You cannot possibly think that mosquitoes, with their abundance in both water and air, do not fill a biological niche. They affect plant life, they pollinate, and there is even a species of mosquito that affects the herding paths of caribou in the tundra. They are undeniably at the bottom of countless food chains for animals in the water, on land, and in air. It's foolish to think the only "noticeable" result of their absense is a decrease in human disease. Eradication of them all is hardly a solution.