Realized niches smaller primarily due to:
“Competitive interactions” for realized niches smaller primarily due to. This relationship follows from the ecological mechanism represented by the terms in the item, not merely from an association between their names. A niche is the multidimensional set of abiotic conditions, resources, and biotic relationships under which a population can persist. The fundamental niche reflects physiological and resource limits, while the realized niche is modified by competitors, consumers, mutualists, and dispersal barriers. The remaining alternatives—“Mutation rates”, “Genetic variability”, “High resources”—refer to different states, processes, or scales and therefore do not express the same causal relationship. Niche differentiation reduces overlap by separating species along resource, space, or time axes. Such partitioning can stabilize coexistence when each species performs relatively better under the conditions it uses most strongly. The cited framing is therefore most useful when treated as a conditional biological claim, with assumptions about scale and environmental context kept explicit.
Ref: Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 13