VARIATION IN INSULAR ENDEMISM BETWEEN TAXA 243out this temporal model, we describe the
observations on which it is based, and then
examine what we expect in terms of critical rates,
illustrating the model in a simple graphic, of
similar format and terms to MacArthur and
Wilson’s (1967) EMIB graphic (Fig. 4.1).
Analyses of species richness variation reviewed
in Chapter 4 provide broad support for the idea
of an environmentally-determined carrying
capacity(K)for richness. Yet, many oceanic islands
fail to attain their hypothetical carrying capacity as
they are too young or have had too little time
since some past disturbance event(s) in which to
rebuild their diversity (Heaney 2000).
Remote islands receive colonists so rarely that
immigration and speciation occur on similar
timescales, and thus increasing proportions of the
biota of remote islands are made up of endemic
species. Within archipelagos such as Hawaii and
the Canaries, young islands should present the
greatest opportunities in terms of the physical
attributes of size and topographic range, and the
biological attribute of ‘vacant’ niche space (see
figure). This implies opportunities for radiation
through sympatric, parapatric, and allopatric
mechanisms (see text for details).
Following attainment of their maximum
altitude, such islands are prone to suffer
catastrophic losses due to mega-landslides,
subsidence, and erosion (Chapter 2). In time, their
eruptive activity slows, and although erosion then
dominates, once their elevational range declines
below about 1000 m these catastrophic losses in
area become less likely (Hürlimannet al. 2004), so
that the attrition of the island slows as the islands
attain old age. Their eventual fate is to slip back
into the sea, or, in tropical seas, to persist as
atolls.
Opportunities for phylogenesis (maximal values
of KR), and especially for adaptive radiation, can
thus be expected to be greatest at a fairly early
stage in the process of biotic accumulation on a
remote island, when there are sufficiently
complete ecological systems to provide adaptive
opportunities, but still plenty of ‘unoccupied’
niches. As opportunity allows, so speciation is
initiated, and the rate of production of SIEs
increases to an early peak. As the island ages,
the complex eruptive and erosive history of the
stage may provide additional opportunities for
non-adaptive radiation by within-island isolation
(vicariance or peripheral isolation) (cf. Carson
et al. 1990).
From this point in the island’s history, we can
consider two scenarios (using the notation in the
figure, and with E being the extinction rate):
1. As R gradually approaches the hypothetical
value of K, Smight be expected to decline even
as R continues to increase.
2 If the notion of a fixedK is considered an illu-
sory concept (because increasing diversity allows
more diversity to evolve, as MacArthur and Wilson
1967, Emerson and Kolm 2005a), then S must still
eventuallyfall with the decline and submergence
of the island itself. At this stage however fast
evolution may be working, K is diminishing (e.g.
see Stuessy et al. 1998), and so E(IS) and the
inevitable slide back into the ocean will necessarily
draw R (and with it S) down to zero (see figure).
Note that in practice, the hypothetical carrying
capacity of a volcanic island over the course of its
existence can thus be predicted to describe
something like a parabolic function, but skewed
to present a peak capacity early on, followed by a
far more extended period of gradual decline. This
smooth curve will be interrupted by the
occasional catastrophe (eruptions, mega-
landslides; Chapter 2).
Within a single oceanic archipelago such as the
Canaries or Hawaii, it is therefore to be expected
that the islands with the greatest K and R values
will tend to be those of greatest size, elevation
and topographic complexity, and will thus also be
relatively young. Hence, within the Canaries, as
shown in this chapter, Tenerife, much of which is
of2 Ma and the oldest parts of which are
8 Ma, has greatest diversity. During ‘late youth’,
islands have been in existence long enough to
have sequentially built their substantial size, long
enough to have accrued colonists, and long
enough for speciation events to have occurred in
a wide range of colonists. Hence, high
proportions of SIEs are to be expected within such
an island.
Hence, we expect to find that the relative
contribution of IandSto the richness of a remote
island will vary through time as shown in the