Figures 7.15 and 7.22 demonstrate how downward shifts of high-altitude vegetation (primarily forests) during glacial maxima could have created avenues of dispersal: such a lowering would allow a plant or animal species, previously isolated on individual peaks, to cross ridges and migrate along mountain ranges.

This mechanism is precisely the one invoked by many authors to account for the intracontinental dispersal of vegetation types and associated animals.

When followed by isolation as mesic biomes contracted, it could result in taxonomic disjunctions (Simpson 1975; Vuilleumier and Simberloff 1980).

Glacial cycles may have had similar effects on the distributions of many marine organisms (Figure 7.25).

In the case of cold-water stenothermal species, relatively warm tropical waters serve as an effective physiological barrier to dispersal, limiting their distributions to the middle or high latitudes of the Northern or Southern Hemisphere

During glacial maxima, however, cooling of marine waters could have allowed range expansion into the lower latitudes.

Subsequent rewarming of tropical waters during interglacials could have again caused range contraction and possibly bipolar distributions of these species (Figure 7.25C).

As we noted earlier, eustatic and isostatic changes in sea level greatly altered opportunities for biotic exchange for both terrestrial and marine biotas(see Figure 7.9).

Greatly reduced sea levels during the Wisconsin created extensive landbridges, such as Beringia (connecting Siberia and North America), the Sunda Shelf (connecting Malaysia, and Indonesia), and the Arafura Sea and Bass Straits (conncting New Guinea, Australia, and Tasmania).

While these landbridges eliminated and fragmented marine biotas, they served as important dispersal corridors for terrestrial organisms, In most cases, however, biotic exchange was asymmetrical, withe more species migrating from larger(species-rich) to smaller areas than vice versa (e.g., from Siberia to Alaska; from Southeast Asia to the "islands" of the Sunda Shelf; from Australia to Tasmania). Biotic Exchange of terrestrial organisms across Beringia (see below) contributed significantly to the similarity between Nearctic and Palearctic biotas.

Our own species used this glacial landbridge to colonize North America from Siberia.

Similary, biotic exchange of marine organisms often tended to be asymmetrical, depending on the size and diversity of each species pool and the ocean currents and other factors influencing dispersal.

While Beringia served as a dispersal corridor for terrestrial organisms during glacial maxima, the Bering Strait was an important corridor for the dispersal of marine life during interglacial periods.

Again, more species migrated from the larger, more species-rich region - that is, from the Pacific basin northward.

During the late Cenozoic, 125 species of marine invertebrates invaded the Arctic-Atlantic region from the Pacific, while no more than 16 species colonized in the reverse direction(Durham and Mac Neil 1967; see also Vermeij 1991).

In Chapter 6 we discussed the tectonic events that ultimately formed a Central American landbridge between North and South America (about 3.5 million years B.P.).

The resultant waves of biotic exchange between Nearctic and Neotropical boitas, referred to as the Great American Interchange, were made possible not just by tectonic events, but by eustatic changes and vegetative shifts associated with glacial cycles.

During glacial maxima, the lowering of sea levels increased both the area and the elevation of the Central American landbridge. Perhaps just as important, the relatively dry conditions that prevailed during glacial maxia caused savannas to expand toward the equator and form a continuous habitat corridor for the migration of many species adapted to these open habitats (savannas and shortgrass prairiel see Webb 1991).

We shall return to the profound effects of these waves of biotic interchange in Chapter 16.

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G5 changed APP sorting like Apple's iPhone.

All app is background.

Most of all, Android phone is App folder.

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Android have the widget.

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This posting is only my English typing practice.

Now Start.

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Because dry deserts are notoriously poor environments for fossilization, 

it has been difficult tot reconstruct the biotic history of deserts.

Is a particular desert old or young? 

Did deserts move closer to the equator during glacio-plu-vial periods,

or did they stay where they were,

but with greatly contracted areas? 

Are the desert communities seen today the same ones that existed before and during the Pleistocene?

Where did desert vegetation survive during the mesic pluvial periods?

Many of these questions remain largely unanswered, 

but they are the focus of much current research.

Traditionally, the history of deserts has been inferred mainly from theoretical paleoclimatological models,

reinforced wherever possible by fossils, especially pollen records from the sediments of pluvial lakes.

Neither provides a very reliable account of the small-scale history of deserts.

In recent decades, a new type of fossil data has been used to reconstruct the vegetational history of one complex region:

the desert zoned of the semiarid and arid southwestern United States ( e.g., Betancourt et al. 1990; Grayson 1993).

Pack rats(Neotoma) are abundant rodents in xeric habitats.

They hoard plant materials in large caches, which are sometimes protected in caves or under rock ledges.

The remains of these caches become solid structures, called middens, 

and persist for thousands of years if kept dry.

They are excellent sources of plant fossils because they provide a relatively complete and often quite continuous sample of the plants growing within 100m of the rats' den during its occupation.

By collection pack rat middens at different elevations and locations and dating the materials using radiocarbon methods, researchers can reconstruct the shifts in elevation and composition of vegetation types and deduce past climatic regimes(i.e., those back to 40,000 years B.P.).

Data from pack rat middens are continuing to e synthesized ( Van Devender 1977; VAn Devender and Spaulding 1979; Wells 1979l Betancourt et al. 1990; see also Fritts 1976),but some general trends are apparent. 

It is clear that there have been major changes in the vegetation at all elevations throughout the arid Southwest within the last 20,000 years. In general, climates in this region were substantially cooler and wetter during the most recent(Wisconsin) glacial maximum.

During this period, vegetation zones were displaced as much as 500 to 1000 m below their present limits(Figure 7.22).

The change to the present configuration of climates and vegetation types occurred primarily within the last 8000 to 12,000 years.

In general, pack rat middens reveal a history consistent with that found in pollen records and geological studies of pluvial lakes, but they provide a much more detailed picture of vegetation changes. 

It is now clear that plant communities did not simply move as entire entitles up and down the mountains.

Rather, they changed dramatically in composition.

For example, during the most recent glacial maximum (21,000 to 15,000 years B.P.),

most plant species in the Grand Canyon occurred 600 to 1000 m lower than at the present time.

This finding indicates a cooler, wetter climate (Cole 1982), Many of the species that inhabited areas along the rim of the Grand Canyon, however, are no longer found in similar communities in the same region today.

In fact, the community contained several species that are presently found in northeastern Nevada and northwestern Utah, at least 500 km to the north (Cole 1982).

Many of the plants now dominant in the coniferous forests of the inter mountain West (e.g., ponderosa pine and pinon pine) were rare and restricted in glacial times. On the other hand, species that were much more widespread 10,000 to 30,000 years ago are now narrowly distrivuted or no longer occur in this region.

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The main Actress is Song Hye Kyo and Kim Ji Won.

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Yoo Si Jin & Seo Dae Young & Yoon Myung Joo are army.

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Thank you.