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because immigration is very slow to distant
archipelagos, and is little influenced by island size
in such remote contexts, thus producing relatively
flat intraarchipelago ISARS. Land-bridge islands, in
contrast, may have steeper species–area curves
because of the predominance of extinction, as over
long time-periods, members of the assemblage
stranded by island formation suffer occasional
attrition, supplemented by little immigration
(Fig. 6.3b, Millien-Parra and Jaeger 1999).
In such cases, the fauna at the point of isolation
has been termed supersaturated, i.e. it contains
more species than at true equilibrium; thereafter, the
dominance of extinction over immigration results
(in theory) in relaxationof species number, declin-
ing towards eventual, hypothetical equilibrium.
Classic illustrations of this effect have also come
from non-volant mammals occupying montane
habitats in the Great Basin of western North
America. These studies have been selected for their
illustrative value, and by no means all non-equilibr-
ial systems are products of Quaternary climate
change. The distinction between true oceanic
islands and land-bridge islands is, as Lawlor (1986)
has shown, a crucial one for understanding
species–area relations of non-volant mammals

of remote islands, and analyses which fail to

distinguish the two groups are thus likely to be

misleading.

Implications for endemics?

If environmental fluctuations can be important to

some small to moderate-sized near-continental

islands, why do we not hear more of the adverse

effects of natural disturbance in remote oceanic

islands? Might they not be anticipated to be worst

affected, being unable to replenish species stocks

swiftly after such events as cyclones? Perhaps the

effects of variability are largely ‘built in’ (evolution-

arily) to their ecologies?

Miskelly (1990) reports widespread reproductive

failure and delayed breeding by Snares Island snipe

(Coenocorypha aucklandica) and black tit (Petroica

macrocephala dannefaerdi), two endemic land birds,

on the Snares Islands, in the New Zealand sub-

Antarctic. Snipe also suffered unusually high adult

mortality. He attributed these phenomena to the

pronounced El Niño–Southern Oscillation (ENSO)

event of 1982–83, which produced heavy rainfall

and significantly lowered temperatures on the

Snares Islands, thereby reducing the invertebrate

TEMPORAL VARIATION IN ISLAND CARRYING CAPACITIES 159

Number of species

Area

Number of species

Area

(a) (b)

Landbridge islands Landbridge islands

Mainland

Oceanic islands

Oceanic islands

Mainland

Figure 6.3Equilibrium and non-equilibrium representations of island species–area relationships for a hypothetical archipelago, as summarized
by Lawlor (1986). (a) By an equilibrium argument, the effects of recurrent colonization and extinction produce steepening of the species–area
curves with increasing distance, as extinction (arrows) has greater relative impact on remoter islands, for which immigration is increasingly
difficult, (b) In the non-equilibrium model, oceanic islands may be undersaturated, as immigration (open arrows) is too slow to ‘fill’ the islands
(extinction is relatively unimportant), whereas land-bridge islands undergo ‘relaxation’, i.e. their richness patterns are extinction dominated (solid
arrows).