Mob spawn seems to be randomly assigned to points on the map on generation and how often this respawn happens depends on some factor like how many player structures are affecting their spawns...

Try this, build a ton of workbenches all around the outer perimeter of the meadows and have a smaller area in the center that is a wide field that has no player buildings. Then you end up with say mobs spawning in said area because they can't spawn elsewhere.

In further regions away from the center of the map, mobs spawns more often, and especially when you have two biomes with mobs killing each other, resetting the mob count.

Meadow+Swamp creates a situation where at night undeads outright goes around murdering all the animals and chases them around.