How is surplus related to the different subsistence modes?

In the different subsistence modes, surplus plays a
critical factor in an economic system.  I will clarify my
statement.

Foraging (Hunting &
Gathering):  Foods have a short shelf-life and will spoil rapidly if not
preserved or eaten.  So, people either kill or harvest exactly what they will eat when
they need it, or they sell or give away any extra they don't consume.  The extra is
called surplus and is the basis of economics.  I gather
enough for myself and family and sell the surplus to you. 
In turn, you gather a different product and sell any
surplus to
me. 

Cultivation (Horticulture &
Subsistence Agriculture):  In the beginning, farmers planted crops to
provide for themselves and their animals.  They initially grew enough to eat fresh
during the summer and preserved the rest to carry them through the winter months.  Then,
they discovered that if they planted extra of their crop
(surplus), they could sell it and make a profit.  Farms
grew larger and larger to produce more and more surplus to
make more and more money!

Pastoralism
(Nomadism & Ranching):  Whether people stay in one place to raise
animals (ranching) or follow the animals around from pasture to pasture (nomadism), the
animals are bred for one purpose:  to provide people with food, clothing, and shelter
materials.  Some animals were huge and had more meat than could be consumed by one
person!  So, the extra was surplus and was sold to many
people.   The raising and selling of animals for profit has became a very lucrative
business. 

Distribution & Exchange: 
As more and more surplus products became
available, there needed to be better ways of getting the products to the people who
needed them.   So, companies were formed to distribute (or spread) the products around
the country (or world).  Today's distributors have the means to buy, preserve, store,
ship, and sell the surplus products that others make. 
Sometimes distributors will simply exchange one surplus
product for another surplus product of equal
value. 

Surplus has the unique
way of regulating the economy in that the more of something there is, the lower the
price will be for that item.  It keeps prices low and the economy
healthy.