104 SPECIES NUMBERS GAMES: THE MACROECOLOGY OF ISLAND BIOTAS
expectation that, on the whole, the rarest species on
a particular island are most likely to become extinct.
This view, although basically stochastic, is at least
part way along the axis towards a model of struc-
tured turnover. Now, if, in general, turnover at equi-
librium involves a subset of fugitive populations,
while another, larger group of species, mostly of
larger populations, is scarcely or not at all involved
in turnover, then the turnover is highly heteroge-
neous. Both Williamson (1981, 1983, 1989a,b) and
Schoener and Spiller (1987) have concluded that
most empirical data point to turnover mostly being
of such fugitive, or ephemeral species. Whether this
is consistent with the EMIB or not is an arguable
point: at best, if this really is an accurate description
of turnover at equilibrium, then it suggests that the
theory is ‘true but trivial’ (Williamson 1989a).
How is equilibrium approached? Can you
assume that the more species an island has, the
smaller the populations? Do the curves for immi-
gration rate and extinction rate follow the trends
described by MacArthur and Wilson of smooth
decline and increase, respectively? Actually, as
they put it themselves (1967, p. 22): ‘these refine-
ments in shape of the two curves are not essential
to the basic theory. So long as the curves are
monotonic, and regardless of their precise shape,
several new inferences of general significance con-
cerning equilibrial biotas can be drawn’. This
acknowledges that it is the monotonicity of the rate
changes which is critical. It is possible to test
whether, indeed, immigration and extinction
trends are monotonic. The answer in at least
two cases appears to be ‘no’. Rey’s study illustrates
this in a simple system. Observations from the
Krakatau islands illustrate much the same point in
a complex system.
The Krakatau islands were effectively sterilized
in volcanic eruptions in 1883. Recolonization of the
three islands in the group commenced shortly after.
It started in a limited number of places and gradu-
ally claimed more of the island area. Niche space
and carrying capacity increased as the system
developed. Successional processes kicked in, and
habitat space waxed and waned as grasslands
developed and then diminished—overwhelmed by
a covering of forest. Bush and Whittaker (1991)
demonstrated that, if the lists of species present are
taken at face value, the trends in immigration and
extinction from the lists (not the real, unmeasurable
IandEon the islands) behave non-monotonically
for birds, butterflies, and plants. Bush and
Whittaker’s (1991) bird data were later shown to be
incomplete for the latest survey periods, but even
after correction (Thornton et al. 1993), the non-
monotonicity of trends remained (Bush and
Whittaker 1993). It therefore appears that for
complex islands, there is more ecological structure
to the assemblage of an island ecosystem than is
allowed for in the EMIB (see Chapter 5).What causes extinctions?
What causes extinctions, and thus turnover, is one
of the crunch issues connected with the equilib-
rium theory. MacArthur and Wilson (1967) were
not explicit. There is no doubt that they regarded
it as, in general, a function of population size, but
to what extent does their theory assume it to be
random (and homogeneous), as opposed to a
deterministic outcome of species–species interac-
tions, particularly competition? The ‘narrow’
answer is that the EMIB is essentially a stochastic
formulation (above), but the ‘broad view’ answer
is that their text acknowledges the importance of
competition. For instance, the data for the recolo-
nization of Krakatau by plants (up to 1932) failed
to fit their expectations. One of the two explana-
tions they offered invoked a successional pattern
in which ‘Later plant communities are dependent
on earlier, pioneer communities for their success-
ful establishment. Yet when they do become estab-
lished, they do not wholly extirpate the pioneer
communities.. .’ (MacArthur and Wilson 1967, p.
50). This invocation of competitive effects is a
community-wide, rather than one-on-one, form of
competition, and is sometimes given the label dif-
fuse competition. Wilson (1995, p. 265), writing
retrospectively about his field reconnaissance
before the mangrove islets experiments, recollects
‘For ants the pattern was consistent with competi-
tive exclusion. Below a certain island size, the col-
onization of some species appeared to preclude
the establishment of others... ‘.