Biodiversity should be conserved because without biodiversity, animals would only have one thing to eat species to eat. This would mean that if one species dies out, it's predator would also die because the predators wouldn't have anything to feed on.
"If you remove enough of the key pieces on which the framework is based then the whole picture may be in danger of collapsing. We have no idea how many key 'pieces' we can afford to lose before this might happen, nor even in many cases, which are the key pieces. The ecological arguments for conserving biodiversity are therefore based on the premise that we need to preserve biodiversity in order to maintain our own life support systems." -