If you build a public space port you are opening the possibility of natives from nearby planets moving to your base. You also open the door for them to leave your base and move to another base that you own or a base belong to an enemy empire.
If you have natives on your base with a happiness levels above 70% and have an active public space port a percentage of natives from all planets in a 300 LY radius will move to your base. Natives will only move to the base if there is at least one native of the same type on the base already.
If there are two bases within 300 LY of each other and both have active public space ports and both have the same type of natives, the natives on the least happiest base will move to the happiest base. This exchange can take place between two bases that you own or between a base you own and an enemy base.
If there are less than 200 natives on the planet or base that natives are exiting they will all leave. If there are more than 200 natives between 5% and 10% will be leaving.
How do you keep natives happy? They do not like it when the base is attacked by enemy ships. They like public space ports and low tax rates. Ultra conservative is the favorite tax rate for all natives.
Enemy incite disorder spy attacks can cause the happiness levels to drop.
Privateer bases within 200 ly cause a drain of 10 points of native happiness.
When a Privateer base within 400 ly gets destroyed the happiness of the natives increases.
A Rebel base within 200 ly with more than one hundred thousand colonists will drain away happiness from your base and add the happiness to their own natives.
The Lizards own a ship device called the Psi-Opps Hisser that will increase the happiness of the natives under the ship.
The Mind Crusher ship device drives the native happiness of any base within 10 ly down.
The Solar Gamma Ray ship device can drive the native happiness down on any base the ship is parked over.
Feb 2nd, 2004