Re: Practically zero
And if you really believe that is how the natural world works please stay away from strychnine (lethal dose to an 80 killo human is less than 0.1 grams - that's around 1.2 parts per million), nicotine (lethal dose less than 1 gram, or around ten parts per million) and a whole host of other poisons where the deadly dose is much lower than what you sweepingly consider to be 'practically zero'. And that's before we get to the real nastiness of compounds like ricin at a few tens of milligrams.
The ecosystem of the planet involves and requires a great many more living things to work in harmony than just talking primates, and is likely to be similarly or more sensitive to some perturbations.We know from measurements that you really don't have to change the CO2 levels very much at all to alter the acidity of the sea or the rate of growth of certain types of plant by immediately measurable factors.