Since you mentioned CO<sub>2</sub>, I'm guessing you are using [this answer][1] from Tim B. 

If so, then think of dolphin moles. To store up large stores of CO<sub>2</sub> while still being able to breathe may require the bone grass to go on a crazy oxygen converting frenzy. This would create an abundance of outside oxygen while the plants stored up CO<sub>2</sub>. Only during this time do the moles appear for air. The bone grass has learned to ignore the moles as the miles always disappear before they can pump out sufficient CO<sub>2</sub>. The mole dolphins don't have to surface for a while and are alive and well. If you want the moles to tunnel into people's lungs, it would be a bit impractical as the person collapsed because it has too much CO<sub>2</sub>... Perhaps this is what the mole eats? 

Also just a side note. The CO<sub>2</sub> could be used as a defense against fire. Whenever you try to light the bone grass, it suffocates the fire!


  [1]: http://worldbuilding.stackexchange.com/a/38493/7129