If one species is lost from a food web all the others are affected. In the food web above, if the mosquito larvae were removed the ducks will have less food and the floating algae will be eaten less. So the duck population is likely to decrease and the floating algae population may increase.
Other populations will also be affected. For instance, the water flea population might be expected to increase as there will be more floating algae to eat. The snails do not appear to have any connection to the mosquito larvae, and yet their population may decrease if the hungry ducks eat more of them.