Keystone mistletoe:
When you build a stone arch, the top center stone is called the “keystone.”  My parents’ home has a stone arch over the front door.  The door is heavy cypress, iron banded with classical nautical battle lanterns flanking.  The stone of the arch is mostly rough marble.  The keystone is a fossil sea fan of a species long since extinct.  Somebody was in a romantic mood when the front of that house was designed. 

The keystone is usually a bit larger than the other stones.  Sometimes I wonder why it is singled out.  The arch I described is fasted together with mortar and does not depend on the shape of the stone for its integrity.  A more classical design would have the stone looking like or actually depending on each other’s shape in order for the arch to stand.  In that case, of course were the keystone to be removed, the arch would fall.  But it would also fall if any other stone was removed.  Why the big deal on the keystone.  I suppose placing it marks a moment of triumph in the construction.

The situation is a little odder when the structure is not an arch but a dome.  The first great dome I know of is the Pantheon of Rome, the temple to all the gods.  When I was there it was a bright sunny day, and it was also raining.  The center of the dome is missing.  There is a big skylight.  Rain was coming straight down through the skylight and splashing in a circle on the stone floor.  At the same time sunlight was coming down obliquely to light up a different circle.  Where to two overlapped, the rain dancing in the sun was most beautiful.

I digress and probably repeat.

It is hard to build a simple hemispherical dome.  The Eskimo used to make them out of ice.  Buckminster Fuller figured out how to build one out of bits of tetrahedra.  There was a big steel tank for gas in the shape of a sphere I knew as a child.  The Jefferson Memorial is built to look like the man’s home, and the engineering problems were dealt with either by this renowned polymath or by his highly skilled servants. But generally the top of the dome approximates an unsupported level surface.  This is not stable.  It’s easier to leave a hole.  The hole can be covered as in the capitol building by putting up another little building with its own – much smaller – dome.  Another way is,  like at the Kremlin, to have the top of the dome extend up into a point, looking rather like an onion. 

The most obvious solution would be just to put a little dome over the hole.  But that would look like a breast, and I don’t know of a building that uses that shape. 

So while the keystone is the strength of an arch it is a weakness in a dome. 

That much said, a keystone species is a species that has such a profound effect on an ecosystem that were it to be removed the ecosystem would be greatly altered.  Wooly mammoths come to mind.  Or grass in the Everglades.  Or coral in a reef.  Silly old me.  It turns out that nobody has actually been able to prove that there is such a thing as a keystone species. 

I am fond of saying, “If a population is able to enlarge without limit, eventually it will be destroyed by speciation effects, and if it is a keystone species the whole ecosystem can go.”  Saying that was fun while it lasted.  At least now they have demonstrated the existence of at least one keystone species, the ever popular mistletoe.  (Under the Mistletoe ECONOMIST vol. 404 n0. 8793 July 14, 2012 page 72)

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