We have two main questions: why did the losses occur, and what were the ecological consequences
His answer is that the answers are deeply intertwined.
He looks at a marine megafaunal species: Steller’s sea cow. A dugongoid serenian radiated into N Pacific with onset of last ice age. It winked out at end of Pleistocene, with the exception of the Commander Island where they persisted until 1768 (off east coast of Russia). This was the only island group not occupied by aboriginal peoples. Russians trappers discovered the Commander Islands in the early 18th century.
Why did they go extinct? Direct overkill? Or sea otter-kelp cascade? When sea otters are abundant, sea urchins are rare, and kelp is abundant as it is nor browsed by urchins. There are two states: a kelp state and an urchin state, with rapid transition between the states. The threshold otter density seems top be about 6 otters/km2. Sea cows are browsers, urchins are much more destructive as browsers
Turvey and Risley (Biol. Lett. Royal Society 2006) concluded human kill rates are sufficient to explain their extinction. Sea otter-kelp hypothesis not needed.
Jim Estes disagrees with this conclusion. The last sea otter was recorded on the island over 1743-1754, well before the extinction of the sea cow. Did sea otter loss drive the sea cow to extinction? We have no records of how 18th century kelp changed, but the collapse has mirrors with what happened with sea otter collapse in the late 20th century. Overall there is a 12-fold decline in kelp caused by a loss of sea otters (Estes et al. 1998). This decline in kelp would cause a 90% decline in food supply for sea cows.
How might this have influenced the sea cow population? Create demographical model of trajectory of sea cow population assuming that not a single sea cow was hunted. The annual sea cow starvation rate because of a loss of sea grasses would be 0.74-0.91. If the sea cow population was around 1500 at the start, how long would it take to drive them to extinction. He argues the changes in the ecosystem alone were sufficient to cause the extinction even with zero hunting.
Conclusion: Food webs are deeply interconnected things. When we talk about extinction we have to thing about ecological interactions. Many megafaunal extinctions may be associated with loss of keystone predators - more complex and subtle than a simple model of direct hunting.
His answer is that the answers are deeply intertwined.
He looks at a marine megafaunal species: Steller’s sea cow. A dugongoid serenian radiated into N Pacific with onset of last ice age. It winked out at end of Pleistocene, with the exception of the Commander Island where they persisted until 1768 (off east coast of Russia). This was the only island group not occupied by aboriginal peoples. Russians trappers discovered the Commander Islands in the early 18th century.
Why did they go extinct? Direct overkill? Or sea otter-kelp cascade? When sea otters are abundant, sea urchins are rare, and kelp is abundant as it is nor browsed by urchins. There are two states: a kelp state and an urchin state, with rapid transition between the states. The threshold otter density seems top be about 6 otters/km2. Sea cows are browsers, urchins are much more destructive as browsers
Turvey and Risley (Biol. Lett. Royal Society 2006) concluded human kill rates are sufficient to explain their extinction. Sea otter-kelp hypothesis not needed.
Jim Estes disagrees with this conclusion. The last sea otter was recorded on the island over 1743-1754, well before the extinction of the sea cow. Did sea otter loss drive the sea cow to extinction? We have no records of how 18th century kelp changed, but the collapse has mirrors with what happened with sea otter collapse in the late 20th century. Overall there is a 12-fold decline in kelp caused by a loss of sea otters (Estes et al. 1998). This decline in kelp would cause a 90% decline in food supply for sea cows.
How might this have influenced the sea cow population? Create demographical model of trajectory of sea cow population assuming that not a single sea cow was hunted. The annual sea cow starvation rate because of a loss of sea grasses would be 0.74-0.91. If the sea cow population was around 1500 at the start, how long would it take to drive them to extinction. He argues the changes in the ecosystem alone were sufficient to cause the extinction even with zero hunting.
Conclusion: Food webs are deeply interconnected things. When we talk about extinction we have to thing about ecological interactions. Many megafaunal extinctions may be associated with loss of keystone predators - more complex and subtle than a simple model of direct hunting.