To "reign over" something is to have absolute authority and control over it. God has ultimate rule over the earth, and he exercises his authority with loving care. When God delegated some of his authority to the human race, he expected us to take responsibility for the environment and the other creatures that share our planet. We must not be careless and wasteful as we fulfill his charge. God was careful in how he made this earth. We must not be careless in how we take care of it.