68 THE BIOGEOGRAPHY OF ISLAND LIFE
Unfortunately, with the exception of a few well-
studied islands, few groups of insects are suffi-
ciently well known for broad biogeographic
patterns to be established. The Lepidoptera provide
an exception. The tropical Pacific butterfly fauna
consists of 285 species, of which 157 (55%) occur on
continental land masses, i.e. they have not speci-
ated after arrival on the islands. One hundred are
endemic to a single island/archipelago and 28 are
regional endemics, i.e. they are found on more than
one archipelago but not on the mainlands (Adler
and Dudley 1994). Interestingly, butterfly specia-
tion in the Pacific archipelagos is primarily a result
of limited interarchipelago speciation. Intra-archi-
pelago speciation appears important only in the
Bismarcks and Solomons, which contain, respect-
ively, 36 and 35 species endemic at the archipelago
level. Of the other 24 archipelagos in the survey,
only New Caledonia makes double figures, with 11
endemic species, and the others contribute just 18
species between them. The Bismarcks and
Solomons are the two largest archipelagos in land
area and the closest to continental source areas.
Butterflies are generally specialized herbivores,
their larvae feeding only on a narrow range of
plants, or even a single species. On the smaller and
more remote archipelagos, the related host plants
simply may not be available for the evolution of
new plant associations, thereby impeding the
formation of new butterfly species (Adler and
Dudley 1994).
Lizards
In their survey of the 27 oceanic archipelagos and
isolated islands in the tropical Pacific Ocean for
which detailed herpetological data exist, Adler et al.
(1995) record 100 species of skinks in 23 genera, of
which 66 species are endemic to a single
island/archipelago and a further 13 are regional
island endemics. Of the 23 genera, 9 are endemic to
the islands of the tropical Pacific. A few species are
widespread; for example, Emoia cyanuraoccurs on
24 of 27 archipelagos in the survey. The Bismarcks,
Solomons, and New Caledonia appear particularly
rich in skinks, but most archipelagos contain fewer
than 10 species. Hawaii has only 3 species, none of
which is endemic, in stark contrast to the birds,
insects, and plants. Some Atlantic competition in
the biodiversity statistics game is provided again
by the Canaries, which feature the endemic Gallotia
lizards, an interesting monophyletic group
composed of 7 extant species of varying sizes, plus
at least one more extinct species (Izquierdo et al.
2004). Another important group of island lizards
are the Caribbean anolines, which are typically
small, arboreal insectivores. This is one of the
largest and better-studied vertebrate genera: about
300 species of anoline lizards have been described,
half of which occur on Caribbean islands (Losos
1994).Birds
The examples of adaptive radiation best known to
every student of biology from high-school days are
probably the Galápagos finches and the Hawaiian
honeycreepers. Therefore it will come as no sur-
prise that islands are important for bird biodiver-
sity. In excess of 1750 species are confined to
islands, representing about 17% of the world’s
species; of these 402, or 23%, are classified as threat-
ened, considerably in excess of the 11% of bird
species worldwide (Johnson and Stattersfield 1990;
Groombridge 1992, p. 245).
Adler (1994) has examined the pattern of bird
species diversity and endemism for 14 tropical
Indian Ocean archipelagos. Probably the most
famous island endemics of all, the dodos of
Mauritius, Rodrigues, and (possibly) Réunion,
although extinct, are included as members of this
data set, which incorporates both extant species
and extinct species known only from subfossils (i.e.
recently extinct species). Using stepwise linear
regression it was found that numbers of each of the
following were related positively to the number of
large islands and total land area: total species num-
ber; number of continental species; and number of
regional endemics. In addition, less remote and
low-lying islands tended to have more continental
species (of the low islands, only Aldabra had its
own endemics), and higher islands tended to have
more local endemics and proportionately fewer
continental species.
Similar results were obtained for the birds of the
tropical Pacific, but the Indian Ocean avifauna