[sadkingbilly]

Mosquitoes are a food source for many animals, including: fish, birds, spiders, and bats.

Maybe those animals could find something else to eat, but there sure are a lot of mosquitoes - imagine how many get eaten.

[StavrosK]

Are you sure? An article I read said that mosquitoes are so lean that they aren't even worth the energy expenditure of hunting them.