How does competition affect niches?
there is two vision of niches, one with only abiotic factors in consideration and an other one with biotic factor like competition in consideration.
When you only consider abiotic factors like temperatur, salinity etc.. you get to find a range of those factors where one species can live. this will be the fundamentale niche. But in nature there is more tha only abiotic factors, species intercat with each other and that can affect their niches.
Competition is an interaction that is negative for both the participant. If a specie can't handle the competition for a ressource against an other species thoses ressources won't be part of the loosing specie's niches.
That new niche, with the new limite due to competition will be the realized niche.