To mention just one issue, there is a difference between fundamental niches and realized niches. What does that mean? Well, it could be that a plant, for example, could happily grow under wet, mesic and dry conditions if it were the only plant around. However, there is another plant that can also grow under mesic conditions but it is much more competitive than the first plant. That means that if they occur in different areas, the first plant will grow in all three habitats and the second one only in the mesic habitat, but if they happen to occur in the same area, you will find the first species in wet and dry places and the second one in the mesic places where it excludes the first.
For illustration, this is how well both species do along a moisture gradient, which we could call their fundamental niches:
And this is how many individuals of each species you would really be able to find along the moisture gradient if they both occur together, which would show their realized niches:
What this shows is first that no organism should be expected to come with a clearly defined role but that it has to settle into one depending on what other organisms are around. Also, which of these two is the niche relevant for the Ecological Species Concept?
If we go with the realized niche, for example because we have so far only observed the species together in nature and don't realize that the orange species would feel even happier under mesic conditions than where it is actually forced to live, we would probably intuit that the orange populations in dry and the wet habitats are two Ecospecies. Perhaps there is a bit of circular reasoning involved on my part (i.e. I may be smuggling the criteria of the Biological Species Concept into my argument), but this seems plainly absurd to me. But if we take the fundamental niche as a guide, what use is that if it may never be realized in nature?
