The world is filled with wrong thinking. No matter how strongly or confident one is about a perceived fact, if it is wrong, IT IS WRONG.

Is it possible to maintain a starter in such a way as to greatly alter the ratio of yeast (fungi) to LAB (bacteria)? The question deals not with the production of CO2, acids, or other byproducts, but with the various populations of the 2 mentioned microbes.

Until recently I assumed that a starter could be managed (through food, temperature, and hydration) to increase the ratio of yeast microbes over the LAB microbes. Recently obtained new information from a trusted friend puts that thought in serious question.

Why does the answer to this question matter?

1. If the relative population ratio of each set of microbes remain basically unchanged, then our focus is best targeted towards the byproducts produced by the two main groups of microbes. 
2. If we can swing the relative population ratio of each set of microbes our focus may be to prepare an environment that causes one group of microbes to multiply at a faster rate relative to other.

IF #1 is true, it seems that we can alter our starters characteristics rather quickly.

• I have long believed that a non-acidic starter can produce sour tasting bread, by simply providing the proper environment during the bulk ferment. Think about this. We know we can allow our starter to over ferment. And if we do, the starter will be extremely acidic in short amount of time and also in a single feed. Isn’t a levain just a big starter?
• If the relative microbe reproduction of both groups are of no consequence, then our focus is bested targeted towards enabling or hindering the metabolism(s) of one or both groups of microbes. For example - as the metabolism of the yeast increase, so does the production of CO2 and other byproducts.

We are fortunate to have a number of scientifically trained bakers on this forum, of which I am NOT one. My brain is untrained, but at times my mind takes me to deep places :-)

*NOTE - the word relative is used because the ratio of yeast to LAB greatly favors the latter.

- A thought for those that have not considered this
We are taught that once the starter’s PH drops to a certain level that the LAB are no longer able to reproduce. BUT the yeast are not negatively affected and continue to multiply. I used to wonder, if this is true how does the starter continue to grow much more acidic. The answer - even though they don’t multiply their population, the LAB continue to produce acids.

If the paragraph above is true, then doesn’t that prove that the population of yeast microbes can increase their population relative to the LAB?

I am thinking out loud and hope to learn from the replies to this post.