[Randallw]

Yes but the percentage of oxygen is not breathed in by any indiviual, thus that once they run out of their percentage they expire tipping the balance. If there is more CO2 being generated than oxygen, the humans will go along breathing enough until the total amount of oxygen is depeleted at which time the humans will all drop dead enmasse. This all of course avoids the issue that if the atmosphere is an equal amount of CO2 to Oxygen neither species should be able to breathe properly, I think.