part of the mechanism for the cycle as put forward
by Ricklefs and Cox (1972) was the evolutionary
reaction, or counter-adaptation, of the pre-existing
island biota to the newly arrived species, such that
over time they begin to exploit or compete with the
immigrant more effectively, thus lowering its com-
petitive ability. The subsequent arrival of further
immigrant species will then tend to push the earlier
colonists into progressively fewer habitats and
reduce their population densities. By this reason-
ing, range of habitat use and population density
should each be diminished in species at an
advanced stage of the taxon cycle.
Ricklefs and Cox (1978) examined these proposi-
tions from their earlier paper on the basis of
standardized frequency counts taken in nine major
habitats on three islands: Jamaica, St Lucia, and
St Kitts. Each species was first assigned to one of
their four taxon cycle categories (Table 9.1a). Non-
passerines did not show distinct trends in relation
to stage of cycle, but the data for passerines were
more interesting. The most common stage I species
demonstrated habitat breadths and abundances
rarely attained by mainland species, indicating that
colonization of islands must involve some degree
of ecological release (cf. Cox and Ricklefs 1977),
although the phenomena appeared to be confounded
by variation in the proportion of species in each
stage of the taxon cycle. Passerine species at ‘later’
stages of the cycle tend to have more restricted
habitat distributions and reduced population den-
sities. Early-stage species tend to occupy open, low-
land habitats, whereas late-stage species tend to be
restricted to montane or mature forest habitats.
Similar findings were reported for birds on the
Solomon Islands by Greenslade (1968). Ricklefs and
Cox (1978) found these trends to be clear for Jamaica,
with 35 passerines, but they are difficult to detect
on St Kitts, with 13 species, none of which is endemic.
The Ricklefs and Cox model was subsequently
criticized by Pregill and Olson (1981), who contested
that the four stages are simply criteria that define a
set of distributional patterns and that almost any
species in any archipelago would fall into one of
these categories without necessarily following the
progression from stage I to IV in turn. To take a par-
ticular island, many of the endemics of Jamaica,
especially those endemic at the generic level, are212 EMERGENT MODELS OF ISLAND EVOLUTION
Table 9.1The taxon cycle as applied to the avifauna of the Caribbean by Ricklefs and Cox (1972, 1978) (from Ricklefs
1989)
(a)Characteristics of distribution of birds in the four stages of the taxon cycle
Stage of cycle Distribution among islands Differentiation between island populations
I Expanding or widespread Island populations similar to each other
II Widespread over many neighbouring Widespread differentiation of populations on
islands different islands
III Range fragmented due to extinction Widespread differentiation
IV Endemic to one island N/A
(b)Number of species of passerine bird on each of three islands
Jamaica St Lucia St KittsStage I 5 8 6
Stage II 10 7 6
Stage III 8 9 2
Stage IV 12 2 0
Total 35 26 14
Area (km^2 ) 11 526 603 168
Elevation (m) 2257 950 1315