commonly occur in parallel with those of the main-
land. Only for a few sedentary bird species on the
largest and most isolated Baltic island is there evi-
dence of dynamic independence.
●Should the criteria for independence expressed
above be fulfilled for long periods, the island popu-
lations become increasingly independent as genetic
units, and this allows for the second scale of popu-
lation processes, differentiation. Exemplification of
this comes from several studies, two cases being the
various subspecies of wrens (Troglodytes troglodytes)
on different North Atlantic archipelagos (Williamson
1981), and the studies of Berry (e.g. 1986) on the
founder effect in small mammals on offshore
islands around Britain. The relevance of the equi-
librium theory would seem to be fairly slight in
such circumstances.
●The natural progression in this scheme is to the
evolutionary scale, by which Haila (1990) means
processes leading to the divergence of species, i.e.
taxonomic differentiation to a greater degree than
indicated by the term ‘subspecies’ or ‘variety’ or by
slight niche shifts. Empirical support for the propo-
sition that remote islands typically reach and main-
tain the condition of dynamic equilibrium is patchy
at best (Chapter 4).
Of these four scales, the EMIB is restricted mainly
to the population dynamicsscale, in that the other
scales do not appear to satisfy the criteria necessary
for its operation. The scales of Haila’s framework
grade one to another, such that particular studies
may be at the interface, and the applicability of the
theory uncertain. Even within a single taxon, such as
birds, or bats on the Krakatau islands, some species
populations may effectively be bound by the confines
of their island, while others may have wider territo-
ries inclusive of the islands under study. It is not,
then, that the theory holds for all islands within a
certain space–time configuration, but that the effects
of the processes it represents may be prominent
within this conceptual territory, whereas elsewhere
(in the remaining subject space) they are subsumed
by other dominant processes. It also follows from
these arguments that it should be considerably easier
to develop models based on individual species
(e.g. Lomolino 2000a,b) than whole-system models.
Haila’s (1990) scale framework is predominantly
a biological one. He makes reference to environ-
mental dynamics, but not prominently. It will,
however, be apparent that particular forms of
environmental change and disturbance will also
have characteristic scale patterns, which determine
their relevance to interpretation of the differing
scales of island ecological process. In short, it is not
sufficient to assume ecological systems to be in
equilibrium, this must be demonstrated (Weins
1984), and in systems for which abiotic forcing
dominates, more complex, non-equilibrium models
may be required.Residency and hierarchical interdependency:
further illustrations from Krakatau
One of the features imposed by the equilibrium
paradigm has been the restriction of focus to
species that are resident on islands. Although this
has not always been adhered to, it does result, for
example, in seabirds and (often) migrant birds
being excluded from consideration in island eco-
logical studies. Such species may be of huge signif-
icance to the ecology of an island and may also act
to introduce other fauna and flora.
An illustration of the problems concerning
notions of residency comes from the fruit bats of
Krakatau. The largest flying fox species is Pteropus
vampyrus. A colony of several hundred individuals
was observed intermittently on the islands during
the 1980s and 1990s. Pteropushave been observed
to gather in large roosts on islands but fly to
mainland sites in order to feed (and vice versa), and
their nightly range can be as much as 70 km
(Dammerman 1948; Tidemann et al. 1990). Within
mainland areas Pteropusis also known to be highly
mobile, exhibiting movements that may represent
non-seasonal nomadism in response to variation in
food availability, rather than regular seasonal
migration. Krakatau may thus provide just one
roost site of a number used by the same colony
within a larger geographic area. Their intermittent
use of it may reflect variation in food supply in
the region, human interference at other roost
sites, and avoidance of periodic ash falls on
Krakatau. These animals and other, smaller fruit148 SCALE AND ISLAND ECOLOGICAL THEORY: TOWARDS A NEW SYNTHESIS