There remain those, such as Rosenzweig (1995),
who defend the theory and argue that it ‘holds up
well’. He contends that it is inaccessibility that
defines ‘islandness’ in a biological sense, and that it
follows from this that a biological definition of an
island is preferable to physical definitions such as
being surrounded by water. His definition is: ‘An
island is a self-contained region whose species
originate entirely by immigration from outside the
region’ (p. 211). This is a restrictive definition
that would seem to rule out numerous real islands
that have high proportions of endemic species.
Moreover, the term ‘self-contained region’ is also
important. By it, Rosenzweig means that each
species should be a source species, i.e. it has an
average net rate of reproduction sufficient to main-
tain positive population. The alternative condition
is that species are sink species, which are depend-
ent on influx from outside to maintain their popu-
lations. That is, species which only maintain their
presence through continual supplementary immi-
gration should be discounted, or if they are in abun-
dance presumably the study system should not be
considered insular. This definition, if rigorously
applied, would see a high proportion of study
systems excluded from any test of the EMIB. This
is, in practice, to define islandness by how well
the EMIB applies to an island. We prefer to state the
case the other way around: the equilibrium model
holds only for a limited subset of islands. Can we
generalize as to which?
6.2 Scale and the dynamics of island biotas
The processes invoked in the EMIB as determining
how richness of an island may change from one
point in time to the next are immigration, specia-
tion, and local extinction (Box 4.1), which of course
has to be correct. It is also reasonable to suppose
a general tendency towards equilibrium values,
but that this condition will be approximated to
greater or lesser degrees for particular islands and
archipelagos as other forces and factors come into
play. The degree of fit is argued here to be broadly
related to scale, and in particular the range of scale
in area and isolation of the island system
considered. This line of thinking was developed in
a seminal paper by Haila (1990), who argued that
the key is to clarify what is mean by an ‘island’,
relative to the ecological processes at work (note the
parallels with Rosenzweig’s thinking, above). The
processes themselves form a continuum, with char-
acteristic coupled space–time scales, as shown in
Fig. 1.1. Haila distinguishes (1) the individual scale;
(2) the population scale 1—dynamics; (3) the
population scale 2—differentiation; and (4) the
evolutionary scale. He illustrated this framework
with examples taken from northern European arch-
ipelagos, although his data provide only indirect
evidence of some of the processes at work.
●On the individual scale, a patch of land is an
island if some crucial phase of the life cycle of indi-
vidual organisms obligatorily takes places within
its boundaries. Consider those birds which breed
on an island but overwinter elsewhere, such that
their life cycle is influenced by factors external to
the island. Birds of prey, on the other hand, may
include several small islands of an archipelago
within their territory. In such circumstances indi-
viduals may be viewed as an integral part of the
regional population, such that the insularity of the
environment on this individualscale has few popu-
lation or community-level consequences, other
than through being inferior or superior places for
reproduction.
●When islands are larger and more isolated, they
may support populations that are dynamically
independent of those on other land areas. At this
point the relevance of the EMIB model may be
apparent. To be an island on the population
dynamic scale, two criteria are identified: first, the
whole life cycle of the organisms concerned must
be confined within the island and, secondly, the
island population must be able to demonstrate
independence from the mainland dynamics for
several generations. For instance, there is no
evidence that bird populations on single islands
from the Åland archipelago are dynamically
independent. Although thse islands are about
40 km from the Swedish mainland and 70 km from
mainland Finland, and up to some tens of square
kilometres in area, island population dynamicsSCALE AND THE DYNAMICS OF ISLAND BIOTAS 147