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

Abstract

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.

Impact of the extinct megaherbivore Steller's sea cow (Hydrodamalis gigas) on kelp forest resilience

“…Individuals attained very large sizes, up to 9 m in length, and are estimated to have weighed up to 10 tons. H. gigas was an obligate kelp feeder, apparently incapable of submerging as observed by Steller (Steller, 1751) because of its high buoyancy, and therefore browsed the giant kelp canopy available at the surface (Estes et al, 2016;Bullen et al, 2021). There are no recorded observations of the species beyond the Aleutian Islands, but fossils of H. gigas, and other members of the genus that might represent different species, are known from the Pliocene of California and Baja California, and the post-Pliocene of Japan (Domning, 1976).…”

History's legacy: Why future progress in ecology demands a view of the past

“…Steller's sea cow ( Hydrodamalis gigas ), which became extinct around the year 1750, was a major consumer of canopy kelps. Besides fertilizing the water and increasing kelp productivity, this 4000 kg sea cow opened up the canopy, allowing light to penetrate to a diverse algal understory (Bullen et al, 2021). The kelps can tolerate high rates of consumption by growing at the base of the blades rather than at their tips.…”

Impact of the extinct megaherbivore Steller’s sea cow (Hydrodamalis gigas) on kelp forest resilience

“…Given our understanding that these animals often contribute significantly to the dynamics of their communities, including stability, resilience and state transitions, it is reasonable to speculate that H. gigas (and congeneric species) could have played a significant ecological role in kelp forest communities. Recently, Bullen et al [24] presented six hypotheses of how H. gigas may have structured giant kelp communities. Several of the hypotheses predict that grazing of kelp fronds by sea cows opened up the canopy, increasing light intensity in both the water column and on the sea bottom (Fig.…”