I think I disagree, although I have not thought this through. Clearly, a
2000 gallon pond is not a lake, but surely, we can create microcosm of
this
great macrocosm that we live in?

We can mimic it IMHO, but it needs our constant attention to not let it
"evolve" into a stinking swamp...

Much depends on stocking levels of course, there are perfect examples of
ponds with just a few fish (no koi), ample oxygenating plants and other
plants, that survive without even being filtered (or fed !)

Even then, you can expect the occasional PH crash or low oxygen situation
causing all the fish to die. As they would in nature.

Most ponds need some human intervention (a pump, airpump, occasional
cleaning, water changes, netting) to make for continually happy fish.

Throw in one koi and things get a lot further from nature.

Nothing is ever 100% true, I realize that, there's a great story in Peter
Waddington's Koi Kichi where he has a very sick koi with holes in it,
ulcers, fins all shredded, a gonner. He throws it in a small pond in his
backyard (no filtration, no airpump) to let it die in peace. He then
forgets about it, and after 3 years he wants to drain that pond and clean it
out. First he finds a kid's bike, en then suddenly he sees something
moving... yup, that same koi, grown to 20 inches, no deformaties at all,
almost show quality...

He then continues to say we shouldn't stop our filters or throw bikes in our
pond ;-)

Theo