0198566123.pdf

a secondary stage 1 species, and expanding in its
distribution (Fig. 9.2).

Caribbean birds

Ricklefs and Cox (1972) put forward a taxon cycle
model to account for the biogeography of the
avifauna of the Lesser Antilles. Their version dis-
tinguishes an additional stage (Table 9.1, Fig. 9.3),
but the basics are shared with the ant study.
Stage I The first stage is the invasion of an island
or archipelago by a mainland species. In the
Caribbean, this means a species spreading across
many islands. It is thus taken to be a dispersive
form with interchange occurring between main-
land and islands and among islands. The species is
likely to be one of disturbed or coastal habitats, as
such species typically have good dispersive ability.
There is therefore little differentiation initially
between mainland and island forms.
Stage IIThe colonist may then expand its niche,
invading other habitats, and becoming more
generalized in the use of resources. Species at this
stage have more spotty distributions as selection

against mobility reduces gene flow. They gradually

evolve local forms and become restricted to a sub-

set of the islands. Species then become vulnerable

to being out-competed in their original colonizing

niche by further specialized colonists and may

become restricted to interior forest habitats so that

their niche breadth narrows again.

Stage IIIAs they proceed through the stages of

the cycle, species become highly differentiated

endemics that ultimately become extinct and are

replaced by new colonists from the mainland. Stage

III species thus evidence a longer history of evolu-

tion in isolation, being found as scattered endemic

forms.

Stage IVThe fourth stage of the cycle is when a

highly differentiated endemic species persists as a

relict on a single island. From here the next step is

extinction.

One reason for the success of colonists is that a

recent arrival may have left behind predators, par-

asites, and competitors on colonizing an island,

enabling it to flourish despite the existence of local

species with a longer period of evolutionary adjust-

ment to the conditions on that island. An important

THE TAXON CYCLE 211

Inner

rain

forest

Marginal

habitats

South-eastern Asia Melanesian Islands

Time

A

B

C

D

E

F

Stage 1

Stage 2

Stage 3

Figure 9.2The inferred taxon cycle of ant species groups in Melanesia, in which is traced the hypothesized histories of groups derived
originally from Asia. The following sequence was postulated: stage 1—species adapt to marginal habitats (A) in the mainland source region
(SE Asia) and then cross the ocean to colonize these habitats (B) in New Guinea. These colonizing populations may become extinct in time (C).
Alternatively, the cycle enters stage 2—the ants invade the inner rain forests of New Guinea and/or surrounding islands (D). If successful in
re-adapting to inner rain forest habitats they in due course diverge (E) to species level. Stage 3—diversification progresses within Melanesia,
while the group remaining in Asia may, in cases, be contracting its range, such that the lineage becomes centred in Melanesia. A few members of
these lineages, especially those on New Guinea, may re-adapt to the marginal habitats (F), and expand secondarily. (Redrawn from Wilson 1959.)