Give an example of a fundamental vs. a realized niche?

Fundamental niche can be defined as the set of conditions in which the species can survive and reproduce. This is the niche which a species or organism must occupy. The realized niche is the set of condition, used by the species to survive and reproduce. This is the niche which a species or organism actually occupies.

For example, an animal has an omnivorous diet. It can find many habitats and sufficient food to survive in a region. This is fundamental niche. A predator population is introduced into the region where the omnivorous animal lives, this is likely to reduce the amount of food and the animal may not find suitable habitat to live. This is an example of realized niche.