapid transitions in the functional ecology of seaweed communities in response to well-documented biotic and abiotic disturbances. A biotic disturbance that killed coral created seaweed communities with traits maximized for resistance to herbivory, while a physical disturbance resulted in a replacement community maximizing resistance to physical disturbance traits. Overall, we document transient shifts in community functional traits in response to shifts in environmental drivers, but also evidence for a stable attractor characterized by tall, less herbivore-resistant communities. Uncovering these shifts in ecological strategies in response to major disturbances demonstrates that combining trait-based approaches with long-term data provides a powerful tool for furthering understanding of how functional changes in communities are driven by environmental change.