Trade, Transportation, and Communications. Spatial Interaction

Before the arrival of the European explorers in North America, many Native American societies were dependent on the American bison or "buffalo" for food and clothing. These cultures were nomadic, following the seasonal migration of the bison. Just as pastoral nomads today use many parts of the animals that they herd. Plains peoples used the bison for food, clothing, tools, and other artifacts. With the introduction of European horses and guns, the formidable tasks of hunting and killing a bison were greatly expedited.

The guns and horses that diffused to the central United States had different points of origin. Spaniards had imported horses into Mexico and their other colonies in Latin America. Native Americans living near Spanish settlements bought horses from the Spaniards or captured escaped horses. Guns, on the other hand, were first obtained by Native Americans living in the Great Lakes region. They had been brought there by French fur traders who sold or traded them to local Native Americans who helped them secure valuable animal pelts.

Because guns and horses diffused into the Great Plains grasslands from different places, the relative value of guns and horses varied greatly throughout the region. To the south, horses were relatively easy to obtain while guns were scarce. Thus, a single gun might be worth five horses. In the central part of the region, a gun and a horse were valued equally. Farther north, horses were in short supply white guns were more plentiful. While a gun would be worth five horses to a Native American in Texas, a horse in Wisconsin would be worth five guns.

Spatial Interaction. The example of guns and horses illustrates the importance of geography in the exchange of goods. Trade and exchange are forms of spatial interaction, a term used to describe the movement of people, goods, and information between one place and another. Aggregate migration patterns were discussed in terms of spatial interaction. Analysis of spatial interaction in general, however, focuses on those conditions needed for interaction to take place.

Certain conditions must be met in order for spatial interaction to occur. These include complementarity and transferability. The absence of intervening opportunity also plays an important role. Complementarity implies that something supplied at one place is in demand at another. Geographical separation of supply and demand generates an incentive for places to interact with one another.

Even with complementarity, spatial interaction will occur only if cost-effective transportation between places is available. The condition that costs be acceptable in order for exchange to occur is known as transferability. If transportation is unavailable or too expensive, there will be no interaction. Remarkable improvements in transportation technology have dramatically reduced the friction of distance between places. Reduced friction of distance is associated with increased transferability.

The amount of interaction occurring between two places also depends on the location of alternative supplies or demands. If a demand can be met more conveniently in one place than in another, interaction with the less convenient place will be reduced. The more conveniently located place represents an intervening opportunity; that is, it alters the complementarity between the two places. Thus, movement between two places is modified or even eliminated because of the existence of intervening opportunities.

The concept of intervening opportunity has important implications for transportation, communication, trade, migration, and other forms of spatial interaction. Trade patterns are greatly affected by the presence or absence of intervening opportunities. There is little demand in North America for citrus fruit grown in the Mediterranean region. The low demand for Mediterranean fruit products is explained by competition from the much closer citrus-producing areas of Florida, Texas, California, and Latin America.

Migration behavior is also influenced by intervening opportunities. The relatively small African-American populations of such Northern cities as Boston, Milwaukee, and Minneapolis is related to nearby intervening opportunities. New York, Chicago, and Washington are larger and closer to the South. Size and proximity, among other things, made these metropolitan areas more attractive to African-American migrants from the South.