My impression is that mosquitoes seem to thrive best in degraded ecosystems where there aren't enough birds and fish and other small insects to snack on them an

My impression is that mosquitoes seem to thrive best in degraded ecosystems where there aren't enough birds and fish and other small insects to snack on them and keep the population low. But even then, they're so small that I don't think they provide enough calories to larger animals. As far as I know the only insects for whom mosquitoes are actually a worthwhile food source are other mosquitoes (the non-annoying ones that drink nectar like to eat the kinds that suck blood). So there may be a consequence in terms of being able to pollinate certain plants. But it's also likely the niche would be filled by other insects, though maybe that wouldn't be any less desirable.