188 ARRIVAL AND CHANGE
Body size of mainland population (g)Evolutionary shift from
flying passerine to bau
plan and ecological
strategies of
a large, ground-dwelling
herbivoreab0.000.50Si1.001.502.001 10 100 1 000 10 000 1 00 000 1 000 000Passerine birdsOp OuUngulates (Artiodacty
la)Figure 7.5Lomolino’s (2005, Fig. 6) graphic of the island rule shows the rule for two different bauplans and a possible transition between
them. In essence, small forms tend to get larger (arrow at point a) and larger forms get smaller (arrow at point b) but following different lines,
matching separate optima, dependent on the fundamental body plan and ecological role. Most size changes are gradual but occasionally major
evolutionary transformations occur, involving shifts in bauplan and ecological role in response to the distinctive island opportunities and selective
forces, e.g. the flight loss and gigantism of the now extinct moa of New Zealand. An evolutionary transition from a passerine bird bauplan to that
typical of a large ungulate (large non-volant herbivore) would involve a transition from the left-hand line, with optimum Op(passerine) to the
right-hand line, with optimum Ou(ungulate).
within Lomolino’s model as a result of a shift to
another bauplan, i.e. to another niche (implying
both size and function change). As an example,
New Zealand moas were giant flightless birds that,
in the absence of terrestrial mammals, essentially
acquired the bauplan of ungulates (Fig. 7.5;
Lomolino 2005).
It is also possible that there might be a form of
non-random founding effect (termed immigrant
selection by Brown and Lomolino 1998) involved in
some size shifts. For instance, in active dispersal
events, the founder group might comprise signifi-
cantly larger individuals from within the mainland
population, better able to survive a long-distance
journey to an island. Or, conversely, significantly
smaller plant propagules may be better predis-
posed to arrive on a new island if a passive disper-
sal event has to occur (e.g. Cody and Overton 1996,
above). However, it is hard to see how immigrant
selection would assist in generating the size reduc-
tions in larger mammals. Support instead for the
overwhelming role of selection comes from a recent
study of anthropogenically fragmented forest in
Denmark, where body size changes consistent with
the island rule were reported within just 175 years
(Schmidt and Jensen 2003, cited in Lomolino 2005).
Fossil and subfossil remains provide further
exemplification of the island rule. For instance, the
Mediterranean islands once featured an endemic
fauna of dwarf hippopotami, elephants and deer,
and giant rodents (the so called HEDR fauna),
which survived from the Pliocene to the sub-Recent
period (Reyment 1983). Schüle (1993) argued that
for animals like elephants, which start large, islands
provided insufficient resources to sustain such
large body weights. Even infrequent scarcity could