The main ways in which sharks have been shown to regulate their ecosystem and why they are referred to as a Keystone Species - https://en.wikipedia.org/wiki/Keystone_species - are: 1. Controlling population numbers and quality of their prey species - http://www.fmap.ca/ramweb/papers-total/Myers_etal_2007_Science.pdf -. 2. Causing tropic cascades by modifying prey behaviour patterns