Wow man...
No....the Praries aren't even close to being called a monoculture...
They "are" an actual diverse ecosystem...
Mono means "ONE" ....not multiple.
No other plants are growing in the wheat fields here. No insects are tolerated...
THAT is a monoculture...
It is an artificially enforced unnatural construct...upon nature.
By the same reckoning you are using...
concerning the Praries...alone...
The Sahara and Gobi and Amazon Rainforest are monocultures...because they "look" like just one thing..."an endless seeming expanse of the same thing"

May as well call the oceans monocultures ...since a glance at the surface of them ...they look pretty much uniform too...

And as far as your lake drying up...
that is a natural movement to a marsh....and then ultimately...a meadow...and then most likely into a thicket...of trees..
Happens here all the time ...
Lakelets dry up...turn into swamp or marsh...
Got one occurring down at the bottom of my valley ...here...in fact...
It was...a long time ago...a very big lake...
Over the centuries it has gooten smaller...and is turning into marsh...
But that don't make the process to something monocultural...

As soon as we clear a space and plant only 1 thing...in it...and then make sure the yield and quality isn't disturbed by a foreign (to the monoculture) pest....it is a monoculture.
The biodiversity of that area is damaged.
Trying to argue the fine point and say fungi and bacteria exist ...is really stretching...
they exist as long as they have no negative impact on the crop.
The moment they do...they are considered a negative impact and are dealt with....

Organic?
There is a really questionable term...that has been warped for the use of marketing.