(1986), in discussing the mammalian fauna of a set
of land-bridge islands, aired the notion that it may
be possible to recognize a continuum, from isolated
land-bridge islands having non-equilibrial faunas,
to those of intermediate isolation with both relict
species and those in equilibrium, to islands so near,
and/or so small, that all species are in equilibrium.
The options sketched out in Fig. 6.1 are also sim-
ilar to the distinctions drawn about 30 years ago by
Caswell (1978) (see also Weins 1984; Schoener 1986;
Case and Cody 1987). The scale of Caswell’s appli-
cation is rather different from those generally
under discussion here, but the parallel is apparent
nonetheless. He set out to develop models of
predator (or disturbance)-mediated coexistence.
He distinguished between open systems (involving
migration between ‘cells’) and closed systems (no
migration), and between equilibrium and non-
equilibrium systems, concluding that it would not
do to suggest that communities as a whole are
entirely equilibrium or entirely nonequilibrium
systems. ‘Perhaps a community consists of a core of
dominant species, which interact strongly enough
among themselves to arrive at equilibrium,
surrounded by a larger set of nonequilibrium
species playing out their roles against the backdrop
of the equilibrium species’ (Caswell 1978,
pp. 149–50).
6.4 Temporal variation in island carrying capacities
The dynamic equilibrium paradigm anticipates
that island species number will in general stabilize
and show little temporal fluctuation. Here we focus
on some of the mechanisms that may lead to fluc-
tuations in richness through time, relating mostly
to the idea that an island’s carrying capacities may
change, but also noting that both island area and
degree of isolation may also change over time
(Table 6.3).
The most obvious cause of change in isolation is
alteration in relative sea level, but effective isolation
may be influenced by more subtle environmental
changes. Many accounts of species turnover rates
have treated ocean waters as a constant barrier to
dispersal. Scott’s (1994) observations of an invasion
of San Clemente island, 80 km from the Californian
mainland, led him to question this assumption. In
the late summer of 1984, 26 black-shouldered kites
(Elanus caeuleus) took up temporary residence on
the island for a period of several months, although
previously only one or two kites had been recorded
during 19 autumn/winter cycles of bird observa-
tion. Scott suggested that the 1984 irruption may
have been a consequence of an unusual pattern of
Catalina eddies—which are seasonal cyclonic
winds—possibly linked to an El Niño event. Each
pulse of kites arriving on the island coincided with
the first or second day of a Catalina eddy. It appears
that the eddy system is like a door to San Clemente
island that irregularly opens and closes. The same
El Niño influenced carrying capacity for Darwin’s
finches in the Galápagos (Chapter 9). From such
simple empirical observations, it might be reason-
able to postulate that fluctuations in the weather
might produce variations in rates of immigration,
extinction, and turnover.The prevalence and implications of intense disturbance events
Abbott and Black (1980), in their study of vascular
plant species on 76 aeolianite limestone islets,
attributed slight changes in the turnover patterns to
an increase in rainfall and to the passage of a
cyclone. In discussing other small island studies,
they note that ‘...as most cays occur in tropical
regions, they are subject to frequent cyclonic or
hurricane disturbances which often cause waves
temporarily to inundate cays and wash vegetation
away’ (p. 405) (see also Sauer 1969). Similar find-
ings were reported by Buckley (1981, 1982) in his
analyses of plants on sand cays and shingle islands
on the northern Great Barrier Reef.
Periodic intense disturbance is a natural feature
of many islands across the globe (Whittaker 1995),
and has increasingly been afforced by human
influences. In illustration, hurricanes are frequent,
devastating events throughout the region 10–20
north and south of the equator (which covers a lot
of islands) (e.g. Nunn 1994). For the Caribbean,156 SCALE AND ISLAND ECOLOGICAL THEORY: TOWARDS A NEW SYNTHESIS