Biology Reference
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various taxa. Such information is meager and has not been fully analyzed
with respect to the modes and dispersal potential of the propagules, the
availability of climatically suitable habitats, or the questionable reliabil-
ity of some molecularly based divergence times. But for the moment, we
can note that in the Malpighiaceae six disjunctions occurred between 60
and 31 Ma in the middle Paleocene to early Oligocene and at 17 Ma in the
early Miocene (Davis et al. 2004). There were also disjunctions in Sym-
phonia at 17.36 Ma, plus or minus 1.53 million years (Dick et al. 2003);
in Senecio at 4-3 Ma (Coleman et al. 2003); in Lupinus at 4-3 Ma, as well
as at 8 Ma (Käss and Wink 1997); and in Hypochaeris no later than 3.5
Ma (Tremetsberger et al. 2005, 2006). Future studies will demonstrate if
the correlations are just coincidence. But, if they are not, they suggest one
more way some plants with small wind-dispersed propagules may have oc-
casionally moved more readily between Africa and South America in the
Tertiary.
REFUGIA
The refugium concept was proposed by Jürgen Haffer (1969, 1970, 1974,
1982). It adds to the several factors operating to varying degrees, sometimes
simultaneously, that enhanced biodiversity in the Amazon lowlands. As an
amateur ornithologist, in the fi nest sense of that word, Haffer was aware
that the number of bird species was not uniformly distributed across the
Amazon Basin. There were pockets of high species concentration, and there
were intervening areas where species diversity was less. As a professional
geologist, he was also aware that the species-rich areas often corresponded
with topographic and geologic features that would have perpetuated moist
conditions during periods of dryness. These included sites at the confl uency
of major rivers, the base of slopes, in shallow basins, deep river valleys, gal-
lery forests, and areas underlain by hardpan or caliche. He suggested that
in the drier intervals of the Quaternary, caatingas, cerrado, savanna, and/or
grasslands in and around the basin would have expanded into the lowlands,
and elements of the rain forest would have become isolated into moist re-
fugia. With the return of moist conditions, the rain forest expanded and
coalesced into the extensive plant formation that now covers most of the
basin, while the drier vegetation returned to the surrounding slopes and to
edaphic enclaves within the forest (e.g., sandstone outcrops and hill tops).
An interesting analysis, not yet completed, will be to determine if there are
dry elements within the moist vegetation still retracting toward favorable
dry habitats.
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