На тему: flora and fauna of the usa нужен эссе)

To begin the conversation about animals and pland word of North America is better to start with explanation of climate conditions and gografical situation of the region, in order to clear understanding of such a wide diversity of spices.

USA encompasses about 21.5 million km², between latitudes 26° and 85° N, and longitudes 15° W and 173° E, and it stretches from the Florida Keys northward to Ellesmere Island, and from Greenland westward to Attu Island in the Aleutian Archipelago. Widest in the north, the continent narrows sharply at the Gulf of Mexico. South of the United States border with Mexico, it tapers gradually to the Isthmus of Panama. It is surrounded by three oceans---the Arctic, Pacific, and Atlantic, respectively to the north, west, and east---and by the Gulf of Mexico to the south. It is separated from northeast Asia by the Pacific Ocean, and by the epicontinental Bering Sea, the Chukchi Sea, and the connecting Bering Strait. The Greenland and Norwegian seas, as well as the North Atlantic Ocean, separate North America from Europe and link the Atlantic to the Arctic Ocean; the Denmark Strait divides Greenland from Iceland. The Strait of Florida divides North America from the West Indies (Cuba).

Climate, physiography, and geology play major roles in determining the distributions of present-day soil classes, vegetation types, floras, and faunas. Biogeographers agree that climate is the primary factor in the control of these distributions. Climate determines the erosional and soil-forming processes that occur, and the life forms that are able to survive at a given locale, all of which may be affected secondarily by the types of bedrock and surficial deposits encountered in the area. In turn, relief influences climatic patterns through elevation above sea level and its effects on wind patterns and rainfall.

Geoclimatic changes that occurred throughout Earth history have affected the distribution of biotas through time. Climate has changed under cosmic influences, such as the Milankovitch cycles. The climate has also been affected by the relative position of the drifting continents, because drift implies latitudinal shifts, changes in the distribution of landmasses relative to oceans and oceanic currents, and modifications in the position of mountain ranges relative to airflow patterns. For instance, the Tertiary opening of the Atlantic onto the Arctic Ocean, and the establishment of the circumantarctic current with the opening of the Drake Passage between South America and Antarctica, played a significant role in subsequent climatic cooling.

The deep oceanic conveyor belt (a bottom sea current that links all the oceans) was presumably modified by changes in continental distribution and may have affected climate. W.F. Ruddiman and J.E. Kutzbach (1991) proposed that the 3-km uplift of the high plateaus in Tibet and in western North America in the Pliocene-Pleistocene were instrumental in provoking the late Tertiary trend of climatic cooling. Finally, the pathways by which biotas have been able to spread between continents were also affected by the existence of bridges. Such dynamic factors influenced the evolution of life on the North American continent.

First we take a look about the plant word and we begine by examining the history of North America vegetation. A discussion of the history of the vegetation of North America most logically begins with the events of the late Upper Cretaceous epoch, 70--60 Ma (million years ago). By then, the angiosperms and other major present-day groups were clearly established as dominant in the world's terrestrial flora. The continents were closer together than they are at present, and indeed, Eurasia and North America were still conjoined across the northern Atlantic. The plate tectonic forces that have placed the continents in their present configurations, however, were already in motion.

Our knowledge of the botanical events of the past rests on an interpretation of the fossil record, which for vascular plants occurs in two forms. Macrofossils are structures such as leaves, stems, fruits, seeds, wood, and flowers, whereas plant microfossils representing terrestrial or freshwater aquatic macrophytic vegetation include pollen grains, spores, and phytoliths (crystals formed within living plants). Paleobotany (including specialized approaches such as dendrochronology and analysis of pack-rat middens) has come to imply the study of plant macrofossils, and paleopalynology designates studies concerned with plant microfossils.