The island immaturity - speciation pulse model of island evolution: an alternative to the ‘‘diversity begets diversity’’ model
Fecha
2007Resumen
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).