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|Chapter 1: Variety of Live|
The variation of the living nature on the planet earth - the biodiversity or diversity of life- is still overwhelming. Our planet supports between 3 and 30 million species of plants, animals, fungi, single-celled prokaryotes such as bacteria, and single-celled eukaryotes such as protozoans. Of this total, only about 1.4 million species have been named so far, and fewer than 1 percent have been studied for their ecological relationships and their role in ecosystems. A little more than half the named species are insects, which dominate terrestrial and freshwater communities worldwide; the laboratories of systematists are filled with insect species yet to be named and described. Hence, the relationships of organisms to their environments and the roles that species play in the biosphere are only beginning to be understood. From which does this richness arise and what is its genesis? This chapter gives some answers to these questions.
Despite numerous conservation efforts in the past, evidence pointed to a continued decline in almost all species worldwide. The 1996 Red List of Threatened Animals issued by the International Union for Conservation of Nature and Natural Resources identified 5,205 species in danger of extinction. In tropical forests alone, for example, biologists estimated that three species were being extinguished every hour. Much of the decline or mass extinction was caused by the destruction of habitats, especially logging. Only 6% of the Earth's forests were formally protected, which left the remaining 33.6 million sq km (13 million sq mi) vulnerable to exploitation (see adverse human impacts).
Although the idea of conservation or protection is probably as old as the human species, the use of the word in its present context is relatively recent. Over the years conservation has acquired many connotations: to some it has meant the protection of wild nature, to others the sustained production of useful materials from the resources of the Earth. The most widely accepted definition, presented in 1980 in World Conservation Strategy by the International Union for Conservation of Nature and Natural Resources, is that of "the management of human use of the biosphere so that it may yield the greatest sustainable benefit while maintaining its potential to meet the needs and aspirations of future generations." The document defines the objectives of the conservation of living resources as: maintenance of essential ecological processes and life-support systems, preservation of genetic diversity, and guarantee of the sustainable use of species and ecosystems. More generally, conservation involves practices that perpetuate the resources of the Earth on which human beings depend and that maintain the diversity of living organisms that share the planet. This includes such activities as the protection and restoration of endangered species, the careful use or recycling of scarce mineral resources, the rational use of energy resources, and the sustainable use of soils and living resources.
Conservation is necessarily based on a knowledge of ecology (= the science concerned with the relationship between life and the environment), but ecology itself is based on a wide variety of disciplines, and conservation involves human feelings, beliefs, and attitudes as well as science and technology (see ecological backgrounds).
The natural world among us is extremely rich and variable in forms, colors, odours, behaviors, movements etc. To understand this richness and variability it is necessary to collect this unities into classes, types and categories. Biodiversity deals with biological and geographical unities such as genes, chromosomes, species, families, biogeographic regions. Therefore understanding natures richness requires both distinction and description of these biological and biogeographical unities.
Taxonomy is that part of biological science that deals with the variation of living organisms. In a systematic way taxonomists catalogue organisms into phyla, orders, families, genera and species, etc. These taxonomic unites are called taxa (singular taxon). From the theory of evolution taxonomists know that taxa are not 'static' unities. In fact taxa are extremely variable both in space and in time. An important role thereby is played by genetics, ecology, biogeography, paleontology and the theories of 'heredity' and 'population dynamics'.
Biodiversity has many facets. In most definitions biodiversity is an addition sum of genetic, taxonomic and ecosystem diversity. Genetic diversity embraces the variation in genetic material, such as genes and chromosomes. Taxonomic diversity, mostly interpretated as the variation among and within species (also the human species), includes the variation of taxonomic unities such as fyla, families, genera, etc. Ecosystem diversity or even better biogeographic diversity concerns with the variation in biogeographic regions, landscapes and habitats.
One has to realize that biodiversity is always concerned with the variability of the living nature within a specific area or region. The idea of biodiversity gets its content merely within a time or space connection.
With genetic, taxonomic and biogeographic diversity not al aspects of biodiversity are named. There are many more forms of variation, such as seasonal variation, non-genetical variation caused by environmental influences (fenotypical variation). There is also the variation due to differences between divergent stages of live (ontogenetic diversity) and modes of live (cultural diversity). Nevertheless the named three aspects of biodiversity are considered to be the most important dimensions of biodiversity. Lets have a more detailed look on genetic, taxonomic and biogeographic diversity.
In the past, little attention was paid to genetic variety among wild species, although it is in fact crucial to the continued abundance of biological forms, the development of species diversity (evolution) and the functioning of the biosfeer, the ecosystems and the biological communities. The extent of diversity within a species depends upon the number of individuals, its geographical range, the degree of isolation of individual populations and its particular genetic system. An important role is also played by natural and anthropogenic processes of selection as well as factors influencing spatial and temporal changes in the genetic stock of the species or population. Genetic variety is essential to the ability of species and populations to adapt to changing environmental conditions and is therefore a prerequisite for their survival.
In sexually reproducing species, each local population contains a distinct combination of genes. As a result, a species is a collection of populations that differ genetically from one another to a greater or lesser degree. These genetic differences manifest themselves as differences among populations in morphology, physiology, behaviour, and life histories; in other words, genetic characteristics (genotype) affect expressed characteristics (phenotype). Natural selection initially operates on a phenotypic level, favouring or discriminating against expressed characteristics. The gene pool (= total aggregate of genes in a population at a certain time) is affected as organisms with phenotypes that are compatible with the environment are more likely to survive for longer periods, during which time they can reproduce more often and pass on more of their genes.
The amount of genetic diversity within local populations varies tremendously, and much of modern conservation biology is concerned with the maintenance of genetic diversity within populations of plants and animals. Some small, isolated populations of asexual species often have little genetic diversity among individuals, whereas large, sexual populations often have great variation. Two major factors are responsible for this variety: mode of reproduction (sexual and asexual) and population size.
In sexual populations, genes are recombined in each generation, resulting in new genotypes. Offspring in most sexual species inherit half their genes from their mother and half from their father, and their genetic makeup is therefore different from either parent or any other individual in the population. New, favourable mutations that initially appear in separate individuals can be recombined in many ways over time within a sexual population. In contrast, the offspring of an asexual individual are genetically identical to their parent. The only source of new gene combinations in asexual populations is mutation. (= An alteration in the genetic material of a cell that is transmitted to the cell's offspring. Mutation may be spontaneous (the result of accidents in the replication of genetic material) or induced by external factors (e.g., electromagnetic radiation and certain chemicals). Mutations take place in the genes, which are found in the long, chainlike molecules of deoxyribonucleic acid (DNA)).
Asexual populations accumulate genetic variation only at the rate at which their genes mutate. Favourable mutations arising in different asexual individuals have no way of recombining and eventually appearing together in any one individual, as they do in sexual populations. A much wider range of favourable gene combinations, therefore, is likely to be found in sexual than asexual populations.
Over long periods of time, genetic diversity is more easily sustained in large populations than in small populations. Through the effects of random genetic drift (= change in the gene pool of a small population that takes place strictly by chance), a genetic trait can be lost from a small population relatively quickly. For example, many populations have two or more forms of a gene, which are called alleles. Depending on which allele an individual has inherited, a certain phenotype will be produced. If populations remain small for many generations, they may lose all but one form of each gene by chance alone. The lloss of alleles happens from sampling error. As individuals mate, they exchange genes. Imagine that initially half of the population has one form of a particular gene, and the other half of the population has another form of the gene. By chance, in a small population the exchange of genes could result in all individuals of the next generation having the same allele. The only way for this population to contain a variation of this gene again is through mutation of the gene or immigration of individuals from another population. Minimizing the loss of genetic diversity in small populations is one of the major problems faced by conservation biologists. Environments are constantly changing, and natural selection continually sorts through the genetic diversity found within each population, favouring those individuals with phenotypes best suited for the current environment. Without the genetic diversity that allows populations to respond evolutionarily to changes in the physical environment, diseases, predators, and competitors, populations risk extinction.
Recent advances in biochemical and electron microscopic
techniques, as well as in testing that investigates the genetic relatedness among species,
have redefined previously established taxonomic relationships and have fortified support
for a five-kingdom classification of living organisms.
Species diversity is determined not only by the number of
species within a biological community--i.e., species richness--but
also by the relative abundance of individuals in that community. Species abundance
is the number of individuals per species, and relative abundance refers to the evenness of
distribution of individuals among species in a community. Two communities may be equally
rich in species but differ in relative abundance. For example, each community may contain
10 species and 500 individuals, but in one community all species are equally common (e.g.,
50 individuals of each species), while in the second community one species significantly
outnumbers the other four. These components of species diversity respond differently to
various environmental conditions. A region that does not have a wide variety of habitats
usually is species-poor; however, the few species that are able to occupy the region may
be abundant because competition with other species for resources will be reduced.
Global gradients also affect species richness. The most
obvious gradient is latitudinal: there are more species in the tropics than in the
temperate or polar zones. Ecological factors commonly are used to account for this
gradation. Higher temperatures, greater climate predictability, and longer growing seasons
all conspire to create a more inviting habitat, permitting a greater diversity of species.
Tropical rainforests are the richest habitat of all, tropical grasslands exhibit more
diversity than temperate grasslands, and deserts in tropical or subtropical regions are
populated by a wider range of species than are temperate deserts.
How the unique distributions of animals and plants in various biomes came to be is not explicable purely through present climatic factors and latitudinal zonation. Geologic events such as continental drift and past climatic conditions must be taken into consideration as well. This is the approach used in historical biogeography to study the distributions of flora and fauna throughout the world.
Biodiversity on the planet earth is the net-result of two major processes: speciation and extinction. To understand and protect biodiversity it will be necessary to get some knowledge of both processes.
Speciation is the formation of new and distinct species in the course of evolution. In individual cases, it involves the splitting of a single evolutionary lineage into two or more genetically independent ones. Speciation occurs in three different ways: allopatric, parapatric and sympatric speciation. Allopatric speciation is speciation by geographic isolation. By parapatric speciation there is no complete geographic isolation. Sympatric speciation is speciation without georgaphic isolation. The three different ways of speciation have in common that species come into existence gradually (gradual speciation). Apart from gradual speciation we also know of sudden speciation (for instance chromosomal speciation).
An important phenomenon by speciation is 'adaptive radiation', an 'outburst' of speciation. Most adaptive radiations can be attributed to the existence of new ecological opportunities. One may think on the colonisation of ecological 'empty' regions (such as islands in oceans) after extinctions on a large scale (mass-extinctions).
Extinction is the dying out or termination of a race or species. Extinction occurs when a species can no longer reproduce at replacement levels. Most extinctions are thought to have resulted from environmental changes that affected the species in either of two ways. The doomed species might not have been able to adapt to the changed environment and thus perished without descendants; or it may have adapted but, in the process, may have evolved into a distinctly new species. The effect of humans on the environment, through hunting, collecting, and habitat destruction, has become a significant factor in plant and animal extinctions.
Although extinction is an ongoing feature of the Earth's flora and fauna (the vast majority of species ever to have lived are extinct), the fossil record reveals the occurrence of a number of mass extinctions, each involving the demise of vast numbers of species. One such mass extinction occurred at the end of the Cretaceous period, some 66,000,000 years ago, when the dinosaurs and much of the marine life of the day perished. Evidence points to the impact of an asteroid hitting the Earth as the cause of the Cretaceous extinctions. It is suspected that catastrophic events--such as an asteroid impact--may have triggered other mass extinctions as well. In fact, mass extinctions appear to have taken place approximately every 26,000,000 years, which has led some paleontologists to propose that a cyclical cosmic event causes these periodic die-offs.
Evolution is the theory in biology postulating that the various types of animals and plants have their origin in other preexisting types and that the distinguishable differences are due to modifications in successive generations. The theory of evolution is one of the fundamental keystones of modern biological theory.
The virtually infinite variations on life are the fruit of the evolutionary process. All living creatures are related by descent from common ancestors. Humans and other mammals are descended from shrewlike creatures that lived more than 150,000,000 years ago; mammals, birds, reptiles, amphibians, and fishes share as ancestors aquatic worms that lived 600,000,000 years ago; all plants and animals are derived from bacteria-like microorganisms that originated more than 3,000,000,000 years ago. Biological evolution is a process of descent with modification. Lineages of organisms change through generations; diversity arises because the lineages that descend from common ancestors diverge through time.
The 19th-century English naturalist Charles Darwin, whose Origin of Species by Means of Natural Selection (published in 1859), argued that organisms come about by evolution, and he provided a scientific explanation, essentially correct but incomplete, of how evolution occurs and why it is that organisms have features--such as wings, eyes, and kidneys--clearly structured to serve specific functions. Natural selection was the fundamental concept in his explanation. Genetics, a science born in the 20th century, reveals in detail how natural selection works and led to the development of the modern theory of evolution. Since the 1960s a related scientific discipline, molecular biology, has advanced enormously knowledge of biological evolution and has made it possible to investigate detailed problems that seemed completely out of reach a few years earlier--for example, how similar the genes of humans and chimpanzees might be (they differ in about 1 or 2 percent of the units that make up the genes).
Biological evolution is the process of change and diversification of living things over time, and it affects all aspects of their lives--morphology, physiology, behaviour, and ecology. Underlying these changes are changes in the hereditary materials. Hence, in genetic terms, evolution consists of changes in the organism's hereditary makeup. During the process of evolution - the history of life on earth - both processes speciation and (mas-)extinction were very common.
The first lifeforms resulted from the process of evolution somewhere between 4 and 3,5 billion years ago. Afterwards there has been a net-increase of biodiversity. However, in the last decades biodiversity is being significantly reduced by certain human activities.
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