Biology Reference
In-Depth Information
As expected in such a setting, there is no evidence of desert, shrubland/
chaparral-woodland-savanna, or grassland communities.
By the late Miocene, represented by the Gatún Formation, the land
fragments were more consolidated, and they had been elevated somewhat
by subduction, compression forces generated by microshifts between the
North and South American continents, lowering of sea levels, and the ac-
cumulation of lava and ash generated as the Caribbean plate slid slowly to
the east. Two subtle changes in the vegetation resulted from the appearance
of this slightly higher land. One was the initial presence of a few northern
deciduous elements, such as the ecologically broad
Alnus
and
Quercus
. The
great distance involved in migrating from the north was also a likely factor
in determining their relatively late arrival. The other change was the fi rst in-
dication of a slightly drier Pacifi c (southern) side, compared to a wetter At-
lantic (northern) side of Panama. This is recorded in the plant fossil record
by an increase in grass pollen from rare or absent to 7.5 percent. Among the
causes was defl ection of the trade winds by the early Cordillera Central. By
the end of the Pliocene, all twelve ecosystems had long been established
somewhere in the New World, and those absent from southern Central
America are explained by its near-equatorial position, low elevations, and
the insular confi guration of the landscape. In summary, northern Latin
America at the end of the Pliocene included at least elements of ten ecosys-
tems: desert, shrubland/chaparral-woodland-savanna, grassland, mangrove,
beach/strand/dune, aquatic, lowland neotropical rain forest, lower to upper
montane broad-leaved forest, coniferous forest, and páramo.
Like their counterparts to the north, deserts began forming in the late
Miocene and Pliocene from older preadapted elements, and further co-
alesced in the warm interglacials. The collective evidence suggests that
geologically, deserts are among the more recent of the modern New World
ecosystems.
The representatives of shrubland/chaparral-woodland-savanna grow un-
der less severe conditions of cold/heat and dryness than do desert taxa, and
therefore they appeared earlier. Elements of this vegetation were already
present by the Eocene to the north, and probably also in the Sierra Madre
region, although fossils fl oras and associated faunas documenting their
presence are few. During the Oligocene, they were coalescing in basins and
leeward habitats, and they began to spread with mid-Miocene drying and
increasing rain shadows, augmented by slope, exposure, and soil. In north-
ern Latin America, this middle to late Miocene dry shrubby community,
or tropical dry forest, was modernized by continued drying in the Pliocene
and Quaternary.
