0198566123.pdf

Samoa. Wilson bundled the species into three
stages. Stage 1 species are those that dominate in the
marginal habitats: open lowland forest, grassland,
and littoral habitats (Fig. 9.2). They have greater
ecological amplitudes than the other species, as
they also occur in other habitats. They tend to be
trail-making ants, nesting in the soil. Stage 1 species
typically show a continuous distribution with no
tendency to break into locally distinct races. Stage 2
of the process sees ants ‘returning’ to the dominant
vegetation types, of interior and montane forest,
within which they are more likely to be found
nesting in logs or similar habitats. If they succeed in
adapting to the inner rain forests they eventually
differentiate to species level within Melanesia,
forming superspecies or species groups. As they
differentiate, they are liable to exhibit reduced gene
flow across their populations. Stage 3 species occupy
similar habitats to those of stage 2, but evolution has
now proceeded to the point where the species group
is centred on Melanesia and lacks close relatives in
Asia. This may be, in part, a function of change in
Melanesia, and in part of the contraction in the
group remaining in Asia. Progress within the cycle
is thus marked by a general pattern of restriction of

species to narrower ranges of environments within

the island interiors. Meanwhile, these lineages have

been replaced by a new wave of colonists occupy-

ing the beach and disturbed habitats.

Pivotal to this inferred evolutionary system is

that the process is driven by the continuing (albeit

infrequent) arrival of new colonist species. Their

colonization initiates interactions that push earlier

colonists from the open habitats. This is because the

earlier colonists are likely to have become more

generalized in the time since they themselves colo-

nized, as they spread into additional habitats, thus

losing competitive ability in their original habitats

(Brown and Gibson 1983). This model is persuasive

in that it provides answers to some seemingly

puzzling phenomena. For example, it helps explain

why species inhabiting interior forests of the

oceanic islands are more closely related to species

of disturbed habitats on New Guinea, than to those

species that have similar niches and which occur in

mature forest on New Guinea. Although largely a

unidirectional model, it was suggested that some

lineages did find a way out of their alleyway,

perhaps temporarily. Occasionally, a stage 3 species

may readapt to the marginal habitats, becoming

210 EMERGENT MODELS OF ISLAND EVOLUTION

Per cent

Increasing faunal size

Littoral Savanna Monsoon

forest

Lowland

rain forest

(marginal)

Lowland

rain forest

(interior)

Montane

rain forest

100

0

50

Figure 9.1Proportions of ponerine ant species in different habitats, as a function of their geographical distribution. The marginal habitats, to
the left, contain both smaller absolute numbers of species and higher percentages of widespread species. Dark columns, species widespread in
Melanesia; stippled columns, species restricted to single archipelagos in Melanesia but belonging to groups centred in Asia or Australia; blank
columns, species restricted to single archipelagos and belonging to Melanesia-centred species groups. (Redrawn from Wilson 1959.)