Biology Reference
In-Depth Information
vidual species and restricted communities within a more prominent vegeta-
tion type. The latter three are probably the most common; that is, “it may
often be easier for lineages to move than it is for them to evolve” (Donoghue
2008, 11549). Moreover, there is “the tendency for species to retain their
ancient ecology . . . over tens of millions of years and across continents”
(Crisp et al. 2009, 754). For example, gallery forests and their associated
fauna provide mesic assemblages within desert environments. Such mo-
saics of vegetation mean that when conditions change, in whatever direc-
tion, some groups of species will likely be favored and thus be available to
move and coalesce into plant formations suitable to the new environment.
After each reshuffl ing, the exact composition of the associations may not
be the same, but the formation will likely be recognizable except after the
most drastic and enduring of environmental changes. In the region of the
Atacama Desert, slope, exposure, and coarse soils supported dry elements
within the mesic environment that preceded desert conditions.
The fi rst event to favor desert formation along the coast is indicated
by a sharp drop in global sea level around 30 Ma. This was the result of
several factors, including drift of Antarctica toward a more polar position,
the cooling trend in global temperatures after the Late Paleocene Thermal
Maximum / Early Eocene Climatic Optimum (LPTM/EECL), and the sepa-
ration of South America from Antarctica through widening of the Drake
Passage (currently 800 km across) beginning just prior to about 30 Ma. The
opening of the passage allowed a west-to-east-fl owing circumpolar current
to develop that thermally isolated Antarctica from warmer tropical waters
and served as a positive feedback to southward drift and cooling tempera-
tures. The combination of these events led to the appearance of signifi cant
continental ice in the Gamburtsev Mountains of Antarctica about 34 Ma
(Bo et al. 2009). At that time, atmospheric CO
2
concentration had declined
from approximately 1600 ppmv in the mid-Cretaceous and more than
1125 ppmv at the LPTM/EECL to about 800 ppmv, that is, to about three
times that of pre-industrial revolution values (280 ppmv) and roughly
twice current values (380 ppmv). At 30 Ma, continental ice was signifi cant
in extent, as refl ected in the drop in sea level, and it reached present-day
volumes at about 15 Ma. It should be recalled that the formation and melt-
ing of sea ice, such as the extensive ice sheet covering the Arctic Ocean, has
little effect on ocean levels, much like the melting of an ice cube in a glass
of water. When extensive amounts of ice are anchored on land, however,
their coming and going has a great impact. It is estimated that if all the
Greenland, Iceland, valley glaciers, and Antarctic ice were to melt, sea level
would rise by about 80.5 m (Greenland and Antarctica would account for
