154 SCALE AND ISLAND ECOLOGICAL THEORY: TOWARDS A NEW SYNTHESIS
hypothesis (or EMIB) is of degree, relating to the
causation of turnover and the biotic response times
(Whittaker 1995). In fact, MacArthur and Wilson
(1967) acknowledged the implications of environ-
mental disturbances for particular island systems.
For instance, they recognized the following phe-
nomena: the occurrence of successional turnover;
the influence of storms on immigration rates; and
that much extinction, especially that resulting from
storms, drought, and invasion of new competitors,
is accompanied by a severe reduction in the carry-
ing capacity of the environment. Nonetheless, they
tended to downplay the general significance of dis-
turbance, arguing that only equilibrium models are
likely to lead to new knowledge concerning the
dynamics of immigration and extinction. But, as
others have noted, where turnover is the product
principally of abiotic forcing, turnover patterns
may be poorly predicted by equilibrium models
(Caswell 1978; Heaney 1986; Whittaker 1995;
Morrison 1997; Shepherd and Brantley 2005).
Figure 6.2 is Bush and Whittaker’s (1993) attempt
to generalize the Krakatau island ecological trends
and to set out alternative trajectories, allowing for
taxa of differing ecological roles and response
times. It encapsulates the temporally bumpy rates
associated with successional processes and pro-
vides for two possible scenarios whereby equilib-
rium might be reached. The data for rate changes
indicate that equilibrium might be approached by a
declining immigration rate, combined with two
alternative extinction trends. In the first, extinction
rate is low and turnover rather heterogeneous
(interactive). In the second, it is essentially
squeezed out of the system altogether (non-
interactive). Given the problems of collecting ade-
quate survey data, and the high degree of
pseudoturnover which is a feature of empirical
data from such large and complex systems, the two
conditions may not be readily distinguishable in
practice. Close examination of the Krakatau plant
data demonstrated that a high proportion of
apparent turnover involved species best regarded
as ephemerals, which had not really colonized in
the first place (Whittaker et al. 2000). The third form
of projection recognizes that the islands experience
environmental change, which may be dramatic and
destructive. The longer the period before equilib-
rium is attained, the greater the likelihood that an
event of intermediate or high magnitude will cast
the system away from equilibrium and into the per-
petual or dynamic non-equilibrium condition of
Fig. 6.1. In this projection, the biota chases perpetu-
ally moving environmental ‘goalposts’.
The relevance of the three projections in Fig. 6.2
to particular Krakatau taxa depends on features of
their ecology such as generation times and disper-
sal attributes, and their position within the biological
and successional hierarchy (cf. Schoener 1986).
Within the conceptual space of Fig. 6.1, different taxa
or guilds (or the same taxa in different island con-
texts) may thus occupy different positions: the rep-
tiles arguably towards the static non-equilibrium;
the plants of Anak Krakatau towards the dynamic
non-equilibrium; the coastal flora at the archipelago
level towards the static equilibrial position; and the
birds of Rakata, at least at times, relatively close to
a dynamic equilibrium but with less homogeneous
patterns of turnover than indicated for a strict
EMIB position. The two diagrams are thus comple-
mentary, in showing the conceptual space (Fig. 6.1)
and temporal features characteristic of different
points within it for one exemplar system (Fig. 6.2).
Dynamic non-equilibrium, or multistate models
have become prominent in several subfields of eco-
logy in recent decades (e.g. savannah ecology), but
thus far relatively few non-equilibrium models
appear to have been developed in island ecology
(see e.g. Caswell 1978; Weins 1984; Williamson
1988; Villa et al. 1992; Russell et al. 1995; Whittaker
1995). Incorporating environmental disturbance
into island ecological models and modelling pres-
ents significant but not insurmountable difficulties.
An example is provided by Villa et al. (1992). They
constructed a model island consisting of a habitat
map of cells containing individuals, a list of the
species involved, and their life history characteris-
tics. The model involved colonization of the island,
and varying degrees of perturbing events that
caused the mortality of individuals. The model was
highly simplified, but nonetheless allowed the test-
ing of ideas. One intuitively appealing result was
that the global attainment of equilibrium within the
simulations was strongly dependent on the rate of