Answer
Resource prices determine money incomes and help ration resources. Resources are a Derived Demand. They are demanded when a producer requires them to supply a good or service. Resourced demand curves slope downwards because they reflect the demand curves of the producers to which they are being supplied. As the producer's demand curve slopes downward, so is the quantity of resources demanded to make the product.
Work Step by Step
Derived demand is the definition of a relationship between a primary product and the resources that go into making it.
It's pretty straight-forward to assume that as your primary product declines on the demand curve, the supplies used to make it will also decline. If I use aluminum to make an I phone and as I raise the prices, the quantity demanded goes down....I will demand aluminum in a smaller quantity.