What is used to separate a species' realized niche from its fundamental niche?

Question

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Competition for food with another species is the factor that is most likely to separate a species' realized niche from its fundamental niche. This is because when two species compete for the same food resources in an ecosystem, they are forced to use a narrower range of resources than they otherwise would. This can result in a species occupying a smaller niche than it would be capable of in the absence of competition.

What Is Used to Separate a Species' Realized Niche from Its