Polar bear will not spawn unless they were pre-generated with the world, once dead no new one will spawn.
Create a Superflat world with the following preset
3;minecraft:bedrock,45*minecraft:dirt,minecraft:grass;12Kill all other currently existing entities
/kill @e[type=!player]Wait some time
You will get rabbits spawning but no polar bears
If you create a normal world and then use something like mcedit to remove all blocks then just put grass down in the ice plains only rabbits spawn. Putting grass can be done manually or with fill command or making structure block for it.
For all above i recommend changing gamerules for daylightcycle and weather such you only dealing with polar bears and rabbits you can also setup a counter system with the command blocks
Comments 7
It is possible but everyone said to me golden bunnies spawn on sand i proved that wrong and it is grass, as far as i know, all passive mobs spawn on grass. Same issue with parrots which i have reported until it confirmed will never get fixed.
Invalid ticket. Polar bears do spawn even after the initial world generation on grass blocks as intended. They are just 10 times as rare as rabbits. Tested in 1.12 with the given super flat preset.
Flaming i will retest this then i hope i am wrong but if not i will need to know if you done anything differently and the length of time you used. I use command block counters to accurately test it.
Cannot reproduce, polar bears are being spawned, although not too often. I tried it both with your preset and the "Snowy Kingdom" preset.
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Could polar bears spawn on a different block than grass?