Question

How can competition limit a population's growth

Answer 1

there could be a decrease in one organism, which could limit another populations food, which means more would die

Answer 2

A competition can lead to direct wars/fights between members of populations, in which people (or animals) will die (if there is only limited food, only limited number of people can survive...). So a competition for water in a very dry area can directly lead to deaths.

Alternatively, those that loose the competition might die as a result of not getting the resource they were competing over.