and its tributaries is sufficient to isolate populations
of birds whose individual members are reluctant to
cross such a wide expanse of water. Jürgen Haffer
(1985) documented that the Amazon River effectively
isolates two similar species of antbirds (fig. 8- 3). The
Dusky Antbird (Cercomacra tyrannina) occurs north
of the Amazon, while the similar Blackish Antbird (C.
nigrescens) occurs south of the river. Both species occur
together only along a section of the northern bank of the
river, but here the Blackish inhabits wet várzea forests,
and the Dusky favors second- growth vegetation of the
terra firme forest. A similar pattern is evident in the
distribution of some curassow species (family Cracidae).
Time, Endemism, and Refugia
How does the high diversity within tropical regions relate
to geologic time— to long periods when speciation might
exceed extinction? When South America is compared
with other major tropical regions, we find that bird species
richness and plant species richness are unequivocally
highest in South America. But how did that happen?
What factors are responsible? South American species
richness, because of how extreme it is, poses vexing
questions for evolutionary biologists and biogeographers.
Since the 1970s, a debate has been ongoing between those
who believe most speciation in South America is recent,
dating primarily to events in the Pleistocene (1.64 million
to 10,000 years ago) and immediately before, and those
who argue that the data do not support such assertions
and that much of the speciation occurred millions of
years before the Pleistocene.
It is unlikely that the equatorial tropics were
climatically stable and constant throughout the
Pleistocene, undisturbed by the giant glaciers bearing
down upon northern temperate areas. Thomas Belt, in
1874, discussed possible effects of northern glaciation
on the tropics. More recently, studies by Jürgen Haffer,
Ghillean Prance, and Paul Colinvaux have, in various
ways, examined geomorphology (the historical
development of present landforms), paleobotany
(the study of past patterns of plant distribution), and
biogeography, and collectively suggest that dramatic
changes occurred in Amazonia during the Pleistocene.
One hypothesis to account for vicariant events
leading to widespread speciation states that during
Pleistocene glacial advances in northern latitudes, the
tropics became cooler and drier. During part of the
Pleistocene, temperature in the Ecuadoran foothills,
east of the Andes, was 4– 6° C (7– 11° F) cooler than at
present. The cooling altered and shifted the distribution
of ecosystems (warming is doing the same thing today).
Ecosystems such as dry forests and open savannas
enlarged, and moist forests contracted. Large continuous
tracts of lowland rain forest were fragmented into
forest “islands” of varying sizes surrounded by “seas”
of savanna or dry woodland. This scenario, in which
savanna expansion created vicariance among forest
tracts, is called the refugia (or refuge) model. Because of
the repeated shrinking and fragmenting of forests, forest
organisms became repeatedly geographically isolated
from populations in other forest areas. The Amazon
Basin became a climatically dynamic “archipelago” of
variably sized rain forest islands (refuges, or refugia)
that promoted speciation among plants and animals.
A classic study by Jürgen Haffer (1974) based on the
current distribution of certain kinds of birds postulate
that at least nine major and numerous smaller forest
island refugia were present in Amazonia during the
Pleistocene. The implication is that many taxonomic
groups went through periods of rapid speciation because
there were repeated episodes of rain forest shrinkage and
expansion. During interglacial periods forests expanded
and secondary contact was established between newly
speciated populations, explaining why so many extremely
similar species can be found today in Amazonia.
The refugia model has been the subject of considerable
debate. Evidence supporting it is based mostly on
present distribution and diversity patterns. Ghillean
Prance has examined woody plant diversity, for
example, and concluded that 26 probable forest refugia
existed for these plants. Other studies have supported
the refugia concept for groups such as primates, birds,
and heliconid butterflies. But it is important to note that
centers of endemism, the presumed Pleistocene refuges,
do not convincingly overlap among taxa, a reality that
lessens support for the refugia model. Supporters of the
refugia model respond that different taxa have different
dispersal powers and different generation times and
thus would be expected to differ somewhat with regard
to the degree of regional endemism. Paul Colinvaux
pointed out that at least one study showed that refugium
locations for plants coincide with areas in which
sampling of plants for herbarium specimens has been
historically most intense. This, of course, would suggest
that the refugia, at least some of them, are artifacts of
uneven sampling effort.130 chapter 8 evolutionary cornucopia