When a substance, like water, moves from an area of high concentration to low concentration.
In other words, the substance goes from where it is to where it isn’t.
Yes because the cell that the organism is made out of is its life source. For example, bacteria is a single celled organism.
Well, depending on how long the migration is, many of the animals (mainly birds) could get sick and die, lowering the population. They could be shot by hunters, lowering the population. The children of the population could get left behind and lose their way, lowering the population. I'm not entirely sure how the population could grow other than them finding and staying with another group of their own kind. hope I was helpful! :)