Biology Reference
In-Depth Information
Barbados grackles showed strong similarity to those of Trinidad, a more
distant but still reasonable source area.
Thus, Barbados has received its fauna at various times and from differ-
ent sources. It is an incomplete fauna, lacking at least ten species present
on St.Vincent and St. Lucia for which habitat seemingly suitable is pres-
ent. No resident hawk is present on Barbados, for example.Thus, although
the species present on Barbados have differentiated, presumably in an
adaptive manner, the set of species present is far from a fully integrated
tropical land bird fauna.
On an even longer time scale, the insular nature of the Lesser Antilles
has resulted in a land bird fauna that is still not in equilibrium in an evo-
lutionary sense. Based on mtDNA fingerprinting of 37 of the 65 land
birds of the Lesser Antilles and their nearest relatives in South America or
the Greater Antilles, this fauna began to invade only about 7.5-10.0 mil-
lion yr ago (Ricklefs and Bermingham 2001). Modeling of the rates of
colonization and extinction that best account for the pattern of genetic
divergences of these 37 species suggested that over this period of faunal
accumulation no significant rate of general extinction has prevailed.
Rather, the pattern suggested that at about 550,000-750,000 yr ago,
either a mass extinction of more than 90% of the extant lineages occurred
or the rate of colonization increased about 13-fold to a new level that has
continued until recent time.The timing of the extinction event or colo-
nization change corresponds roughly to the beginning of Pleistocene
glaciation, when drier climates may have modified habitat conditions and
lowered sea levels increased land areas and connected some islands that are
now separated.
This general analysis suggested that for insular areas such as the Lesser
Antilles, truly equilibrium conditions for an overall bird fauna are unlikely
ever to develop. On the other hand, it confirmed that older taxa have
much more restricted island and habitat distributions, indicative of sub-
stantial evolutionary adjustment to island environments (Ricklefs and
Bermingham 1999).These adjustments are evidently influenced not only
by general habitat conditions but also by interactions with other species
of birds, including later arrivals, and also with biotic agents such as para-
sites (Apanius et al. 2000). Thus, on a time scale of several million years,
integration of invading species is evident, even though the overall biota is
still not at equilibrium.
That the bird faunas of older West Indian islands are well integrated,
as suggested by the studies of Tasmanian birds described earlier, is shown
by comparative ecological and morphological analyses (Cox and Ricklefs
