Just had a Cravers population take off the planet. There was no explanation and no warning. Does anyone know how does migration work and whether there will be some more meaningful feedback in-game when it happens?

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I don't know if there will be a more meaningful explanation in the future. You just need to understand how food and happiness works and the game wants you to check the system yourself to find out what.

Food is a big thing for pops, and you want to keep up with the improvements for food at all times as your pop gets larger.

Happiness is probably more important  as it effects EVERYTHING and all your resources (FIDS). So if your system is unhappy, hover your cursor over the food icon and see a -140 or so to food due to the unhappiness.

Currently there is very little way for the carvers to thrive due to all the approval and disapproval modifiers currently in the game.

Here are some thoughts on the carvers: https://www.games2gether.com/endless-space-2/forum/66-game-design/thread/21365-cravers-approval-and-government