The fundamental is the theoretical niche of an organism given that there are no limiting factors on the environment or resources the organism can use/inhabit.
A fundamental niche is the niche the organism would have if there were no limiting factors, such as predators, competitors, parasites, and disease.
A fundamental niche differs from a realized niche because limiting factors often exist in the real world. The niche the organism actually occupies in the wild is called the realized niche.
For example, let’s say an animal has a diverse, omnivorous diet. It can travel in many habitats and find sufficient food. However, if a predator is introduced in one of those habitats, the animal will avoid that area and no longer feed in that habitat. Thus, its realized niche differs from its fundamental niche.
Below we have an example where competition is the limiting factor for a of barnacle: