0198566123.pdf

6.3 Forms of equilibria and non-equilibria

We are not dealing with a well-stabilized situation of long
standing. This may account for some of our difficulties in
fitting existing situations into a theory which concerns
itself only with final equilibrium.

(Preston 1962, p. 429)
We have argued that there is an important dis-
tinction between equilibrium and non-equilibrium
conditions. There is an equally important distinc-
tion between dynamic and static models of island
ecology. These notions provide for a variety of com-
binations, as shown in Fig. 6.1, in which we high-
light four extremes: dynamic equilibrium, static
equilibrium, dynamic non-equilibrium, and static
non-equilibrium. Some example systems that
appear to match the different conditions are pro-
vided in Table 6.2.
Thedynamic equilibrium hypothesiscorre-
sponds to the EMIB, the properties of which have
already been considered at length. The core EMIB
model is essentially stochastic (i.e. occupying the
extreme bottom left corner of Fig. 6.1), but
MacArthur and Wilson (1967) recognized in their
book that some structuring of turnover commonly
occurred, hence their fuller theory could be
assigned a position a little to the right within the

figure. By either view, their ideas are strongly asso-

ciated with the dynamic equilibrium corner.

Figure 6.1 recognizes a continuum from purely sto-

chastic (homogeneous) turnover, through increas-

ingly heterogeneous turnover to the ideas of

habitat determinism with minimal turnover. This

end of the axis describes the static equilibrium

hypothesis, which may be characterized crudely

as where the key controls of species presence/

absence appear to be habitat controls (as Martin

et al. 1995) and where species turnover measured

on a timescale of generations is insignificant. This

is not to deny compositional change over time, but

that in the absence of human interference the

turnover is unmeasurable, giving the appearance

of stasis.

It is possible to reconcile ideas of habitat deter-

minism with the occurrence of systematic variation

in species richness related to area and isolation,

providing that habitat structure itself is signifi-

cantly influenced by the area and isolation of an

island (Martin et al. 1995). Hence, once again, the

pattern of turnover is central to distinguishing

between alternative hypotheses. The emphasis on

habitat controls has often been associated with

David Lack, who based his analyses on studies of

island birds (e.g. Lack 1969, 1976). He argued that

the failure of birds to establish on islands comes

150 SCALE AND ISLAND ECOLOGICAL THEORY: TOWARDS A NEW SYNTHESIS

Environmental

dynamics faster

than biotic

Biotic dynamics faster

than environmental,

which are low amplitude

Biotic dynamics

resistant to immigration

and to environmental

dynamics

Where does

your island fit?

Biotic dynamics lag

greatly behind

environmental

dynamics, but are

not wholly resistant

Dynamic Static

Equilibrial

Non-

equilibrial

Figure 6.1A representation of the conceptual extremes

of island species turnover. The dynamic equilibrium

condition corresponds to MacArthur and Wilson’s (1963,

1967) theory; the static equilibrium equates to Lack’s (e.g.

1969) ideas on island turnover of birds; the static non-

equilibrium to Brown’s (1971) work on non-equilibrium

mountain tops; and the dynamic non-equilibrium to Bush

and Whittaker’s (1991, 1993) interpretations of Krakatau

plant and butterfly data. Considering a single taxon,

different positions in this diagram may correspond to

different islands or archipelagos, and different taxa in the

same island group may also correspond to different

positions.