We know that if there are…

We know that if there are not regulations in place, the government of the day may choose to ignore our environment and all the inhabitants (not just the people). While current governments may believe that they wouldn't do anything to jeopardize species at risk, who knows what government will be in place in the future.
We must have a Green Belt around the GTA both to protect our water supply but also to protect farmland and all of the life that exists in forests, swamps and meadows in our region.
The role of government is to protect those that can't protect themselves against those that have power to ruin for us what is good and beautiful in our environment.

There is not justification for placing economic interests over the environment as the justification (if there is one) is only short term and the long term destruction of our environment is extremely difficult to repair if at all possible.
The economic value of green space will only become more valuable in the future. No one ever wished a park didn't exist or that a bird that was extinct didn't matter. Our environment is all interconnected. If we loose one species, it upsets the existence of all the other species.