What do we know about the effects of humans on extinction of plant and animal species on earth?

We know that if a plant goes extinct it will cause a chain reaction. For example, If grass dies then what will the cow eat? Nothing. If the cows die what will humans eat? Ok humans will probably eat something else, but cow serve many purposes such as milk as well as meat, which would cut out a huge portion of what most people eat every day 

If you eat to much it could kill you.