Optimal CO2 for plants

In greenhouse experiments, the optimal CO2 for plants is in the range of 600-1600 PPM. Anything more than that seems to have a detrimental effect.

The plants at 600 PPM also grow much better in elevated temperatures as well.

So, as with the oxygen discussion, this just goes to exemplify the evolved fragility of Earth's ecosystem?

It's only fragile for unfit species. Survival of the fittest.

For sure, in so much as that which can adapt.

But I was referring to the overall balance of the Earth's atmosphere.

That which sustains the only known planetary ecosystem.

You would have to ask the life that survived massive asteroid impacts about "fragility of the ecosystem"

After an asteroid wiped out much of life on Earth, mammals—responding to changes in plants—grew in size and diversity surprisingly quickly. ... Pea and bean species from the "protein bar period" provided protein-rich meals that further boosted mammalian size and diversity.

Or ask them about the key to their success.