Sawdust supply falling

From WSJ:

The sawdust article offers a great case for supply and demand analysis, with four good points. The first point is regarding the shift in the supply of sawdust that is the impetus for the recent increases in sawdust prices. With a decrease in the equilibrium quantity of new home construction, the supply of wood byproducts, including sawdust, has decreased. The result is an increase in the price of these byproducts. The second is a movement along the supply function. With an increase in the price of sawdust, entrepreneurial types now find it profitable to scavenge forest floors for scraps, which are left by logging companies, that can be processed into sawdust. The third is a movement along the demand function. With an increase in prices, byproduct consumers are switching from sawdust to, for example, processed cow manure, almond hulls and walnut shells. The fourth point is about the demand for employment in the sawdust industry. With an increase in the price of sawdust, firms that distribute the material have laid off workers because the workers and the purchased sawdust are complementary inputs.

EXTRA CREDIT QUESTION: Why is there no such thing as a shortage?