The concept in which an ecological role or space in a habitat is not filled by a species.
The concept of vacant niche is still a matter of controversy. If niche is regarded as an ecological space or a role that a species fills in an ecosystem, the absence of such species then results in vacant niche. If niche is defined based on Hutchinsonian niche framework wherein the presence of species is needed to define it, the possibility of vacant niche becomes illogical and unacceptable.
See also: niche