The ghost of a giant – Six hypotheses for how an extinct megaherbivore structured kelp forests across the North Pacific Rim

Aim The global decline of megafauna is believed to have had significant and widespread ecological impacts. One such extinction of likely important consequence is the 18th century extinction of the Steller’s sea cow (Hydrodamalis gigas); however, little has been written about how the loss of this megaherbivore may have impacted coastal ecosystem dynamics. Drawing on historical evidence, sea cow biology, kelp forest ecology, and the ecology of extant sirenians, we propose several discrete hypotheses about the effects Steller’s sea cows may have had on kelp forest dynamics of the North Pacific. Location North Pacific Ocean. Time period Pre‐1760s. Major taxa studied Steller’s sea cow (Hydrodamalis gigas). Results & conclusions The evidence we review suggests that Steller’s sea cows exerted substantial direct and indirect influences on kelp forests, likely affecting the physical ecosystem structure, productivity, nutrient cycling, species interactions, and export of nutrients to surrounding ecosystems. This suggests that kelp forest dynamics and resilience were already significantly altered prior to the influence of more recent and well‐known stressors, such as industrial fishing and climate change, and illustrates the important ecological roles that are lost with megafaunal extinction.