Fact 1: Bottleneck effect occurs when a population suddenly gets much smaller.

Fact 2: Population growth is reduced by external migration.

Given the two facts above, answer the question "Bottleneck effect can occur with " with the following options: 
- external migration
 - direct damage to the lungs
 - Movement of an air mass
 - competition
 - if humans disturb bears
 - competing for resources
 - Plant growth is reduced
 - Temperature changing


 

 external migration        