Maintenance of biodiversity on islands.
MacArthur and Wilson's theory of island biogeography predicts that island species richness should increase with island area. This prediction generally holds among large islands, but among small islands species richness often varies independently of island area, producing the so-called 'small-island effect' and an overall biphasic species-area relationship (SAR). Here, we develop a unified theory that explains the biphasic island SAR. Our theory's key postulate is that as island area increases, the total number of immigrants increases faster than niche diversity. A parsimonious mechanistic model approximating these processes reproduces a biphasic SAR and provides excellent fits to 100 archipelago datasets. In the light of our theory, the biphasic island SAR can be interpreted as arising from a transition from a niche-structured regime on small islands to a colonization-extinction balance regime on large islands. The first regime is characteristic of classic deterministic niche theories; the second regime is characteristic of stochastic theories including the theory of island biogeography and neutral theory. The data furthermore confirm our theory's key prediction that the transition between the two SAR regimes should occur at smaller areas, where immigration is stronger (i.e. for taxa that are better dispersers and for archipelagos that are less isolated).
Department of Biological Sciences, National University of Singapore, Singapore 117543, Republic of Singapore Smithsonian Tropical Research Institute, PO Box 0843-03092, Balboa, Ancón, Republic of Panama ryan.chis@gmail.com.
Ryan A
RA
Chisholm
Department of Biological Sciences, National University of Singapore, Singapore 117543, Republic of Singapore.
Tak
T
Fung
Department of Biological Sciences, National University of Singapore, Singapore 117543, Republic of Singapore.
Deepthi
D
Chimalakonda
Department of Plant Biology, University of Illinois Urbana, IL 61801, USA.
James P
JP
O'Dwyer
D000818:Animals; D044822:Biodiversity; D017753:Ecosystem; D053476:Extinction, Biological; D049810:Genetic Speciation; D005843:Geography; D062312:Islands; D008954:Models, Biological; D013269:Stochastic Processes
biodiversity; colonization–extinction balance; island biogeography; niche structure; small-island effect