If vegatation were to decrease then primary consumers that are herbivores which are usually the prey will start to decrease too, because there food source is becoming limited. It's like a domino effect, if something is unbalanced in the ecosystem then the whole ecosystem could collapse.
This means to come up with your own hypothesis or assumptions of something because there is some missing information or not clearly stated.
You would add H+ ions if you added an acid. Thus lowering the pH.
For air masses to form fronts, they must collide with each other. B.
