The island immaturity - speciation pulse model of island evolution: an alternative to the ‘‘diversity begets diversity’’ model
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Islands have long provided model systems in which ecologists and evolutionary biologists have developed, tested and refined models for species diversity (Whittaker and Ferna´ndez-Palacios 2007). In two recent papers, Emerson and Kolm (2005a, b) have presented and discussed multiple regression analyses from two oceanic archipelagos, the Canaries and Hawaii, demonstrating for plants and arthropods that islands of greater species richness also have higher proportions of single island endemics (SIEs). They claim this as evidence that higher species richness of a taxon drives higher rates of diversification in that taxon, i.e. that ‘‘diversity begets diversity’’. Their analysis is interesting, but given that it is an analysis of proportions of SIEs not rate of species production, it is ultimately inconclusive as to mechanisms leading to the relationship. It might tell us, as inferred by Emerson and Kolm (2005a, b), that high species richness creates the conditions for high rates of speciation through: 1) competitive interactions, 2) genetic drift due to small population sizes, and 3) greater community structural complexity. But it could also be that the relationship is a by-product of circumstances not adequately captured in their analyses. Herein, we develop an alternative model, positing that the opportunities for speciation have a broadly predictable relationship to the life cycle of oceanic islands. We term our model the island immaturity speciation pulse (IISP) model of island evolution. Intrinsic to this model is that opportunity drives speciation rate and that opportunity is greatest at a relatively early stage of an island’s life cycle, when intrinsic carrying capacity exceeds species richness by the greatest margin, i.e. when there is greatest ‘‘vacant niche space’’. As islands mature, both richness and endemism increase in tandem, but as islands decline in their old age, opportunities for speciation diminish, in tandem with a reduced carrying capacity (and reduced numbers of SIEs). Our argument is that the mechanisms identified by Emerson and Kolm (2005a, b), whilst each having a role in island evolution, make for an incomplete set of key island mechanisms and that in particular they neglect the likely importance of competitive release early in the life cycle of an island, and of the subsequent decline in carrying capacity, for the proportions of single island endemics (see Peck et al. 1999). In setting out the IISP model, we describe the observations on which it is based, and then examine what we expect in terms of critical rates, and emergent patterns of SIEs, comparing our model with that put forward by Emerson and Kolm (2005a, b). We illustrate our model with reference to data for the arthropods and plants of the Canary Islands (cf. Emerson and Kolm 2005a).