- External factors - Extrinsic factors - habitat, food, interaction with other organisms - Internal factors - Intrinsic factors - biological stress due to overcrowding, maturity, body size, hormonal status. They are mainly biotic

1. Density-dependent factors whose effect decreases as the population increases.
2. Density-independent factors: their effect is independent of population size. They do NOT affect population size. Examples: drought, early freeze, landslides, hurricanes. They are mainly abiotic
3. Density-independent factors: their effect is dependent on the size of the population.

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