As a population grows there is a struggle for available resources.
 Intraspecific competition occurs when organisms of the same species compete for resources (food, space, mates) in their habitat. As populations grow the amount of available food decreases per individual, which could result in a decrease in the growth and reproductive success of the population.

Predation is when a predator catches, kills and consumes a prey. In a predator-prey relationship, the population of the predator depends on the population of the prey.   If the prey population decreases the predator population will also decrease, as the amount of food has been reduced.   When the predator’s population decreases the amount of prey will increase as there is less predators around to consume them. 

Disease can greatly affect the populations that are dense or overcrowded, since it is easier to be passed from one individual to another.