Pollution

When we cut down the last tree !
When we fish the last fish !
When we pollute the last river !

(then) we will understand that we can not / cannot eat money. 
But, unfortunately, this warning will then have no meaning! 

<em>Very interesting idea!  Thanks.  You "cut down" or "fell" a tree to get the timber / wood.  "Cut" on its own does not quite express this. Well done. Michael</em>