0198566123.pdf

Chappell Island in the Bass Strait eat an abundant,
high-quality resource, which requires little effort to
locate, but which is highly seasonal: chicks of mut-
ton birds. Here, larger body size might enable
greater fat storage, thus enhancing survival of indi-
viduals through long periods of fasting.
The size shifts and associated morphological
changes, like other ‘micro-evolutionary’ processes/
patterns reviewed in the present chapter, some-
times lead to island populations being recognized
as subspecies, or full species, but do not necessarily
do so. The island rule is, as Lomolino (2005) writes,
an emergent pattern, and although numerous par-
ticular mechanisms might be involved, varying
from taxon to taxon, the generality of the pattern
(occurring in many taxa, all over the globe) sug-
gests that a relatively powerful underlying selec-
tive force must apply, and that this in turn must
relate to island disharmony. Of the various factors
discussed, those Lomolino holds to be most impor-
tant are summarized in Figure 7.6.

Changes in fecundity and behaviour

Vertebrates colonizing islands also frequently dis-
play changes in behaviour (Sorensen 1977; Stamps
and Buechner 1985). For birds we have already

discussed examples of changes in feeding niche,

loss of flight, loss of defensive instincts on remote

islands, and a general tendency to follow the island

rule. To provide an example of a behavioural

change, Lack observed that birds on the Orkney

isles select a wider range of nesting sites than on the

British mainland (Table 7.2). Lack (1947b) also

noted that island birds frequently have reduced

fecundity, expressed as smaller clutch sizes than

their continental relatives. The same trend has been

reported for island lizards and mammals (Stamps

and Buechner 1985), often combined with the pro-

duction of larger offspring. Further trends that have

been claimed for insular animals include a diminu-

tion of sexual dimorphism, a tendency towards

melanism, and the relaxation of territoriality

(Stamps and Buechner 1985; Wiggins et al. 1998).

Small clutch sizes

Bird clutch sizes are known (1) to increase with

latitude, and (2) to decrease on islands when

compared with continental areas at the same lati-

tude (Lack 1947b; Ricklefs 1980; Blondel 1985).

Furthermore, clutch sizes decrease with decreasing

island area and with increasing isolation within the

same archipelago (Higuchi 1976), and at least for

some related bird species, clutch size also decreases

190 ARRIVAL AND CHANGE

Dwarfism

Gigantism

Relative importance of forces promoting

Body size of populations on the mainland

Immigrant selection

Intensified intraspecific competition

Ecological release from large

competitors and predators

Intensified intraspecific competition

Ecological release from predators

Resource limitation and

specialization for insular niches

Figure 7.6Factors hypothesized to be important in generating the island rule in island vertebrates and how they vary in relation to body size.
(Redrawn from Fig. 9 of Lomolino 2005.)