Bots have become a major part of trading in 2008. (Idea behind Automatic Trading Systems goes back to 1949.) I am pulling this number from my ass but 80 to 90% of trades done in stock exchanges are done by ATS. (Given in 2014 this number was 75% and there is no reason for it to shrink but grow.)
So think about how much volume of the traditional stock exchanges is from just bots trading stocks back and forth. Just a thought.