Answer:
Conflicts with other tribes.
Step-by-step explanation:
When Settlers traded with one tribe only, the gain in prosperity that is obtained by leveraging on the local resources will only be felt by that one tribes.
The other tribes felt the deteriorating resources (since they live within the same region), but do not receive the same benefit in return as that one tribes. As a result, this will gradually build up resentment towards that one tribes or the European settlers. This what might lead to wars/conflicts.