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.
Experience has shown that most elements comprising a fossil assemblage are broadly consistent in terms of habitat preference or they can be sorted into subsets reflecting habitat diversity (viz. elevational gradients). This organization gives rise to the concept of paleocommunities from which it is possible to deduce past climates paleophysiography and biogeographic patterns. Such reconstructions are based on a direct comparison and presumed general equivalency of most members of a fossil flora with modern analogs (composition of the flora) on the observation that present-day plants with certain morphological attributes (e.g. leaf physiognomy) are found in certain habitats and on the assumption that most fossil plants with similar morphological attributes occurred in comparable habitats. For example modern plant assemblages containing many large-leaved entire-margined species with drip-tips typically occur in humid tropical habitats; therefore a fossil flora with many similar leaf types is taken to indicate a humid tropical paleoenvironment. The composition of a fossil flora based on the combined inventories provided by macro- and microfossil remains leaf physiognomy and dendrochronology are all valuable methods for studying vegetational history and reconstructing the environments that influenced the development of North American vegetation through time.
The modern history of systematic botany and floristics in North America began when the first Europeans landed on these shores and began to collect objects of curiosity. It is imperative to use the term "modern " for long before colonization of the New World by Europeans the Native Americans who had arrived millennia earlier had developed their own systems of classification means of identification and associated nomenclature. Unlike that of their European counterparts their knowledge was transferred by the spoken rather than the printed word and was mostly lost as their civilizations fell to the invaders. To a great degree it was not until the twentieth century that Native Americans were recognized as knowledgeable about their plants. By then European thought dominated botany and the Native American's botanical understanding was passed on only in an occasional native name retained in a Latinized form.
It was not until Columbus's second voyage in 1493 that New World plants and animals were taken across the Atlantic. For the European scientific community the unfamiliar specimens were a source both of great intellectual curiosity and of philosophical concern. The curiosities were clearly different from their Old World counterparts and in some instances they were entirely novel. The likes and near-likes could be associated but the distinctly different were philosophically troublesome.
The Spanish of the early sixteenth century were the first to describe the flora of the New World. Gonzalo Fernández de Oviedo y Valdes (1478--1557) visited several of the Caribbean islands and portions of Central America trying to fit the tropical vegetation he observed into a classification scheme that recognized only six species of trees with persistent green leaves. Oviedo had become acquainted with native New World plants of equal or even greater value than those introduced to the New World by the Spanish and he urged their use. He was ignored.
Nicolas Bautista Monardes (1493--1578) never saw the New World. His interests were the new medicines and new remedies he felt certain existed. He classified plants according to their medicinal properties and for the American ones he often retained the native names. He accepted treatments recommended by the Amerinds but as a firm believer in the Doctrine of Signatures he occasionally modified them.
The missionary Jose d'Acosta (1539--1600) spent 20 years in Peru returning to Spain in 1588 to publish various works on the New World. He urged scholars to regard the majority of living things in the New World as unique and not to assign them established European names. He described numerous native economic and medicinal plants and commented on the diversity of potatoes tomatoes and chili peppers he had found in the market; he also mentioned cacao and coca.
During this period intellectual thought often was dominated by religious dogma. Scholarly investigations in the natural sciences began primarily in northwestern Europe. The first naturalists often had to flee the upheavals of the Protestant Reformation and as a result many traveled widely and learned from others. In this way a more unified system of classification and nomenclature began to develop.
Herbals those great tomes illustrated with woodcuts were the primary botanical publications of the age. At first they were little more than restatements of Dioscorides or other classical authors but as the herbals were developed over the next two centuries new species and remedies were incorporated including the wonders of the New World. Of equal importance was the development of botanic gardens first established in Pisa in 1543. These soon became centers of scientific importance because not only could plants of faraway places be seen but their medicinal properties could be determined also.
The following discussion begins with the floras of the Maastrichtian stage of the Upper Cretaceous 70--65 Ma and progresses forward in time through the Tertiary to 2 Ma the end of the Pliocene epoch i.e. to the advent of the Pleistocene the "Ice Ages." Within each section the fossil floras are discussed in a sequence that begins with the southeastern corner of the continent and proceeds westward and around the continent in a clockwise fashion.
In describing paleoevents degrees of latitude and longitude unless otherwise noted are given in terms of present-day locations of the poles and continents even though the North American continent has moved slightly relative to the poles during the Tertiary and to the present.
The flora of North America includes a large number of conspicuous plants that are called "weeds." The concept of weed is not precisely defined for it has both a sociological and a biological component. From the sociological perspective a weed is simply a plant that is growing where someone wishes it were not and therefore a weed may be regarded casually as a "plant-out-of-place." By that definition a rose growing in a wheat field would be a weed; a rose in a garden would not. Some plants however have the genetic endowment to inhabit and thrive in places of continual disturbance most especially in areas that are repeatedly affected by the activities of humankind. These plants are biologically "weedy " and they are sometimes termed colonizing or invasive plants. These biological weeds are the focus of next paragraph.
Weeds have a measurable effect on the affairs of society and therefore they have attracted much attention. Weeds occur in all growth forms and in many lifestyles. The majority of weeds are flowering plants and a high proportion of them share some or all of the following characteristics: short life cycle rapid growth rate high level of energy allocated to reproduction efficient dispersal mechanisms high population growth rate wide distribution seeds with long life spans and flexible use of environmental resources. He noted that a plant with but few of these attributes is less likely to be successful as a weed than is a plant with all or most of them; therefore the variation ranges from casual local weeds to aggressive widespread weeds.
The most troublesome and aggressive weeds are those foreign or alien species that have invaded the North American continent from regions elsewhere in the world. By comparison fewer and less aggressive weeds are native species. Analysis of the geographical components of a large number of weeds usually shows over 60% to be foreign species. The distinction between foreign species and native species is not always clear and it is not easy to measure the impact of those foreign or alien plants on the native vegetation. Several factors contribute to this lack of precision.
Botanists assume that species have a "place of origin " where at some time the species are differentiated from the ancestral entities. As time passes a newly formed species migrates into new areas and/or expands its range through the routine mechanisms of seed dispersal seedling establishment and other factors. Undoubtedly some botanical traffic has occurred between North America and other continents since antiquity but clearly colonization following Columbus's voyages to America initiated a significant number of invasions. Some of the historical aspects of plant migration at the hands of humankind are reviewed by V.Muhlenbach (1979).
Foreign or alien species are usually regarded as those that have been brought to North America by human activities in post-Columbian times while native species either originated in North America or had arrived by various means in pre-Columbian times. Although botanists frequently use the term "introduced" for these foreign or alien species in this chapter the term has a more restricted meaning and refers to those species deliberately brought by people into a new region where the plants grow without cultivation. How many species have been transported from their putative places of nativity to North America in post-Columbian times is of course unknown. The historical documentation for these plant movements is often not well known or not yet researched and many times what is known is based on circumstance and inference.
Here are en examples of native American plants. A large Sycamore tree Platanus racemosa played an early role in the establishment of Los Angeles. The central Gabrielino village of Yangna was located near a 60 ft. high 200 ft. wide sycamore which was used for meetings amongst the Gabrielino leaders and was known as the "council tree". The Spanish settlement that later became the pueblo of Los Angeles was located next to Yangna in sight of this stately tree. The settlement was washed away in the Great Flood of 1815 but the sycamore survived. It later died in 1892 and was cut down. A ring count revealed that the tree was 400 years old. It had started its life about the time Columbus first landed in America. Cottonwoods were very common trees along rivers and arroyos in California and the southwest. As cities and farmers have lowered water tables these riparian trees have disappeared from many banks along arroyos in California Nevada and Arizona. The Fremont Cottonwood Populus fremonti was discovered in 1844 near Pyramid Lake by Major John Charles Fremont and Kit Carson. They used its riparian nature to help locate water. Willows are also a common riparian tree in the southwest. Some are actually assisted by floods. The rushing water bends some branches down into a sand bar where they sprout new roots and plants. The Willow Salix leaves were used by Native Californians for medicine. The small branches were used for baskets and the large branches for wood.