How can change in one part of an ecosystem affect other parts of that same ecosystem?

Ok so take any plant in for say a forest for example. If a certain plant dies an animal wont be able to feed off of it . If that animal only eats that plant, it's gonna die off starvation. That species in that  ecosystem would be no more. If all the water evaporated in a lake then all the fish would die and the animals in the forest would need to find something else to drink out of.

i think the answer would be d