Biology Reference
In-Depth Information
sandy habitats. The same sense of dramatic change in climate and vegeta-
tion in Neogene time, described for the Succor Creek fl ora in the Basin and
Range Province in Oregon, is also apparent at the localities in Patagonia.
The collector works in the cold, dry, windy steppe, and from strata in which
this vegetation is rooted are found the remains of
Acacia
,
Anadenanthera
,
and diverse and large Megathroideids typical of warm-temperate environ-
ments. Multiple lines of inquiry then reveal that reduced levels of CO
2
,
thermal isolation of Antarctica, mountain uplift, and changes in ocean and
atmospheric circulation contributed to the development of cooler and sea-
sonal climates between the middle Miocene and the Pliocene. Upon refl ec-
tion, it is of considerable satisfaction to realize we now know a lot about
how, when, and why these changes happened.
In summary, the ecosystems of South America at the end of the Plio-
cene were desert, shrubland/chaparral-woodland-savanna, including the
extensive tropical savannas of Venezuela and steppe in Patagonia; grass-
land, including the pampas of Argentina; mangrove, beach/strand/dune;
freshwater herbaceous bog/marsh/swamp; aquatic; lowland neotropical
rain forest; lower to upper montane broad-leaved forest, with cloud for-
est, elfi n forest, and ceja associations at progressively higher elevations, and
some northern deciduous forest elements now intermingled (e.g.,
Alnus
,
Quercus
,
Juglans
), páramo, and tundra. Similar communities were present
in the Northern Hemisphere at the time and collectively constitute a living
envelope of modern aspect at the ecosystem level.
The assemblage and sequence of appearance of New World ecosystems
was conditioned by the nature and patterns of climate change (e.g., Milan-
kovitch variations) interacting with the mostly independent consequences
of landscape development (atmospheric CO
2
concentration, ocean trans-
port of heat, barriers, and bridges), and all were acting on the reservoir
of new biotypes generated through evolution. The current result of these
processes operating over the past 100 million years is a biological world
that for convenience has been classifi ed into the twelve ecosystems rec-
ognized in this text. A broad outline of their history is summarized in
table 7.1. At present, that history is a big screen picture without high defi -
nition; that is, it is a little blurry. But the number of fossil fl oras and fau-
nas being investigated through innovative techniques is increasing, and the
pattern is becoming more clear.
The geologic record documents that over intervals measured in hundreds
of thousands to millions of years in the Late Cretaceous and Neogene, en-
vironments changed at a comparatively gradual pace that was periodically
interrupted by catastrophic events that altered conditions more suddenly.
