# Relationship between the Tragedy of the Commons and negative externalities?

I've read that the tragedy of the commons can be seen as a negative externality. That is, $$MSC > MPC$$. However, I'm having trouble understanding why the supply curve actually shifts downwards due to this.

I know, graphically, that the $$MSC$$ curve will be above the $$MPC$$ curve. But, what do the external costs mean in this context and how would it all look graphically?

I created a graphical example of this for you in overleaf. As the graph show in presence of externality the supply $$S$$ shifts right and new supply is $$S_E$$. Demand remains unchanged because the externality affects production costs not preferences or other factors affecting demand. The new equilibrium will be at the intersection of $$D$$ and $$S_E$$. In this new equilibrium price is too low and quantity too high compared to social optimum which would be at the intersection of demand and supply without the externality.