What is a quick explanation of the difference between a fundamental and realized niche?
the fundamentale is what one CAN use and the realized is what one actualy do use.
the fudamentale niches found it's bondaries where the concerned organisme can't live anymore.
For instance among a range of temperatur there will be a minimum and maximum of temperatur an organisme can bear. those limites of temperature are the limites of temperatur of the fundamentale niche.
Those limites are under the green curve.
The same goes for any environemental parameter.
But narture is complex and not only the need of one species is in play. If an other species live close it can enter in competition with our organisme, limiting it's acces to some part of it's fundamentale niche. Therefor the realized niche, the one actualy unsed by the organisme, get shorter, as we can see under the red curve.
Enven though some place fit's the species need on an environmetale point of view, the interaction with other organisme may change the avaible place.
(In some case the red curve could be bigger than the green one with "facilitation" where the resence of a species help other to live in some place they couldn't otherwise)