"The second issue, obviously, is how much you can draw.  While the mana of the world is plentiful, it is not limitless; a skilled mage can draw more and quicker, but the amount around you will limit the rate and the more you pull the more quickly any given source will dwindle.  Besides that, living things are troublesome; the greater the will, the more they will innately fight to hold on to their mana.  If they are trained, it becomes more difficult still.  It is typically not worthwhile to try to draw from anything smarter than a tree, and even some old trees can take umbrage.  Further, drawing from a living creature faster than it can regenerate can do it harm, as can emptying it.  Some of you have experienced mana exhaustion before yourselves, and so have some idea.  So, unless you have very clever workings and a plan, the living are not worth using as a source."