In a linear system the whole is the sum of the parts. If 1 produces 5, then 5 produces 25 and 100 produces 500.
But if you add a deathrate due to overgrowding (carrying capacity is the maximum population that can be supported) you get a non-linear system:
n_t+1 = birthrate * [n_t - deaths]
where deaths is defined as n_t^2 / max_population
If you extend it slightly you get the "logistic model" of population growth developed in early 19th century by Verhulst:
n_t+1 = birthrate * n_t - (n_t^2/max_population)
Logistic Population Growth Model (NetLogo)
This model shows a "logistic function", it starts out fast, then slows and ultimately goes flat.
With a certain logistic model, 1 individual produces 7 after 3 steps, but 5 individuals don't produce 35, they produce just 21. So the whole is not just the sum of the parts.