Editing Ecological niche (section)

=== Coexistence under contemporary niche theory ===
Contemporary niche theory provides three requirements that must be met in order for two species (consumers) to coexist:<ref name=":2" />

# The requirement niches of both consumers must overlap.
# Each consumer must outcompete the other for the resource that it needs most. For example, if two plants (P1 and P2) are competing for nitrogen and phosphorus in a given ecosystem, they will only coexist if they are limited by different resources (P1 is limited by nitrogen and P2 is limited by phosphorus, perhaps) and each species must outcompete the other species to get that resource (P1 needs to be better at obtaining nitrogen and P2 needs to be better at obtaining phosphorus). Intuitively, this makes sense from an inverse perspective: If both consumers are limited by the same resource, one of the species will ultimately be the better competitor, and only that species will survive. Furthermore, if P1 was outcompeted for the nitrogen (the resource it needed most) it would not survive. Likewise, if P2 was outcompeted for phosphorus, it would not survive.
# The availability of the limiting resources (nitrogen and phosphorus in the above example) in the environment are equivalent.

These requirements are interesting and controversial because they require any two species to share a certain environment (have overlapping requirement niches) but fundamentally differ the ways that they use (or "impact") that environment. These requirements have repeatedly been violated by nonnative (i.e. [[Introduced species|introduced]] and [[Invasive species|invasive]]) species, which often coexist with new species in their nonnative ranges, but do not appear to be constricted these requirements. In other words, contemporary niche theory predicts that species will be unable to invade new environments outside of their requirement (i.e. realized) niche, yet many examples of this are well-documented.<ref>{{Cite journal|last1=Tingley|first1=Reid|last2=Vallinoto|first2=Marcelo|last3=Sequeira|first3=Fernando|last4=Kearney|first4=Michael R.|date=2014-07-15|title=Realized niche shift during a global biological invasion|journal=Proceedings of the National Academy of Sciences|language=en|volume=111|issue=28|pages=10233–10238|doi=10.1073/pnas.1405766111|issn=0027-8424|pmid=24982155|pmc=4104887 |bibcode=2014PNAS..11110233T |doi-access=free }}</ref><ref name=":3">{{Cite journal|last1=MacDougall|first1=Andrew S.|last2=Gilbert|first2=Benjamin|last3=Levine|first3=Jonathan M.|date=2009|title=Plant invasions and the niche|url=https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2745.2009.01514.x|journal=Journal of Ecology|language=en|volume=97|issue=4|pages=609–615|doi=10.1111/j.1365-2745.2009.01514.x|bibcode=2009JEcol..97..609M |s2cid=49234920 |issn=1365-2745}}</ref> Additionally, contemporary niche theory predicts that species will be unable to establish in environments where other species already consume resources in the same ways as the incoming species, however examples of this are also numerous.<ref>{{Cite journal|last1=Funk|first1=Jennifer L.|last2=Vitousek|first2=Peter M.|date=April 2007|title=Resource-use efficiency and plant invasion in low-resource systems|url=https://www.nature.com/articles/nature05719|journal=Nature|language=en|volume=446|issue=7139|pages=1079–1081|doi=10.1038/nature05719|pmid=17460672 |bibcode=2007Natur.446.1079F |s2cid=4430919 |issn=1476-4687}}</ref><ref name=":3" />