- In a new study, scientists said that the extinct Steller’s sea cow impacted kelp forests in the North Pacific by browsing at the surface, which would have encouraged the growth and strengthening of the algal understory.
- Not only would sea cows have positively impacted kelp forests in the past, but they may have also enhanced their resilience into modern times, according to the authors.
- Globally, kelp forests face many threats, including ocean warming, which can lead to an overabundance of predatory urchins.
- The authors suggest that it might be possible for humans to reproduce the species’ impact on the canopy of kelp forests to enhance kelp forest resilience.
On a Russian-led expedition in the northern Pacific Ocean in the 1740s, German botanist Georg Wilhelm Steller laid eyes on a species that captured his imagination. It looked like a manatee but was bigger than a killer whale, and it grazed similarly to a cow or horse. After Steller described the animal, it became known as Steller’s sea cow (Hydrodamalis gigas).
The sea cow could grow about 9 meters (30 feet) in length and weigh up to 10 metric tons, but this colossal creature had few defenses. Covered with thick blubber, the sea cow could not fully submerge itself in the water, but floated at the surface, grazing on kelp. This tendency made the species vulnerable to hunting.
“These animals are very voracious and eat incessantly, and because they are so greedy they keep their heads always under water, without regard to life and safety,” Steller wrote in his account. “Hence a man in a boat, or swimming naked, can move among them without danger and select at ease the one of the herd he desires to strike — and accomplish it all while they are feeding.”
By 1768, less than three decades after Steller formally described the species, the sea cow was extinct. But in a new paper, some scientists suggest the species left a considerable legacy in the coastal waters of the North Pacific: the enhancement of kelp forests through its eating habits.
The sea cow effect
From Baja California to southeastern Alaska, giant kelp forests sprout from the seabed along the coast. Giant kelp (Macrocystis pyrifera), a kind of algae, can grow up to 60 centimeters (2 ft) daily in ideal conditions, making it one of the fastest-growing species in the world. While the average length of giant kelp is 30 m (100 ft), it can grow up to 53 m (173 ft) — about half the length of a football field.
These vast underwater forests provide multiple benefits: food and habitat for thousands of species, water oxygenation that helps reduce ocean acidification, and the protection of coastlines from powerful waves and storm surges, which are getting stronger and more frequent with human-induced climate change.
But globally, kelp forests are threatened by warming temperatures, overfishing, pollution and other human activities.
In their new paper, scientists argue that the sea cow impacted kelp forests in the North Pacific by browsing at the surface, which would have encouraged the growth and strengthening of different kinds of understory algae. This thick, rich algae would then provide a food source for predatory urchins that can devour kelp, turning the ecosystem into what’s known as “urchin barren.” In California, for instance, it’s estimated that 95% of the kelp has recently disappeared, primarily because of urchin predation.
“Elephants have a huge effect on forests because they can clear so much space and so much vegetation in a very short period of time,” lead author Peter Roopnarine, a paleontologist and global change scientist at the California Academy of Sciences, told Mongabay as a way of comparison to sea cows. “[They] open up the forest canopy, and you get these large patches, and there’s a whole change in the community at that point until large trees regrow. So the question then was really simple: would sea cows have had a similar effect in kelp forests?”
According to Roopnarine and his two co-authors, the answer was yes.
Using mathematical modeling, they hypothesized that kelp forests grazed by sea cows likely acquired more robust defenses against stressors — not only in the past but also in the present. The authors dubbed this impact the “sea cow effect.”
One of the biggest threats to kelp forests today is marine heat waves, like the one that hit the North Pacific in 2013, known as “the Blob,” and which can bring an influx of sea urchins. Species like critically endangered sunflower sea stars (Pycnopodia helianthoides) and endangered sea otters (Enhydra lutris) can help reduce the number of urchins in a kelp forest community. Yet, these species are not always present in kelp forests due to stressors like pollution, hunting, and disease. For sea stars, one of the biggest threats to their survival is sea star wasting disease, an infectious disease that kills starfish rapidly and which scientists think is exacerbated by climate change.
The authors said that even when kelp forests encountered adverse conditions, sea cows would have helped the kelp recover quickly. But the effect would be even more substantial when kelp forests hosted both sea cows and otters, they said.
“There are a lot of examples of species ecologists have come to understand play a really outsized role in maintaining … an ecosystem,” co-author Roxanne Banker, a research associate at the California Academy of Sciences, told Mongabay. “One of the classic examples is the wolves in Yellowstone as they keep deer populations down, which modifies the watershed. And so I think you’ll see this a lot with large vertebrates because they’re able to basically eat a lot of stuff.”
‘There are no pristine ecosystems left’
Max Castorani, a marine ecologist at the University of Virginia, who was not involved in this study, said the new paper provides an interesting perspective on how kelp forest ecosystems functioned in the past when sea cows were around, but added that it has to make a lot of assumptions.
“In the diverse community of kelp forest ecologists, you’ll probably find people who would question some of the assumptions that they made in the study,” Castorani told Mongabay. “But it … isn’t easy to make these kinds of models and not be criticized for the assumptions that you make.”
Castorani added: “I thought it was an interesting study. In general in ecology, there isn’t a lot of emphasis on the paleo perspective, the deep historical perspective on how ecosystems may have functioned in the past, and it’s an interesting way to think about how … kelp forests may have functioned with these sea cows that are now extinct.”
Mark Carr, a marine ecologist professor at the University of California at Santa Cruz, who was also not involved in the study, similarly noted that the study had made several assumptions that can’t be empirically tested, but added the authors’ extensive modeling was the “real strength of the study.” Additionally, he said, the research contained “several very interesting implications for our understanding of species interactions and kelp forest dynamics and considerations for potential management interventions.
“I think these studies that do retrospective analyses of historic natural (pre-human altered) ecosystems are quite interesting and useful because they make us realize the long history over which humans have altered these systems, and consider the implications for how we manage ecosystems now,” Carr told Mongabay in an email. “As the authors mention, we are always subject to ‘sliding baselines’ that constrain our understanding of systems and our scope of management actions to more recent observations of altered systems.”
The new study’s authors propose evaluating an ecosystem through a past-present-future approach to avoid shifting baseline syndrome, which is the idea that environmental changes are measured against reference points in the near past, lowering one’s interpretation of what an ecosystem looked like.
“There really are no pristine ecosystems left … on Earth,” Banker said. “So it’s really important to understand the historical baselines for how these systems used to work and understand how they’ve come to arrive here to improve our predictive power for how we’re affecting them, and how they will continue to be affected by climate change.”
Could we recreate the sea cow’s impact on kelp forests?
Roopnarine said that while no animal interacts with kelp forests in the same way as sea cows once did, it might be possible for humans to reproduce the species’ impact on the canopy of kelp forests, and for such interventions to be used to enhance kelp forest resilience.
“If our hypothesis is correct, if our modeling is correct, then you could maybe artificially recreate some of the sea cow functionality by cropping the kelp or browsing kelp artificially in the same way that the sea cow would have and opening up … more space in the understory,” Roopnarine said. However, he added that such an approach would require a lot of experimental work to get it right.
Carr said commercial kelp harvesting and the collecting of kelp for abalone aquaculture could be a means to achieve the “past role of sea cows.” However, he said the “devil is in the details” since researchers and managers would need to consider a suite of factors, including how canopy thinning for the sake of understory growth may influence other species like fish that use the forest canopy as habitat.
“Overall, it’s a truly impressive study,” Carr said, “and has several very interesting implications for our understanding of species interactions and kelp forest dynamics and considerations for potential management interventions.”
Elizabeth Claire Alberts is a senior staff writer for Mongabay. Follow her on Twitter @ECAlberts.
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McPherson, M. L., Finger, D. J. I., Houskeeper, H. F., Bell, T. W., Carr, M. H., Rogers-Bennett, L., & Kudela, R. M. (2021). Large-scale shift in the structure of a kelp forest ecosystem co-occurs with an epizootic and marine heatwave. Communications Biology, 4(1). doi:10.1038/s42003-021-01827-6
Roopnarine, P. D., Banker, R. M. W., & Sampson, S. D. (2022). Impact of the extinct megaherbivore Steller’s sea cow (Hydrodamalis gigas) on kelp forest resilience. Frontiers in Ecology and Evolution, 10. doi:10.3389/fevo.2022.983558
Steller, G. W. (1751). De bestiis marinis, or, the beasts of the sea. Retrieved from: https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1019&context=libraryscience