Question

Why does the introduction of another herbivore into a food web possibly hurt other members of the food web?

Answer 1

Depending on where that herbivore is on the food web they could eat food meant for other smaller animals making those animals have less food and cause them to die. 

Answer 2

When you introduce another consumer to the food web it then has to compete with other consumers for food, if this animal is alien to the ecosystem then it =can overrule the animals and consumers already there and shut down the whole ecosystem.