Deep History and Biogeography of La Frontera - Design with the Desert: Conservation and Sustainable Development - page 124

Bullsnake, western diamondback rattlesnake, creosotebush ( Larrea tridentata ), and many
other species are widespread in the warm deserts and desert grasslands of North America.
During Pleistocene glacial periods, their ranges were separated by nondesert habitat and
were only established during the present interglacial (the Holocene) in the last 6000 years. 17
In many species, the distributions of infraspecific taxa are often closely tied to the mod-
ern biotic provinces, and likely expanded and contracted as those biomes responded to
glacial-interglacial fluctuations. An interesting possibility is that the evolution of many,
but not all, infraspecific taxa was related to the formation of the modern biomes that they
inhabit, reflecting natural selection that occurred millions of years ago. For example, the
desert grassland kingsnake is widespread in desert grasslands from Texas to Zacatecas
and Arizona but is abruptly replaced by the desert kingsnake in transition to Sonoran
Desert near Tucson.
6.4 Desert Ice Ages
The warmth of the Pliocene ended suddenly at the beginning of the Pleistocene when
the earth entered a new climatic era of cool, continental conditions. Traditionally, four
ice ages or glacial periods, based on terrestrial sedimentary deposits, were recognized
in North America and widely correlated between Europe and South America. However,
recent studies of isotopic climatic indicators in continuous sediment cores from the ocean
floors record 15-20 glacial periods in the last 2.4 million years with the ice ages about 10
times as long as interglacials, which lasted 10,000-20,000 years. 35
In the last glacial period (the Wisconsin), the massive Laurentide ice sheet covered most
of Canada and extended as far south as New York and Ohio. The mixed deciduous for-
est in much of the eastern United States was displaced by boreal forest with spruce ( Picea
spp.) and jack pine ( Pinus banksiana ). 36 Glaciers covered the highest elevations of the Rocky
Mountains and the Sierra Nevada as well as the peaks of the Sierra Madre del Sur in south-
central Mexico. Now-dry playas in the Great Basin and on the Mexican Plateau were large
lakes, and enough water was tied up in ice on land to lower sea level about 328 ft.
6.4.1 Packrat Curators
Although La Frontera is distant from glaciated areas, past glacial climates have resulted in
profound changes in climate and vegetation. Paleoenvironmental insights of remarkable
power have been gleaned from an unlikely source—“packrat middens.” Packrats or wood
rats are medium-sized rodents that carry plant materials and other objects back to their
houses or dens. Some of this material may become cemented into the hard, dark organic
masses called middens that can be preserved indefinitely in dry rockshelters. 37 Detailed
reconstructions of local communities on rocky slopes for the last 45,000 years have been
completed for many desert areas using the abundant, well-preserved plant remains in radio-
carbon-dated middens. 38 Midden assemblages are excellent for documenting past floras and
reconstructing the vegetation on rocky slopes within about 30 m of the rock shelters.
The midden record extends back for several tens of thousands of years of the Wisconsin
glacial and the Holocene. Plant remains in middens document widespread expan-
sions of woodland trees and shrubs down to elevations that now support deserts from