Answer: Food. Drought, diseases or infestations of crops and animals will cause food reduction to everyone equally.
Step-by-step explanation:
The lack of food, for whatever reason will hurt any population.
Space can fixed by efficiency, building up, and not a problem with a less dense population.
Temperature can be regulated by either.
Mates could possibly be an issue for both, under certain circumstances of less females, but more likely to effect less dense populations.