Biodiversity (Community Ecology) / Biosphere


		Biodiversity (Community Ecology) / Biosphere (Top Posts - Science - 070701) Introduction http://www.britannica.com/eb/article?eu=127612 Excerpt: "Study of the organization and functioning of communities, which are assemblages of interacting pop- ulations of the species living within a particular area or habitat. ... The number of interacting species in these communities and the complexity of their relationships exemplify what is meant by the term "biodiversity." Structures arise within communities as species interact, and food chains, food webs, guilds, and other interactive webs are created. These relationships change over evolutionary time as species reciprocally adapt to one another through the process of coevolution. ..." - - - Species (estimated number of known living species) http://hypertextbook.com/facts/2003/FelixNisimov.shtml Excerpt: ... Right now we can only guess that the correct answer for the total number of species lies between 2 and 100 million ... - - - Biotic elements of communities Trophic pyramids and the flow of energy Autotrophs and heterotrophs http://www.britannica.com/eb/article?eu=127612&tocid=70585#70585.toc Excerpt: "All biological communities have a basic structure of interaction that forms a trophic pyramid. The trophic pyramid is made up of trophic levels, and food energy is passed from one level to the next along the food chain. ... The base of the pyramid is composed of species called autotrophs, the primary producers of the ecosystem. They do not obtain energy and nutrients by eating other organ- isms. Instead, they harness solar energy by photosynthesis (photoautotrophs) or, more rarely, chemical energy by oxidation (chemoautotrophs) to make organic substances from inorganic ones. All other organisms in the ecosystem are consumers called heterotrophs, which either directly or indirectly depend on the producers for food energy. ... Transfer of energy through an ecosystem http://www.britannica.com/eb/art?id=70&type=A - - - Food Chains and Food Webs http://www.britannica.com/eb/article?eu=127612&tocid=70589 Excerpt: "Because all species are specialized in their diets, each trophic pyramid is made up of a series of intercon- nected feeding relationships called food chains. Most food chains consist of three or four trophic levels. A typical sequence may be plant, herbivore, carnivore, top carnivore; another sequence is plant, herbivore, parasite of the herbivore, and parasite of the parasite. Many herbivores, detritivores, carnivores, and parasites, however, eat more than one species, and a large number of animal species eat different foods at different stages of their life histories. In addition, many species eat both plants and animals and therefore feed at more than one trophic level. Consequently, food chains combine into highly complex food webs. ... Generalized aquatic food web - parasites, among the most diverse species in the food web, are not shown: http://www.britannica.com/eb/art?id=71&type=A Even a fully constructed food web, however, can provide only a superficial and static view of the structure of bio- logical communities. ..." - - - Guilds and Interaction Webs http://www.britannica.com/eb/article?eu=127612&tocid=70591 Excerpt: "Most communities contain groups of species known as guilds, which exploit the same kinds of resources in comparable ways. The name "guild" emphasizes the fact that these groups are like associations of craftsmen who employ similar techniques in plying their trade. ... Guilds often are composed of groups of closely related species that all arose from a common ancestor. They exploit resources in similar ways as a result of their shared ancestry. ... Species not only eat one another; they compete for resources, forging a variety of interspecific interactions. Many species also interact cooperatively to search for food or avoid predators. These and other nontrophic relationships between species are as important as food chains and food webs in shaping the organization of biological communities. ... Ecological Succession The structure of communities is constantly changing. ... Each new disturbance within a landscape creates an oppor- tunity for a new species to colonize that region. New species also alter the character of the community, creating an envir- onment that is suitable to even newer species. By this pro- cess, known as ecological succession, the structure of the community evolves over time." - - - Ecological Succession - Types of Succession http://www.britannica.com/eb/article?eu=127612&tocid=70594 - - - Stratification and Gradation http://www.britannica.com/eb/article?eu=127612&tocid=70596 - - - Ecological Niches http://www.britannica.com/eb/article?eu=127612&tocid=70598 Excerpt: "An ecological niche encompasses the habits of a species. Essentially it refers to the way a species relates to, or fits in with, its environment. As a species adapts to the physical parameters and biota within the community, natural selection favours the devel- opment of specialized features that allow the species to uniquely exploit the surrounding resources. ..." - - - Community Equilibrium and Species Diversity http://www.britannica.com/eb/article?eu=127612&tocid=70600 Excerpt: "... The relationship between species diversity and community stability highlights the need to maintain the greatest richness possible within biological commun- ities. ..." - - - Interspecific Interactions and the Organization of Communities http://www.britannica.com/eb/article?eu=127612&tocid=70602 Excerpt: "The interactive relationships that arise between populations of different species form the interactive web of communities. These interactions range from antagonistic to cooperative and have either positive, negative, or neutral effects on the species involved. In antagonistic relationships the interaction is detrimental to individuals of either one or both species; in commensal rela- tionships (commensalism) one species benefits while the other remains unaffected; and in mutualistic relationships (mutualism) both species benefit. The organization and stability of biological communities results from the mix of these different kinds of interaction. These relationships between species are not static; they evolve as natural selection continually shapes and reshapes them. The defenses and counterdefenses seen in the relationships between hosts and parasites, or between prey and predators, are snapshots of one point in time during the ongoing pro- cess of the evolution of interactions. As interactions between species evolve, relationships may shift from antagonism to commensalism to mutualism. As a result, the organization of biological communities is no more fixed than are the characteristics of the species or their environments. Charles Darwin called this ever-changing mix of species and their interactions the "entangled bank" and stressed its importance in the evolutionary process. ... Mutualistic interactions between species are just as integral to the organization of biological communities as antagonistic relationships, with some mutualistic interactions forming the most basic elements of many communities. ..." - - - The Pervasiveness of Mutualism http://www.britannica.com/eb/article?eu=127612&tocid=70604 Excerpt: "Some mutualistic relationships are so pervasive that they affect almost all life-forms. ... Most animals rely on the microorganisms in their gut to properly digest and metabolize food. ... The Evolution of Mutualism Although mutualisms benefit all species involved in a rela- tionship, they are built on the same genetically selfish prin- ciples as antagonistic interactions. In fact, many mutualisms appear to have evolved from antagonistic interactions. No species behaves altruistically to promote the good of another species. ... Mutualism results whenever the selfish activities of species happen to benefit each of them. Natural selection continues to reshape these relationships as each species evolves its ability to exploit the other. ..." - - - Mutualism and Cheaters http://www.britannica.com/eb/article?eu=127612&tocid=70606 Excerpt: "Because mutualisms develop through the man- ipulation of other species, they are always susceptible to invasion by "cheaters," those organisms that can exploit an existing relationship without reciprocating an advantage. ..." - - - Antagonism http://www.britannica.com/eb/article?eu=127612&tocid=70608 Excerpt: "Although mutualisms are common in all biological communities, they occur side by side with a wide array of antagonistic interactions. As life has evolved, natural selection has favoured organisms that are able to efficiently extract energy and nutrients from their environment. Because organisms are concentrated packages of energy and nutrients in themselves, they can become the objects of antag- onistic interactions. Moreover, because resources often are limited, natural selec- tion also has favoured the ability of organisms to compete against one another for them. The result has been the evolution of a great diversity of lifestyles. This diversity can be categor- ized in any number of ways, but the edges of all the categories blend with one another. Evolution continues to mix all the different kinds of interspecific interactions into novel ways of life. ... Parasitism, grazing, and predation are the three major ways in which species feed on one another. The parasite lives on and feeds off its host, usually decreasing the host's ability to survive but not killing it outright. Grazing species are not as closely tied to their food source as parasites and often vary their diet between two or more species without directly killing them. Predators, however, capture and kill members of other species for food. Parasitism - Types of parasites Parasitism is thought to be the most common way of life, and parasitic organisms may account for as many as half of all living species. ... The most species-rich groups of organisms are parasites, which, in becoming specialized to live off their hosts alone, eventually become genetically distinct from their species, sometimes to the degree that they are considered a new species (speciation). ... One common type of parasite is the parasitoid, an insect whose larvae feed and develop within or on the bodies of other arthro- pods. ... Parasitoids alone number about 68,000 named species, and most parasitoids have yet to be named and described. Realistic estimates of the total number of described and undes- cribed parasitoid species are about 800,000. ... The number of species of insects that develop as nymphs or larvae on a single plant host may outnumber the parasitoids ... there are at least a few million plant-feeding insect species, but the estimates range from 2 to 30 million. ..." - - - Specialization in Parasites http://www.britannica.com/eb/article?eu=127612&tocid=70611 Excerpt: "... The different ways by which host species are linked to parasites contribute to the complex web of inter- actions that shape the structure of communities. ..." - - - Grazing http://www.britannica.com/eb/article?eu=127612&tocid=70614 Excerpt: "The word "grazing" conjures up images of large mammals moving through seas of grass. Grazing, however, is a form of interspecific interaction that has been adopted by a number of other groups as well. A grazer is defined as any species that moves from one victim to another, feeding on part of each victim without actually killing it outright. The "victim" is to the grazer as prey is to the predator. ... On most continents, reciprocal evolutionary changes, or coevo- lution, between grasses and large grazing mammals have taken place over periods of millions of years. ... Plants have evolved more than 10,000 chemical compounds that are not involved in primary metabolism, and most of these compounds are thought to have evolved as defenses against herbivores and pathogens. ..." - - - Predation http://www.britannica.com/eb/article?eu=127612&tocid=70616 Excerpt: "Predation differs from both parasitism and grazing in that the victims are killed immediately. Predators therefore differ from parasites and grazers in their effects on the dynamics of populations and the organization of communities. ... Cannibalism, in which individuals of the same species prey on one another, also has arisen many times and is common in some animal species. ... Because predators kill their prey immediately, natural selec- tion favours the development of a variety of quick defenses against predators. In contrast, the hosts of parasites and the victims of grazers can respond in other ways. ... the evolu- tion of interactions between parasites and hosts, grazers and victims, and predators and prey all differ from one another as a result of the ways in which the interaction affects the victim. ..." - - - Predation - Effect on Community Structure http://www.britannica.com/eb/article?eu=127612&tocid=70618 - - - Competition http://www.britannica.com/eb/article?eu=127612&tocid=70619#70619.toc Excerpt: "Competition is a powerful form of interaction in the organization of communities, but it differs from other forms of antagonistic and mutualistic relationships in that no species benefits from the interaction. ..." - - - Types of Competition http://www.britannica.com/eb/article?eu=127612&tocid=70620 Excerpt: "Species compete for almost every conceivable kind of resource ... Species may compete for many resources simul- taneously, but often one resource, called the limiting resource because it limits the population growth of each species, is the focus of competition. ... The Effects of Competition Over evolutionary time, the effects of competition on species can vary. In some environments, the effects may be highly asymmetrical, and, at the extreme called amensalism, the sur- vival or growth of one species may be inhibited and the other(s) not affected. The weaker competitor will either go extinct locally, diverge from the other species in its use of resources, or evolve an increased competitive ability. All three outcomes have been observed in natural and experimental populations studied by ecologists. ..." - - - Commensalism and Other Types of Interaction http://www.britannica.com/eb/article?eu=127612&tocid=70622 Excerpt: "In commensal interactions, one species benefits and the other is unaffected. The commensal organism may depend on its host for food, shelter, support, transport, or a combina- tion of these. ... Many other kinds of interaction, however, range from antagon- ism to commensalism to mutualism, depending on the ecolog- ical circumstances. For example, plant-feeding insects may have large detrimental effects on plant survival or reproduction if they attack small or nonvigorous plants but may have little or no effect on large or vigorous plants of the same species. Some human diseases may cause only temporary discomfort or be life-threatening, depending on the age and physical condition of the person. No interaction between species fits neatly into the categories of antagonism, commensalism, or mutualism. The interaction de- pends on the genetic makeup of both species and the age, size, and physical condition of the individuals. Interactions may even depend on the composition of the community in which the inter- action takes place. ... The Coevolutionary Process As pairs or groups of species interact, they evolve in response to each other. These reciprocal evolutionary changes in inter- acting species are called coevolutionary processes, one of the primary methods by which biological communities are organ- ized. Through coevolution local populations of interacting species become adapted to one another, sometimes even evolving into new species." - - - The Study of Coevolution http://www.britannica.com/eb/article?eu=127612&tocid=70625 - - - The Coevolutionary "Arms Race" Versus Reduced Antagonism http://www.britannica.com/eb/article?eu=127612&tocid=70625#70625.htm Excerpt: "Nothing is absolutely predictable about the direction of coevolution. How an interaction coevolves depends not only on the current genetic makeup of the species involved but also on new mutations that arise, the population character- istics of each species, and the community context in which the interaction takes places. ..." - - - Predator-Prey Interactions http://www.britannica.com/eb/article?eu=127612&tocid=70626 Excerpt: "In an evolutionary arms race, natural selection pro- gressively escalates the defenses and counterdefenses of the species. ... Parasite-Host Interactions Parasites and their hosts engage in a similar evolutionary arms race, although in the past parasitologists believed this not to be the case. ... Parasite-host interactions are now understood to evolve toward either increased or decreased antagonism, depending on several important ecological factors. ..." - - - Importance of Interspecific Interactions http://www.britannica.com/eb/article?eu=127612&tocid=70629 Excerpt: "Unfortunately the importance of interspecific inter- actions may become apparent only after the balance of a community has been disrupted, often by human hands and often with serious reverberations. ..." - - - Gene-for-Gene Coevolution http://www.britannica.com/eb/article?eu=127612&tocid=70631 Excerpt: "In some interactions between parasites and hosts, coevolution can take a specific form called gene-for-gene coevolution or matching-gene coevolution. It is a form of reciprocal evolutionary change based on the idea that, if one member of a coevolving relationship has a gene that affects the relationship, the other member has a gene to counter this effect. These genes evolve reciprocally and provide the genetic basis for certain types of coevolution. This relationship has been demonstrated between plants and a number of their parasites ... The Geographic Mosaic Theory of Coevolution The study of evolving interactions in natural biological com- munities has indicated that the long-term dynamics of coevo- lution may occur over large geographic ranges rather than within local populations. ..." - - - Coevolution of One Species With Several Species http://www.britannica.com/eb/article?eu=127612&tocid=70633 Excerpt: "In the process called coevolutionary alternation, one species coevolves with several other species by shifting among the species with which it interacts over many gener- ations. ... Coevolution Among Groups of Species Coevolution often involves even larger numbers of species, but many of these coevolving interactions are much more difficult to study than paired or alternating relationships. ..." - - - Coevolution Among Many Species http://www.britannica.com/eb/article?eu=127612&tocid=70637#70637.toc - - - Evolution of the Biosphere http://www.britannica.com/eb/article?eu=127613&tocid=70638#70638.toc Excerpt: "Life is characteristic of the Earth. The biosphere—which in relation to the diameter of the Earth is an extremely thin, life-supporting layer between the upper troposphere and the superficial layers of porous rocks and sediments—is clearly visible from space ... All known forms of life are based on nucleic acid–protein systems, although life systems involving different chemical components are theoretically possible. Life appears to have developed on the Earth as soon as conditions permitted. ... The abiotic elements of the biosphere have been profoundly shaped by life, just as life has been molded by the environ- mental conditions that surround it. The biosphere has grown over time. Seven hundred million years ago it was a narrow and possibly discontinuous band encompassing only the shallower parts of the ocean. Today it reaches high into the atmosphere and deep into the ocean, invading even the tiny spaces in porous rocks. ... Life is changed through the process of evolution. Evolution is an inevitable consequence of inheritance, genetic variation, and competition arising from the number of individuals exceeding available resources. The result—natural selection—permits the perpetuation of some traits over others. Through billions of years this pro- cess has resulted in a great diversification of life-forms. ... The history of life is characterized by an acceleration of evolutionary change and unpredictable periods of extinction, often followed by rapid diversification. ... The degree of interdependence between organic and inorganic elements of the biosphere and the importance of both negative and positive feedback mechanisms in the maintenance of life increasingly are being recognized. ..." - - - The Development of Life The Building Blocks http://www.britannica.com/eb/article?eu=127613&tocid=70643 Excerpt: "The oldest undisputed fossils are about 3.5 billion years old. Important events in the history of life: http://www.britannica.com/eb/art?id=78&type=A Life seems to have originated about 3.9 to 3.5 billion years ago. The basic chemical building blocks needed to form life are abundant on the Earth as well as elsewhere in the known uni- verse. Life probably first arose through the self-assembly of small, organic molecules into larger ones. ... The First Life-Forms The earliest simple life-forms in the fossil record are prokar- yotes (cellular organisms without a membrane-enclosed nucleus)—namely, the bacteria and cyanobacteria (formerly called blue-green algae). They have been found in rocks called stromatolites ... Stro- matolites formed when colonies of prokaryotes became trapped in sediments ... Until about 2.5 to 2.8 billion years ago, the Earth's atmosphere was largely composed of carbon dioxide. As primitive bacteria and cyanobacteria had, through photosynthesis or related life processes, captured atmospheric carbon, depositing it on the seafloor, carbon was removed from the atmosphere. ... Cyanobacteria were also the first organisms to utilize water as a source of electrons and hydrogen in the photosynthetic pro- cess. Free oxygen was released as a result of this reaction and began to accumulate in the atmosphere, allowing oxygen-depen- dent life-forms to evolve." - - - The Development of Life The Growing Complexity http://www.britannica.com/eb/article?eu=127613&tocid=70645 Excerpt: "... Major changes in the evolution of the biosphere occurred in the late Precambrian (about 700 to 540 million years ago). Before this time, for about 1.4 billion years following their first appearance, single-celled eukaryotes had been the dominant life-form on the Earth. Then, in the late Precambrian, complex multicellular organ- isms (animals or plants composed of large numbers of more or less specialized cells) evolved and diversified rapidly. Challenges to the Development of Life The development of complex life before this time may have been hindered by the atmospheric changes that the biota produced. The prior abundance of carbon dioxide in the atmosphere had provided an insulating, or green- house, effect. As organisms removed this gas from the atmosphere, the greenhouse effect was lessened and the Earth's climate changed. This occurrence is believed to have resulted in severe ice ages that gripped the planet. ... The distribution of life-forms dependent on a nearby shore- line or a terrestrial habitat has been affected by the relative positions of the continents. The cyclic breakup of super- continents has provided many opportunities for evolution to continue in isolation. ..." - - - The Diversification of Life Extinction and Diversification http://www.britannica.com/eb/article?eu=127613&tocid=70647#70647.toc "... The Cambrian Explosion The beginning of the Cambrian Period, now thought to date from 540 rather than 570 million years ago, witnessed an unparalleled explosion of life ... Many of the major phyla that characterize modern animal life—various researchers recognize between 20 and 35— appear to have evolved at that time, possibly over a period of only a few million years. Many other phyla evolved during this time, the great majority of which became extinct during the following 50 to 100 million years. ... The beginning of the Cambrian is marked by the evolution of hard parts such as calcium carbonate shells. These body parts fossilize more easily than soft tissues, and thus the fossil record becomes much more complete after their appearance. Many lineages of animals independently evolved hard parts at about the same time. The reasons for this are still debated, but a leading theory is that the amount of oxygen in the atmos- phere had finally reached levels that allowed large, complex animals to exist. Oxygen levels may also have facilitated the metabolic processes that produce collagen, a protein building block that is the basis for hard structures in the body. ..." - - - The Emergence of Terrestrial Life http://www.britannica.com/eb/article?eu=127613&tocid=70650#70650.toc Excerpt: "Plants had invaded the land by the middle of the Silurian, about 420 million years ago, and by 410 million years ago various arthropods were found on land. Significant events in plant evolution: http://www.britannica.com/eb/art?id=80&type=A By the Middle Devonian (387 to 374 million years ago) true spiders capable of spinning silk had evolved. Winged insects followed some 50 million years later. By the Late Devonian (374 to 360 million years ago) some vertebrates also had emerged onto the land. They were to give rise to the chor- dates—amphibians, reptiles, birds, and mammals. Evolution of the chordates: http://www.britannica.com/eb/art?id=82&type=A Plant Life Terrestrial plants are believed to have evolved from the chloro- phytes, such as the green algae. Their survival on land demanded special adaptations to prevent them from drying out and to aid them in obtaining nutrients and in reproducing. ..." - - - Animal Life http://www.britannica.com/eb/article?eu=127613&tocid=70652#70652.toc Excerpt: "In adapting to life on land the earliest terrestrial verte- brates faced problems similar to those of the plants. Some members of a group of fleshy-finned, air-breathing fish—the crossopterygians—are believed to have been the ancestors of the land-dwelling vertebrates. Eusthenopteron is the best- known of these. ... A Period of Extensive Glaciation and Drought: The Permian Period The interval between the middle of the Carboniferous and the Early Permian is characterized by a prolonged ice age. All the continents were joined into one supercontinent (Pangaea), and a vast ice sheet covered what is now Antarctica, southern Australia, most of India, the southern half of Africa, and much of eastern South America. ... The close of the Permian is marked by perhaps the greatest well-documented extinction event on the Earth. In all, about 96 percent of the marine species vanished ..." - - - The Reptilian Radiation http://www.britannica.com/eb/article?eu=127613&tocid=70655#70655.toc Excerpt: "The earliest reptilian fossils have been found in rocks from the Carboniferous, about 340 million years ago. These early reptiles gave rise to the synapsid reptiles, which became abundant by the Permian. ... One group of synapsids, the therapsids, or mammallike reptiles, gave rise to mammals in the Late Triassic. Primitive diapsid reptiles gave rise to two principal groups, the lepidosaurs ("scaly reptiles"), which includes lizards and snakes, and the archosaurs ("ruling reptiles"), which includes dinosaurs and crocodiles. They first appeared in the Late Carboniferous, about 300 million years ago, and for 60 million years afterward they remained small, with generalized characteristics. Only after the great Permian extinction did they begin to diversify and dominate the environment as they gained in size, abundance, and variety. The Triassic Period (245 to 208 million years ago) began with relatively warm and wet conditions, but as it progressed condi- tions became increasingly hot and dry. ... Lizards were present by the Triassic, while snakes evolved from monitor-like lizards about 120 million years ago during the Early Cretaceous. The archosaurs dominated terrestrial life from the Middle Trias- sic (240 to 230 million years ago) until the end of the Cretaceous. The best-known archosaurs were the dinosaurs, but pterosaurs (flying reptiles), crocodiles, and birds are included in the group. Birds are believed to have evolved from an order of primitive archosaurs, Thecodontia. The earliest fossil evidence of birds is that of the crow-sized Archaeopteryx, from the Late Jurassic. Archaeopteryx skeleton: http://www.ucmp.berkeley.edu/diapsids/birds/archaeopteryx.html The Diversity of Cretaceous Biota During the Cretaceous Period large dinosaurs such as the preda- tory Tyrannosaurus, the herbivorous Triceratops, and the sauro- pod Alamosaurus were dominant forms on land. ... Birds, which first appeared in the Jurassic, and mammals, which evolved in the Triassic, were also in existence but were minor components of the Earth's fauna in contrast to their dominance in the Tertiary Period (66.4 to 1.6 million years ago). It seems likely that various insect groups diversified rapidly at this time in response to the ecological opportunities opened by the spread of flowering plants." - - - The Diversity of Cretaceous Biota Climatic Effects http://www.britannica.com/eb/article?eu=127613&tocid=70657#70657.toc Excerpt: "The maximum development of greenhouse conditions occurred in the Cretaceous and was probably associated with an increase of greenhouse gases such as carbon dioxide in the atmosphere ... There were no polar ice caps during this time, and land within both the Arctic and Antarctic circles was able to support a diversity of plant and animal life. The sea level was consider- ably higher than at present, and the low-lying parts of the continents formed vast but shallow inland seas. ... Mass Extinction The Cretaceous Period came to an abrupt end about 66.4 million years ago with a massive extinction event. Dinosaurs, ammonites and most belemnites (both related to squid and nautiluses), rudist clams, and toothed birds all became extinct. Indeed, all animal species that reached an adult weight of approximately 25 kilograms (55 pounds) at sexual maturity appear to have disappeared at this time. Smaller organisms such as calcareous plankton, glass sponges, freshwater fish, and brachiopods were severely diminished in diversity, as were gymnosperms and angiosperms of the laurel group. The cause of this—one of the world's great extinction events— is still hotly debated. Many biological, climatic, and extrater- restrial factors have been put forward to explain it. The asteroid theory, proposed by Walter and Luis Alvarez about 1980, postulates that the extinction was a result of the Earth's collision with an asteroid about 10 to 20 kilometres in diameter. It is generally supposed that the impact caused vast amounts of particulate matter to be emitted into the upper atmosphere, obscuring the Sun and resulting in a drastic reduction in photosynthetic activity and a global cooling. ... The asteroid theory has promoted renewed interest in extinc- tions in general. ... there is an emerging consensus that extinc- tion events have been more frequent, more catastrophic, and more variable in effect than was previously realized. It is also becoming apparent that, because they randomly influence the survival or extinction of various species, extinctions are one of the major determinants of evolutionary direction." - - - A Period of Transition http://www.britannica.com/eb/article?eu=127613&tocid=70659#70659.toc Excerpt: "The evolutionary and ecological responses of species surviving the Cretaceous extinction appear to have been rapid. One surviving species of fern is thought to have covered 90 percent of the land surface of the Earth within 10,000 years of the catastrophe. ... Mammalian Diversification and Rise to Dominance Among the three groups of modern mammals, egg-laying mono- tremes and marsupials have persisted in relatively small numbers and have been most successful on the southern continents. ... The third mammalian group, the placental mammals, has met with the greatest success, giving rise to flying forms (bats), mar- ine species (whales, sirenians, and seals), and an extraordinary variety of land-based forms. The diversification of the placental mammals was rapid. ..." - - - The Appearance of Primates http://www.britannica.com/eb/article?eu=127613&tocid=70661#70661.toc Excerpt: "Among the placental mammals to appear during the Late Cretaceous were the primates. A small group of large- bodied, tailless species—the apes—eventually diverged to give rise to a bipedal lineage about 3.5 million years ago. The genus Homo evolved from this line 2 million years ago. Between 200,000 and 100,000 years ago modern humans, Homo sapiens, had evolved ... Climatic Fluctuations The period of time following the Late Cretaceous extinction event, the Tertiary Period (66.4 to 23.7 million years ago), was marked by climatic fluctuation with a general trend toward cooling ... Since then the climate has oscillated, culminating about 2.4 million years ago in the onset of the ice ages, with many advances and retreats of the world's ice caps. ..." - - - Effects of Acquatic Changes http://www.britannica.com/eb/article?eu=127613&tocid=70663#70663.toc Excerpt: "In the Late Eocene the temperature of the bottom water of the southern ocean dropped dramatically, by 4° to 5° C. This appears to have been caused by the increasing physical, and thus thermal, isolation of Antarctica and its surrounding seas. ... This ultimately led to the development of the Antarctic Bottom Water—cold, deep, nutrient-rich water that today originates at Antarctica and flows north to all the major oceans of the world. ... The development of the Antarctic Bottom Water has had a profound effect on life in the oceans owing to its novel nutrient-carrying capacity. Because of this ability, it is be- lieved to have led to major changes in nutrient cycling when it was first established. It may, for example, be responsible for the abundance of krill and thus for the evolution of the great mysticete (filter-feeding) whales, which first appeared in the Oligocene (36.6 to 23.7 million years ago). Periods of Glaciation The development of an extensive ice cap in Antarctica six million years ago led to a dramatic fall in sea level. At its height, the terminal Miocene ice age saw Antarctica's Ross Ice Shelf extend about 300 to 400 kilometres north of its present position. This event isolated the Mediterranean Sea, which experienced numerous cycles of evaporation and refilling during subsequent oscillations in temperature. ..." - - - Quaternary Events Megafaunal and Other Extinction Events http://www.britannica.com/eb/article?eu=127613&tocid=70665#70665.toc Excerpt: "During the Pleistocene the diversification of mam- mals continued, accompanied by localized and fewer wide- spread extinction events. In the terminal Pleistocene (50,000 to 10,000 years ago), however, extinction events occurred without a large number of groups of larger vertebrates being replaced. The species that became extinct, which included mammoths, mastodons, ground sloths, and giant beavers, are collectively known as megafauna. ... These extinctions have removed 29 percent of the vertebrate genera weighing more than 40 kilograms from Europe and 73 percent of such genera from North America. Until 1,000 to 2,000 years ago the megafauna of large, long-isolated landmasses such as New Zealand and Madagascar survived. Gigantic birds such as the elephant birds of Madagascar and the moas of New Zealand disappeared after the Pleisto- cene in the past few thousand years. ..." - - - Quaternary Events Impact of Human Activities http://www.britannica.com/eb/article?eu=127613&tocid=70668 Excerpt: "The past 10,000 years have seen dramatic changes in the biosphere. The invention of agriculture and animal hus- bandry and the eventual spread of these practices through- out the world has allowed humans to co-opt a large portion of the available productivity of the Earth. Calculations show that humans currently use approximately 40 percent of the energy of the Sun captured by organisms on land. Use of such an inordinately large proportion of the Earth's productivity by a single animal species is unique in the history of the planet. The human population continues to expand at the rate of approximately 80 million persons per year and may reach 10 billion sometime in the 21st century. Changes to the atmos- phere caused by complex technology and the increasing population threaten to cause major disruptions to the bio- sphere. ..." - - -