Examining the generality of the biphasic transition from niche-structured to immigration-structured communities

Theoretical and empirical studies suggest that as immigration increases, ecological communities transition from a niche-structured regime to an immigration-structured regime. The niche-structured regime is the domain of classic niche models; the immigration-structured regime is the domain of island biogeography and related theories. A recent unified model predicted a biphasic species–area relationship (SAR) arising from the transition between the two regimes, but the generality and scope of this relationship remain unclear. Here we study the transition further to address three key questions: (1) Can MacArthur and Wilson’s classic graphical paradigm of intersecting immigration and extinction curves be adapted to capture the niche-structured regime that occurs at low-immigration rates? (2) Do different ecological models predict a similar biphasic SAR? (3) Can the biphasic island SAR be reconciled with the classic triphasic SAR observed in mainland biogeography? On the first point, we find that the transition can be incorporated into MacArthur and Wilson’s graphical paradigm by forcing the extinction curves sharply downwards at low species richness, reflecting the stabilizing effect of niche processes. On the second point, we confirm that a variety of simple ecological models produce qualitatively similar biphasic SARs. On the third point, we find that a unified model predicts an overall four-phase SAR with the second phase being a shallow niche-structured phase that is rarely observed in mainland SARs, which we hypothesize is because local communities on mainlands are usually in the immigration-structured regime.