links in a people chain

If all life on land were to vanish tomorrow, the creatures in the oceans would replenish and thrive. But if the life in the oceans were to perish then we would perish with them and this is the course we currently hold. We're all capable of understanding this, yet many of us choose to turn a blind eye, to declare what's beneath the waves as out of sight, out of mind. How long do we ignore what's happening? How devoid of life do the seas have to be before we realise that not having fish to eat will be the least of our problems?