Question

What will happen if all carnivores are removed from a forest?

Answer 1

Ecosystems evolve to a point of balanced equilibrium.This is not a moral explanation, simply how nature works.
In a new isolated habitat—a peninsula that suddenly loses its land bridge and without predators—the herbivores eat too many of the plants to sustain themselves and lose the energy to breed. If they are voracious they may starve.In another isolated habitat but one with predators they may kill the all the herbivores, at which point the predators starve.There are many examples of these scenarios all over theworld. Islands with no land animals except birds, whose ancestors arrived on the wing and then without predators, grew too fat and happy to bother retaining the sacrifices required for flight.Continents and very large islands posses more loosely separated habitats. Here, over many generations the herbivores develop breeding rates consistent with the available plant life. Predators too reach a state of equilibrium with their food source—the herbivores.This isn’t automatic, there is no master switch telling rabbits and deer to slow down reproduction as the food runs out. Wolves and fossa don’t decide to take abreak to let the prey numbers recover. Rather, over many generations the insatiable herbivores and predators (in a particular habitat) starve leaving the creatures with more appropriate appetitesto pass on their genes.
So the answer to what would happen if you removed all the carnivores from the forest depends on the forest and the specific creatures within. If there is enough food to sustain a slow breeding population herbivores might things may not change. If the predators were the only thing keeping the herbivores in check but the deer, antelope, kangaroo can move to a nearby forest than that will work out. If the forest is isolated (highways, suburbs and strip malls) and the deer are insatiable and fast-breeding you may end up with situations like overpopulated and starving white-tailed deer in the Northeast.