In ecology<span>, a </span>niche<span> is a term describing the relational position of a </span>species<span> or population in an </span>ecosystem<span>. ... According to the competitive exclusion principle, no two </span>species can<span> occupy the same </span>niche<span> in the same environment for a long time.</span>
An ecological niche is the functional role of the species in an ecosystem. The functional roles includes how individuals of a species derive food, which kind of habitat they acquire for survival, mating seasons in which they bred. In an ecosystem, only one species fill an ecological niche this is because of competitive exclusion principle which says that two or more species having same ecological niche in the same ecosystem compete for common resources in an ecosystem. The one which is more competing and strong derives the resources better than the less competing and weaker species. Hence, in the end of the competition only one species is left filling an ecological niche.
A postulate is just more or less an assumption of a fact or truth. I think Mendel was able to postulate concerning the behavior of unit factors during gamete formation because he studied this area well and base on his readings he was able to imagine what might be happening during this time.
