Is the relation between midi-chlorian count and Force ability a correlation or a causation? If the latter, in which direction?

Question

The midi-chlorians are a sensitive topic for many Star Wars fans, I can imagine there are those who regard the original trilogy as the only canon and refuse to acknowledge the prequel trilogy. One of the main causes is the introduction of the midi-chlorians.

However, as far as I remember, there is no mention in any of the movies that the midi-chlorians are actually the cause of the Force abilities. From that scene in the move you could also interpret that the high midi-chlorian count is an indicator of Force abilities (or the potential for Force abilities).

Can it be, that it is only an indicator? For example, there are bacteria which thrive in acidic environments, or others that thrive in basic or saline environments, so maybe midi-chlorians thrive better in "Force-able" environments?

Is there anything in canon which answers this question? It seems the wiki and others sites take for granted that midi-chlorians are the cause of the Force, but without mentioning any proof.

Answer 1

From The Phantom Menace script , it's clear that the midi-chlorians that act as an intermediary in the communication with the force. (emphasis mine)

ANAKIN : Master, sir...I've been wondering...what are midi-chlorians?
QUI-GON : Midi-chlorians are a microcopic lifeform that reside within all living cells and communicates with the Force.
ANAKIN : They live inside of me?
QUI-GON : In your cells. We are symbionts with the midi-chlorians.
ANAKIN : Symbionts?
QUI-GON : Life forms living together for mutual advantage. Without the
midi-chlorians, life could not exist, and we would have no knowledge of the Force. They continually speak to you, telling you the will of the Force.
ANAKIN : They do??
QUI-GON : When you learn to quiet your mind, you will hear them speaking to you.

Update :

Through, I remembered the case of Grievous :

Dooku provided blood from the frozen body of Jedi Master Sifo-Dyas as a means of keeping Grievous's body alive during his transit to Geonosis [...] While the midi-chlorian-rich blood of Sifo-Dyas may have played a critical role in maintaining Sheelal's life, Grievous saw it as a personal failure that the transfusion did not also give him a degree of sensitivity to the Force.

So, since you could not tack on Force sensitivity to someone by artificially augmenting his midi-chlorian count, it imply there's is some kind of affinity between the host and the midi-chlorian.