Biosphere 2 in Arizona was a closed system such as envisioned on Mars. I have visited this facility and it is very energy intensive to run all the environmental systems. One pump, one control system, one plumbing leak would be disaster. A smoking electric motor would fill the complex with toxic fumes. The backup equipment would be staggering.
Biosphere 2 was an absolute disaster because the project failed to produce enough food to meet the calorie requirements of the participants. The hungry Biospherians began eating seed stocks that had not been grown inside the Biosphere 2. Such a situation will not go well on Mars because of the time it takes to send emergency food or other critical equipment. A colony of one million people would require a perfect system. Any little failure would have big consequences. I wouldn’t risk my life in such a situation.
1,000,000 people who are starving would find some slower moving members of the colony to fill their appetite. And reduce the community’s food needs.