Biology Reference
In-Depth Information
they remained in a transitional glacial/nonglacial range until about 15 Ma
(fi g. 3.4). By that time, ocean waters and land surfaces had cooled signifi -
cantly, and temperate deciduous elements had spread widely. Shortly after-
ward, many New World mountain systems attained their highest elevations,
providing additional temperate habitats. Less moisture was evaporating
from the cooler oceans into the atmosphere, so wet/dry seasonality was
also intensifying and spreading. At about 2.6 Ma, another tipping point
in global temperatures was reached that ushered in the Quaternary glacia-
tions. During all this time, and documented as far back as at least the Late
Triassic (Vollmer et al. 2008), the extraterrestrial Milankovitch variations
were operating; but by the Quaternary, the overriding infl uence of CO
2
,
although still important, had waned to the point that these variations were
unmasked and became more evident as regulators of climate.
Climate-forcing mechanisms interact in a complex system of positive
and negative feedbacks that can reinforce, cancel out, or reverse the many
individual factors—and they have done so throughout geologic time. To
make the point, some of the interactions are sketched in fi gure 3.5, includ-
ing the still unquantifi ed role of solar variations (Foukal et al. 2004). Among
the conclusions reinforced by this diagram are (1) climate is an immensely
complex system, and considering its essential role in sustaining life on the
planet, common sense would dictate it should be pertubated with caution;
(2) a systems (multidisciplinary) approach incorporating data from many
independent lines of inquiry is necessary for understanding the ecosystems
as they respond to the varied factors; and (3) context information is neces-
sary to assess conclusions about ecosystems as a basis for intelligently read-
ing the past, managing the present, and preparing for the future.
PHYLOGENY
Knowledge about the evolutionary relationships among extant plants is
important for determining meaningful patterns of biogeographic distribu-
tion, as opposed to ones based on artifacts of taxonomy. For example, a
different explanation would be required to explain the disjunct occurrence
of a species in two widely separated localities than if taxonomic study re-
vealed the taxon represented two distantly or unrelated species. Another
value of phylogeny is that it can serve as a basis for assessing identifi cations
of fossils from particularly important strata or localities. Fossils preserve
in rocks of different ages, organisms evolve over time, and there should
be some general correspondence between them. Primitive (basal) groups
(e.g., Magnoliids) would be expected in older strata, advanced (derived)
