Things we lost...The Atitlán grebe.

The giant grebe was a podicipedidae native of Guatemala (Podilymbus gigas) which was discovered in 1929. 
It became extinct because of the introduction in the homonyous lake of the smallmouth bass (native of North America) for sport fishing. This fish killed the nestlings and large quantities of grabs and fish which were the main food source of the grebe. 
In 1983 there were only 32 specimens left and in 1989 the last two were sighted. After their disappearance this species was officially declared extinct. 

(Thanks to Lorenzo S. for helping me writing this post)