What are the positive effects of migration?

Animals and people migrate in search of food, or to fulfill a life cycle.

Darwin.

Charles Darwin developed the theory that the fittest survive. Fittest did not mean 'best' on some sort of ethical basis. It meant fittest in the sense that a characteristic fitted best with the current environment. A country with no Immigration is less exposed to other ideas in the world and its survival rate is thus reduced. With some Immigration the best combination of ideas of locals and immigrants survive. With too much immigration the local ideas may be overwhelmed and lost and the survival rate reduced.