A population is a group of organisms of the same group of species that live in a particular geographic area. Each population needs specific resources due to size, reproduction strategies and survival curves. Their ecosystem provided them with resources for growth however there is a limit to what an ecosystem can provide.
If ecosystems had an infinite amount of resources, populations would grow exponentially however we do not see this occur because of carrying capacity. A carrying capacity is the maximum population size of the species that the environment can sustain, given the food, habitat, water, sunlight and other necessities available in the environment.
Once a population hits a carrying capacity because of limiting resources there is an overshoot in the population number and many die offs. There are two types of ways to graph population growth - exponential and logistic growth.