That's funny, you can tell you and I come from two different 'camps'. You have a more 'nano tank' point of view, and I'm from the 'bigger the better' side. I would have said,"the larger the tank, MORE possible it is to create a self-sustaining ecosystem." For example, I could probably fill my 240 with enough micro and macro algae to support one herbivorous fish. So I could build a self-sustaining tank, but it would not be very interesting as a display. But I couldn't do that with a small tank. It couldn't grow enough algae to support a herbivorous fish. It would consume quicker than algae could regrow.
I'd like to hear more about a reef in a jar. How does smaller make it MORE possible?
Take a look at the 2008 Advanced Aquarist article, "Micro-Ecosystems." I think you would enjoy it. The reason I say that smaller is better is that the bigger the ecosystem (and, particularly, the size and complexity of the organisms within it), the more difficult it is to balance all the environmental factors. Though I should probably specify that when I say ecosystem I mean in the food-web sense, rather than the geographical sense. In other words, I would consider a 100g tank full of plankton to be a "small" ecosystem in this context, and a 10g tank with a clownfish and some corals to be a "large" ecosystem.
Of course, in a large tank, it may be possible to create a simple ecosystem with a more complex animal. But consider it from the perspective of keeping relatively simple organisms (like in those Ecospheres you can buy) such as invertebrates, algae, bacteria, and plankton. It is much easier to set up a small ecosystem (<=1g) and balance the ecological factors on a minor scale than it would be to try and balance, say, a 50g full of invertebrates, algae, bacteria, and plankton. On a small scale, calculations are much simpler, and the general ecology (growth rates, reproduction rates, death rates, predation rates, etc) is far simpler to predict; even though these are exponential functions, if you only have, say, 2 shrimp in a small ecosystem, these rates are going to be a lot more linear, and thus easier to analyze, predict, manipulate, and calculate. On a large scale, calculations are much more complicated because these are exponential functions, and higher magnitudes are more difficult to predict, analyze, manipulate, etc.
About the reef-in-a-jar, it has come to my attention that the "Reefbowl" I was originally referring to is not actually a closed, self-sustaining ecosystem. That said, it is an awesome example of an unbelievable microecosystem. However, another example would be the PJ Reefs jar. Also, there are several different examples in the article I mentioned in the first sentence.
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