Schmungle said:
Mosquitos. They're everywhere in summer, their bites itch for days, and they spread diseases.
I'm stealing Schmugle's answer. Mosquitos seem to be one creature which serves no purpose in the larger ecosystem other than serving itself. It makes people sick and kills them in some of the worst deaths imaginable. If all mosquitoes were to suddenly and mysteriously vanish tomorrow, there doesn't seem to be any down the road bad consequences.
INB4 culling the human herd is the important work. No, it's not. We're way past the point where humans collectively have trouble feeding themselves. All of our harm to the larger environment is a matter of policies and habits, not zoology.