Lets not forget the seabird chicks starving because the sand eels they feed on have been hoovered up by trawlers for cat food. The ecological consequences of pet keeping are considerable: it's estimated that in the US cats kill over a billion small mammals a year
http://www.abcbirds.org/abcprograms/.../predation.pdf
and of course there's the St Stephen's Island Wren - the only species to be made extinct by a single individual
BBC - h2g2 - The Story of the Stephens Island Wren