Realized Niche: The survival of the species in an ecological space is called the realized niche of those species. Though there are many factors that are required to measure the ecological space in which the species live and these factors are temperature, soil pH, and nutrients for plants and the relation of all the ecological factors that affect species should also have to determined.
Fundamental Niche: Fundamental niche is measured only in an ideal world. It is the ecological space occupied by species which is devoid of all the competitors and other interactions with the environment i.e., Biotic interaction.
Fundamental Niche is difficult to determine but it can still be obtained in laboratory condition and or it can still be obtained to some extent when a species is introduced in a new region where it surpasses all its competitors and predictors and flourishes well in the new environment.