And then two come along at once

My mind had spent 68 years untroubled by thoughts of Steller's sea cow. But for Christmas 2019 I was given a copy of Ross Barnett's The Missing Lynx, which introduced me to this loyal and gargantuan extinct manatee. It was rendered extinct, wrote Barnett, because it was so easy to hunt (which is where the loyalty came in – the silly things went back to help fellows who'd been harpooned).

Steller's sea cows were large herbivores that had a seal-like appearance with a tail which resembled that of a whale. The Steller's sea cow was named after George Steller who discovered the animal and who described it: "The animal never comes out on shore, but always lives in the water. Its skin is black and thick, like the bark of an old oak, its head in proportion to the body is small, it has no teeth, but only two flat white bones one above, the other below". 

Source

But then I heard Rob Newman, on The Extinction Tapes. He made it clear that (on this rare occasion) it wasn't only man that was vile. Well, vile, but not because of a direct effect on the sea cow population. The vileness was due to the popularity of sea otter fur, as explained in this article in the Proceedings of the National Academy of Sciences of the USA. It's not a simple story, but in a nutshell the hunting of sea otters for their fur, left sea urchins unchecked – and the sea urchins deprived the sea cows of their main (perhaps sole) source of food, kelp. It was not that  they ate an enormous amount, but they cast the plants adrift by gnawing through their moorings. As the National Ocean Service website says:

Sea urchins will often completely remove kelp plants by eating through their holdfasts. Other invertebrates found in kelp forests are sea stars, anemones, crabs, and jellyfish... 

A wide range of marine mammals inhabit kelp forests for protection and food. Sea lions and seals feed on the fish that live in kelp forests. Grey whales have also been observed in kelp forests, most likely using the forest as a safe haven from the predatory killer whale. The grey whale will eat the abundant invertebrates and crustaceans in kelp forests. One of the most important mammals in a kelp forest is the sea otter, who takes refuge from sharks and storms in these forests. The sea otter eats the red sea urchin that can destroy a kelp forest if left to multiply freely. 

Source

This graph from the PNAS article, where the scale on the left shows population density (of sea otters) as a proportion of the maximum, shows what happened and when. 

 

Of course post hoc doesn't have to mean propter hoc. And the hoc in question is so remote in time and space that it's impossible to know what caused what. But the author of the PNAS article is pretty convinced and convincing:

Although the exact timing and even the existence of a kelp forest collapse in the Commander Islands can only be surmised, the phase shift probably occurred soon after the onset of the fur trade in 1743. Sea otters in the Commander Islands had been hunted to virtual extinction by 1753. Although the precise timing of the associated kelp forest to urchin barrens phase shift depends on the exact trajectory of decline in sea otter density, those details are of little consequence to our argument. Sea otters were ecologically extinct by 1753, and the kelp forest collapse therefore preceded that date if our data from the western Aleutians are a reasonable proxy for what happened 250 y earlier in the Commander Islands. If the time course of the sea otter decline in the Commander islands was roughly exponential, then the kelp forest collapse probably occurred around 1750, just 7 y after the onset of the fur trade and 16 y before the last record of a living sea cow

b

PS And my the latest nomination for a Tezzy ...

<GLOSSARY item="TEZZY>
Time-wasting Site of the Year.
</GLOSSARY>

...goes to this, where the 14th century meets the 21st. In the 20th, in the year of my birth, the Equatorie was  misattributed to Chaucer; but recent research has placed the writer as a 14th century monk:

The ... possibility that the Equatorie was Chaucer’s own composition is an issue that has occupied the attention of many scholars; however, this has now been resolved. The writer has been identified as John Westwyk, a Benedictine monk of Tynemouth Priory and St Albans Abbey, whose life was one of dramatic contrasts.

The interactive model equatorium based on the Equatorie offers many hours of play. Don't say you weren't warned; this Tezzy looks as though it might be richly deserved. I haven't put it through its paces though, because of...

Self-isolation Chronicles

My tai chi classes are continuing via Zoom. But yesterday normal service was interrupted because my laptop had a nasty turn.

The backup plan was to continue on the desktop. But this has two big drawbacks.

1. It does not do video, and fixing up a camera for it wouldn't be plain sailing, as it's running Windows 7 (I know...).
2. It is in the study, where there's not enough room to swing a ca...gerbil.

<stop_press>
The day has been saved by a near-obsolete iPad.
</stop_press>

Meanwhile, my local U3A has gone into hibernation. Ironically, the mail announcing that everything was suspended until further notice claimed to be Guaranteed virus-free