Reconciliation of niche and neutral theory has been a core goal of theoretical ecology in recent years. We will present a novel conceptual unification of the two theories from an unexpected direction: island biogeography. The key insight comes from an empirical phenomenon known as the "small-island effect", whereby species richness is independent of island area below a threshold island size. The small-island effect was observed by MacArthur and Wilson in their 1967 treatise on island biogeography, as an exception to their general rule that species richness should increase with island area. The combination of the small-island effect among small islands and the classic increasing species–area relationship (SAR) among large islands leads to an overall biphasic SAR, which has now been observed in many empirical data sets, across different taxa and archipelago types.
Results/Conclusions
Our explanation of the biphasic island SAR is that it arises from a transition between a niche-structured phase among small islands and an immigration-structured phase among large islands. In the niche-structured phase, species richness simply reflects the number of fundamental niches because immigration is too weak to provide more species than niches. But as island area increases past a critical area, immigration becomes strong enough to provide on average more than one species per niche at the immigration–extinction balance. From this point on, further increases in island area lead to higher species richness, and the SAR thus moves into the increasing immigration-structured phase. We provide evidence for our theory in the form of (i) a dynamic model that gives excellent fits to 100 archipelago SARs and (ii) confirmation of predictions about how the critical area should vary across taxa and archipelago types. We conclude by discussing the implications for the roles of niche versus dispersal processes in maintaining diversity of mainland communities.