Bottleneck effect occurs when a population suddenly gets much smaller.
Population growth is reduced by external migration.
Bottleneck effect occurs with external migration.
Bottleneck effect can occur with 
Options:
1) external migration
2) direct damage to the lungs
3) Movement of an air mass
4) competition
5) if humans disturb bears
6) competing for resources
7) Plant growth is reduced
8) Temperature changing