Why are there mosquitos

To balance the food chain.

Explanation:

Mosquitoes actually play a big part in the food chain.. Animals such as frogs eat them, and what if other foods that frogs eat go out(Not saying it's gonna happen, its just a what-if)? They will have to resort to the mosquitoes. And if those were gone, the frogs would go out. Then a certain number of animals maybe maybe not would have to develop a new diet due to the extinction of frogs.  

step 1: nitrogen fixation- conversion of atmospheric no₂ into forms useful to plants by lightning, bacteria, and cyanobacteria.

step 2: nitrification-process of converting ammonia to nitrite or nitrate.

step 3: assimilation-plant roots absorb new form of nitrogen.

step 4: ammonification-conversion of organic nitrogen to ammonia by bacteria involved in decomposition.

step 5: denitrification-bacteria turns the decomposed matter back into n₂o, which is released into atmosphere; repeat cycle.

"it built up in the atmosphere"

i think thats it, idk, i tried looking for this exact q& a for grad. and idk if i got this question right or sorry if im wrong