Biology Reference
In-Depth Information
Another anomaly concerns the relative number of plant versus animal
species moving north and south across the land bridge, and their record
of diversifi cation once they reached the other side (Burnham and Graham
1999). Many mammals moved from north to south where they radiated
and diversifi ed extensively. There are over 200 genera and 400 species of
mice and sigmodontine rodents of North American origin in South Amer-
ica, where they represent roughly 25 percent of the mammalian species
(see summary in Flannery 2001). There are more kinds of canines in
South America than on any other continent, and it has more than half of
the world's camelids. In total, about 53 percent of the modern-day South
American mammalian fauna is derived from North America, and they, or
their ancestors, mostly appeared around the time of completion of the Pan-
ama land bridge. Moving southward were raccoons, bats, shrews, hares,
pocket mice, pocket gophers, squirrels, fi eld mice, snapping turtles, cats,
saber-toothed tiger, puma, panther, weasels, skinks, otters, fox, wolf, bears,
elephantids, tapirs, horses, peckeries, Camelidae, and deer. Moving to the
north was a more modest cadre of armadillos, sloths, porcupines, capybara,
and opossums, and after arriving in North America they underwent limited
diversifi cation. Only around 10 percent of North American mammals have
South American ancestors.
The pattern for plants is just the reverse. Relatively few trees and shrubs
moved from the north into South America. They include
Alnus
,
Populus
,
Quercus
,
Salix
, and
Juglans
, and they underwent comparatively little diver-
sifi cation. In contrast, those moving north diversifi ed extensively and are
prominent members of the tropical and subtropical forests. As noted ear-
lier, about 75 percent of the canopy trees of the Mexican lowland forests
are derived from South America (Wendt 1993). The reasons for the differ-
ences are not known, but some components of the explanation are clear.
The plants moving from north to south were mostly temperate ones mi-
grating in response to cooling temperatures and the gradual emergence of
limited and scattered highlands. Upon reaching South America, these few
temperate taxa encountered extensive lowland tropical basin habitats with
temperate environments restricted along the still emerging Northern An-
des Mountains. The many more tropical taxa moving from south to north
encountered a relatively expansive lowland tropical environment increas-
ing in extent northward because of the funnel-shaped confi guration of the
continent (Flannery 2001). In other words, the pool of temperate plant taxa
in the north diminished toward Central America, while the pool of tropi-
cal plant taxa in the adjacent lowlands of South America was extensive.
