Biology Reference
In-Depth Information
rain shadow and provide a few scattered upland temperate habitats.
Alnus
crosses into the uplands of the Northern Andes around 1 Ma and
Quer-
cus
about 330 kyr (Hooghiemstra and Ran 1994). Thus, in using “the land
bridge,” or any continental connection, as an explanation for distributions,
it is necessary to estimate to the extent possible the ecological characteris-
tics of the organisms involved and when or if these habitats likely existed
in that landscape.
The broad outline of climate, geology, and biotic history is reasonably
well established for the Isthmian region because there is such a diversity of
information available to make the reconstructions. This evidence is, in part,
a consequence of the political, economic, and strategic importance of the
canal region where the underlying geology is of prime concern. The long-
time presence of the Smithsonian Tropical Research Institute and its fi eld
station on Barro Colorado Island in Gatún Lake is another important source
of information for the region. Nonetheless, not all aspects of the biotic his-
tory are fully resolved. For one thing, the fossil terrestrial fauna has a large
component of browsers and grazers, suggesting a shrubland/chaparral-
woodland-savanna and grassland vegetation, while the fossil fl oras suggest
a more dense vegetation of tropical forest.
A similar anomaly is apparent between the fl oras and faunas in the
Miocene fl oras of the American west, and Taggert and Cross (1990) sug-
gested a factor important in the explanation. In areas of volcanic activity,
the eruptions periodically reset the successional clock to barren ground
for relatively brief periods of time in local areas, followed by open recov-
ery vegetation and eventually the reestablishment of climax forest. In such
a setting, the primary component of the fossil fl oras for most of the time
is forest. However, volcanism allows for a shifting mosaic of open herba-
ceous and shrubby successional communities evident only with close, high
resolution sampling. Thus, the mobile herds of vertebrate fauna leave a
record strong in browsers and grazers, while the fossil fl oras preserve pri-
marily deciduous forest. A similar mechanism has been suggested for the
early to middle Miocene Cucaracha Formation of Panama, dated within 1.5
million years to about 16 Ma, and strata of comparable age from the Isth-
mian region (Graham 1988). In addition, paleosol evidence suggests locally
dry habitats, with MAP from 573-916 mm (
147 mm) to 296-1142 mm,
compared to estimates from pollen assemblages of 1200-1500 mm. Both
would allow for forest in and adjacent to coastal and swampy lowlands, with
contemporaneous drier vegetation scattered in the uplands (Retallack and
Kirby 2007).
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