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COMMUNITY ECOLOGY: 



Grassland moved to the southwest, again mixing with 

 other floras. As a consequence, there were additions 

 and subtractions of species in the Grassland. The 

 additions came from hereditary changes in existing 

 southwest species and hybridization between south- 

 west and Grassland species, producing new species. 



The North American Pliocene was very important 

 in reference to modern mammals. At the close of the 

 Pliocene there was little extinction compared to other 

 times. However, the Pleistocene modified this pic- 

 ture considerably. Most forms either became extinct 

 or had their ranges considerably restricted. Forms 

 that became extinct included the large dire wolf, 

 saber-tooth tiger, American lion, mammoth, Colum- 

 bian elephant, imperial elephant, mastodon, tapirs, 

 horses, camels, giant beaver, glyptodons, ground 

 sloths, and many others. A great many of the living 

 groups of hoofed mammals had close relatives that 

 were lost. 



The Pleistocene glaciers did not cover all of the 

 northern United States. Sites in the Pacific and 

 Atlantic coasts did remain free of ice. In fact, animal 

 fossils indicate moderate persistance of an ice-free 

 corridor along the Pacific Coast from the Pacific 

 states to Alaska, across an open Bering land bridge, 

 and down the coast of eastern Asia. 



One of the places having pronounced Pleistocene 

 effects was Europe. During the Ice Age, the flora and 

 fauna of Europe became much reduced. The primary 

 reason for this extinction is believed to have been the 

 presence of east-west mountain chains (Pyranees, 

 Alps, Balkans, Caucasus, and so on). These moun- 

 tains were glaciated early and formed a substantial 

 barrier to southern retreat of the more northern floras 

 and faunas. It is known that certain animals and 

 plants were trapped and killed in the ice retreat (e.g., 

 hairy mammoths are found in Siberian glaciers of 

 today). However, other organisms escaped from the 

 north and found safety in India and Africa. For ex- 

 ample, the Indian elephant probably evolved from 

 hairy mammoths that found suitable habitats in 

 India. Also, the deer are thought to have found refuge 

 and been able to return to Europe. However, a good 

 percentage of Indian mammals (south of the Hima- 

 layas) and African mammals (south of the deserts) 

 are descendants of the old European stock. 



The present fauna of northern Europe is composed 

 mainly of species that could have come back from 

 southern retreats, plus others that are invaders from 

 the south. That portion derived from original stock 



apparently came from species that remained near the 

 northern fringe of their southward-moving habitat 

 and were able to return with the habitat to Europe. 

 The animals that never returned went farther south, 

 and instead of following the retreating ice northward 

 evolved sufficiently to remain in their "new" en- 

 vironments. Moreover, these southern species re- 

 mained south long enough to be trapped by unfavor- 

 able northern areas, such as the Sahara Desert. 



The present European flora and fauna also include 

 certain North American forms. These organisms 

 were able to cross the then continuous land from 

 North America to Asia, traverse Asia, and finally 

 inhabit Europe. All of this took place during Pleisto- 

 cene times. 



Along with general reduction, present-day Euro- 

 pean flora and fauna display inconsistencies in distri- 

 bution. This inconsistency is best observed in south- 

 ern Europe where the plants form a unit called the 

 Mediterranean Flora. This flora developed from 

 Pleistocene species that found a refuge in the Balkan 

 Peninsula. Very few of these Balkan plants re-entered 

 northern Europe. In contrast to this simple explana- 

 tion of over-all plant distribution, that of certain 

 animals is puzzling. For example, the Barbary ape of 

 Gibraltar (native there until recent extinction and 

 more recent introduction) and the Atlas Mountains is 

 not closely related to African apes. Its close relatives 

 are in eastern Asia, so its present range involved dis- 

 tant immigration, perhaps with the creatures that 

 came from North America across the Bering land 

 bridge. However, this is but a single example of the 

 fact that the origin of the European flora and fauna 

 required both simple north-south movements with 

 the ice and more complex east-west invasions. 



The Pleistocene does not provide full explanation 

 for the reduction of organisms in Europe. Another 

 reason is the extensive cultivation and associated re- 

 moval of primitive areas. In contrast to Europe, 

 eastern Asia, even with its denser human population, 

 still has more primitive areas. For example, India 

 has a very large human population, but man is 

 densely packed in various centers of habitation; be- 

 tween human habitats, there are many primitive 

 areas. For this reason and human beliefs which 

 largely prevent killing, animals such as tigers are 

 able to survive in India. On the other hand, the 

 present European primitive areas are so restricted 

 and modified that they could not support such 

 animals. 



