“Compression of realized niche” for competition typically causes. 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—“Expansion of fundamental niche”, “Increased genetic diversity
Ref:
Ecology: Concepts and Applications, Molles, 9th Ed., Ch. 13