What are some ways that population size is limited within an ecosystem?

Populace development is restricted in ecosystems as a result of absence of assets. On the off chance that there were boundless assets, a populace would develop exponentially. 
For instance, if a solitary bacterium were put in a petri dish and was given boundless assets, it would increase to cover the whole world inside a few days. 
Sadly, assets are restricted, so populaces are as well.