What are realized and fundamental niches?
Realized niches detail the reality of the species' niche whereas fundamental niches detail the niche with no competition.
Realized and fundamental niches are two parallel concepts. A niche can be thought of as a species job or role in the ecosystem. Niche encompasses not just a species' habitat but also the trophic level of the species, the role the species plays (competition, predation, parasitism, etc), how the species utilizes its space, and so forth.
A fundamental niche is the niche the species would ideally occupy or could theoretically occupy. This can be thought of as the textbook version of a niche.
In contrast, the realized niche is the niche the species actually occupies. It is the niche the species occupies in absence of other competitors. The realized and the fundamental niche can be identical, but they can also differ depending on the other coexisting species, human actions, issues in obtaining resources, and so forth. The realized niche is more limited or smaller than the fundamental niche.
For example, if an invasive species is introduced into an ecosystem, it will compete with existing species for food, space, and other resources. Thus, the realized niche of those existing species may change and differ from the fundamental niche.