Biology Reference
In-Depth Information
has been suggested by Paul Martin (1982, 1984) because the hunters ar-
rived in North America at about the same time as the extinctions. However,
the importance earlier attributed to hunting has recently diminished be-
cause kill sites are relatively few (see summary by Fagan 2004, 38-41). Cli-
mate change is another factor long-regarded as playing a role in the demise
of the mammoths, giant ground sloths, and the huge kangaroos of Australia
(
Procoptodon
), in part, because the change also was happening at the time
they disappeared. In itself, however, climate was not likely the only factor,
because the fauna had survived many similar fl uctuations throughout the
Pleistocene. A third factor has been proposed by Chris Johnson (2002),
namely, the low birth rate of many of the large plains and savanna mammals
that disappeared. This supposedly rendered them vulnerable to the com-
bined impacts of climate and hunting. Most of those that did survive were,
in fact, arboreal, nocturnal, or lived in dense forests. As noted elsewhere,
it is reassuring that rather than interminably perpetuating the one-cause-
versus-another mode of discussion, a number of plausible factors—overkill,
climate, reproductive rate—are offering a multifaceted explanation ap-
proaching the complexity of the event.
A further hallmark of the Quaternary period is that the dramatic changes
in the environment, their causes, and the biotic responses can be traced with
a relatively high degree of precision. This raises a question about whether
the myriad apparent tipping points and fl uctuations in the Quaternary re-
ally were occurring at a pace faster than in the Cretaceous and Tertiary, or
if their cluster is just an artifact of the higher resolution possible for this
most recent period. There certainly were reversals in the Tertiary, such as
the cool interval of 1-1.5 million years at the Paleocene-Eocene boundary
in the northern Rocky Mountains (Wing 1996), the four warm spells fol-
lowing that boundary in New Zealand (Newton 2007; Nicolo et al. 2007),
and the well-documented warm periods of the middle Miocene and middle
Pliocene. Nonetheless, the evidence is convincing from Greenland ice cores
taken in the 1980s that the past 150,000 years was a wild if not crazy time
of rapid “switch and dial” climate changes (Alley 2000). There was usu-
ally a relatively gradual buildup to glacial conditions, then a precipitous
and chaotic fall into interglacial climates. After the LGM circa 18,000 BP,
Greenland warmed by 12°C-15°C in a matter of several decades. Within
the overall warming trend, there were reversals, and one of the most promi-
nent was the Younger Dryas (named after the European alpine plant
Dryas
octopetala
). It was a cold interval between 12,800 and 11,500 BP and in-
cluded a decrease of 4.5°C
1.3°C in surface marine water temperatures
(e.g., Corrège et al. 2004). There was another drop in temperature at
±
