One of the things I learned today:

Whales feces allows phytoplancton to survive and nourishes them to create about 50% of the oxygen on Earth. So, if you remove Whales from the equation, (if they go extinct) then  basically you remove 50% of the oxygen producing organisms on earth. 

 Because so many species (including Trees) are symbiotic with human survival losing ANY key species could prevent the survival of many other species including humans on earth.