that larger, winter-active species were good
colonizers, whereas the smaller species and hiber-
nators seldom travelled across the ice, and if they
did so, then only for limited distances. In related
studies of incidences of species across 8 mainland
and 19 insular sites, it was found that species that
were known to utilize the ice cover had signifi-
cantly higher insular rankings. Furthermore, within
a particular functional guild, such as the insecti-
vores, the smaller species were found to have lower
insular rankings.
Although these data are strongly suggestive of
immigration effects, species interactions may also
be operative. It is a logical corollary of the EMIB
that in addition to species numbers being a result
of interactive effects of immigration and extinction,
so also should species composition. Lomolino
(1986) seeks to draw a distinction between com-
bined (additive) effects of factors influencing
immigration and extinction, and interactive or
compensatory effects. The latter appears to be, in
essence, merely another name for the rescue effect
of Brown and Kodric-Brown (1977), whereby sup-
plementary immigration of species present but at
low numbers on an island, enables their survival
through to a subsequent census. Thus, where
immigration rates in the strict sense are high, so
too will be rates of supplementary immigration. As
the distinction between the two forms of popula-
tion movement is effectively abandoned in this
study, it might be termed instead the arrival rate.
Species may thus have a high incidence on islands
either where low arrival rates (poor dispersers
and/or distant islands) are compensated for by
low extinction rates (good survivors and/or large
islands) or where high extinction rates are com-
pensated for by high arrival rates. Thus species are
common on those islands where their rate of
arrival is high relative to their rate of loss. Of itself
this is a trivial observation, but when combined
with other ecological features it provides another
option for understanding species compositional
patterns, and one that stresses the dynamism
inherent in the EMIB.
Figure 5.3 summarizes five hypothetical insular
patterns for the distribution of particular species.
First, a species may be distributed randomly
(panel b) within an archipelago. Secondly, it may
have minimum area requirements and not be
dispersal-limited within the archipelago, in which
case it will be present on virtually all islands over a
critical threshold size (panel c). Thirdly, if it is a
poor disperser which has low resource require-
ments relative to insular carrying capacities, it may
occur only on the least isolated islands (panel d).
Fourthly, it may depend on both island size and iso-
lation, but not exhibit compensatory effects, thus
resulting in the block pattern (panel e). Finally, it
may exhibit the compensatory relationships between
immigration and area and thus show the diagonal
pattern of panel (f) of the diagram. Lomolino (1986)
postulated that compensatory effects should be more
evident for archipelagos with a large range of area
and isolation relative to the resource requirements
and vagility (mobility) of the fauna in question. If it
is further assumed that within an ecologically sim-
ilar group of species, larger species have both
greater resource requirements and greater vagility
than smaller species, an interesting prediction
arises from this compensatory model. On the least
isolated archipelagos, the smaller species will have
a higher incidence on the smaller islands. But, as
isolation of a hypothetical archipelago increases,
the low persistence of the smaller species combined
with their poorer vagility means that larger species
should be the more frequent inhabitors of smaller
classes of islands, i.e. their incidence relationships
‘flip over’ as a function of isolation. In short, if
recurrent arrivals and losses are important in shap-
ing the composition of the islands in question, the
incidence of a species as a function of area will also
vary as a function of immigration.
Lomolino developed a form of multiple dis-
criminant analysis which enabled him to distin-
guish between the block effects and compensatory
effects of Fig. 5.3. He applied this procedure to 10
species of mammals, each occupying at least 2 of
19 islands studied in the Thousand Islands region
of the St Lawrence River, New York State.
Circularity of argument in using distributional
patterns to infer causation could be avoided as
independent lines of evidence on species move-
ments, body size, and other features of their aute-
cology were available.120 COMMUNITY ASSEMBLY AND DYNAMICS