Article: Estes, J. A., Burdin, A., & Doak, D. F. (2015). Sea otters, kelp forests, and the extinction of Steller’s sea cow. Proceedings of the National Academy of Sciences, 201502552, doi:10.1073/pnas.1502552112
A long time ago on this very planet, many large creatures similar to the ones we know today roamed the earth. During the time period known as the Pleistocene, megafauna (animals weighing over 44 kg, 97 lbs) like mammoths and giant sloths thrived. Near the end of the Pleistocene, about 9,700 BCE, most of these animals became extinct in the New World (North and South America) for reasons scientists are still puzzling out.
The two most popular theories for the extinctions are climate change and the arrival of humans to the area. The climate was undergoing a shift from cold to warm, known as a glacial recession, because the glacier covering much of the land masses receded poleward. This shift may have happened quickly enough that many species were not able to adapt and died out. However, there hasn’t been much evidence found for this theory in other glacial recession periods, so it may be unlikely.
There is evidence for large animal extinctions after the arrival of humans at several time periods on several continents, so this explanation seems more likely than climate change. The introduction of humans to the area could have affected large animals in many ways. Humans could have directly hunted and overkilled the populations to extinction, or they may have introduced new diseases that wiped out megafauna. Additionally, our species has a tendency to alter the natural environment significantly, so human-induced deforestation or other changes may have destroyed the habitat these animals required.
This study examines another mechanism of human impact on large species. If humans hunted a keystone species to a certain tipping point or extinction, they may have indirectly triggered a collapse of a complex ecosystem, leading to a chain reaction resulting in the extinction of other species. Since it is difficult to study ecosystems that existed thousands of years ago, the authors researched a more recent megafauna extinction event of the Steller’s sea cow in the mid-1700s, for which a few in person observations from Georg Stellar exist.
The Sea Cow
Georg Steller observed the last remaining population of sea cows (Figure 1) in the Commander Islands (the very end of the Aleutian Island chain) in 1741. These animals probably ranged over much of the Pacific Rim, from Northern Japan to Baja California. They were closely related to the dugongs and manatees that still exist today. Based on this similarity and some fossil records, it is likely that the sea cow primarily fed on macroalgae known as kelp (Fig. 1). If something started killing kelp, the sea cows would have struggled to find food.
Another species inhabiting a similar range as the sea cows at that time would have been the sea otter. Sea otters are known for their incredibly soft fur, making them popular targets for fur traders over the years. As such, Maritime records do exist for the population of sea otters in the Commander Islands for the 1700s, but not for sea cows, which were not economically important. The relationship between the population of sea otters and sea cows is somewhat complicated, but I will provide a simplified explanation here.
Sea otters also inhabit kelp forests, but they are carnivores, feeding on many marine invertebrates like clams, mussels, and sea urchins. They are considered a keystone species, one that plays a pivotal role in keeping an ecosystem balanced and healthy. Otters perform this role in kelp forest ecosystems by keeping sea urchin populations low. When enough otters are removed from the kelp forest, say by hunting, sea urchin populations boom and the urchins eat the root systems of kelp, decimating the forest. As explained above, this lack of tasty kelp would likely cause mass starvation of sea cows. If enough sea otters remain in the system, urchin numbers stay low enough that the kelp forest ecosystem maintains a healthy balance.
Although direct records of sea urchin and kelp forest health do not exist for the Commander Islands to directly study their effect on the sea cows in the 1700s, much data on the sea otter-urchin-kelp relationship exists for similar islands in the Aleutians from the 1990s (Fig. 2), when killer whales killed many sea otter populations. This study used that data as a basis for a model to study the effect of sea otter removal on sea cow populations in the Commander Islands. Based on the maritime records of Commander Island otter populations in the 1700s and the model based on nearby islands in the 90s, the researchers could estimate the health of the kelp forests and sea cows in the 1700s.
The researchers determined that the hunting of sea otters would have led to the collapse of the local kelp forests. Local sea otters had been hunted to virtual
extinction by about 1753, and the kelp forest collapse would have occurred several years before then (Fig. 3). Using data on the response of modern day dugongs to sudden food source removal, the model predicted that a population of 1,500 sea cows (a reasonable estimate) would have declined to just one member by 1768, when the creatures were declared extinct (Fig. 4). The mortality (rate of death) for the sea cow population modeled would have been 31%,
which is lower than the observations of dugongs in a similar situation, meaning it is very feasible (the model didn’t predict an outrageous rate of death). The model did not include any emigration rate, meaning that it didn’t account for sea cows leaving the area to try and find greener pastures. It also included no human hunting of sea cows, which most likely occurred.
Based on the model, it is possible that the extinction of Steller’s sea cow could have been completely caused by the hunting of sea otters and the subsequent collapse of the kelp forests. There was almost certainly also hunting and emigration of sea cows, which would only contribute to a faster decline. This information supports the reasoning that not all extinctions of megafauna in the Pleistocene were due to direct human overhunting. Some may have been due to removal of keystone species, upsetting sensitive ecosystem balances and triggering chains of events that led to other extinctions. Humans may not have hunted certain megafauna species at all; some species may have died due to a cascade effect in the ecosystem that was triggered by humans hunting a different species.
Overall, this paper supports the notion that natural ecosystems are very complex, with connections between species that may not be readily apparent. It is important to consider these relationships when deciphering past events, as the most obvious solution may not be the most accurate one. It is also important to remember this type of impact in the future, as we may affect a species before realizing the critical role it played in an ecosystem.