Unit 3 biological diversity




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UNIT 3 BIOLOGICAL DIVERSITY

Structure

    1. Introduction

    2. Objectives

    3. Biological Diversity

      1. Concept and Levels of Biological Diversity

      2. Measurement of Biodiversity

      3. The Current Status of Biodiversity

      4. Distribution of Biodiversity

      5. Evolution of Biodiversity

      6. Role of Biodiversity in Ecosystem Function and Stability

    4. Speciation

      1. Concept of Species

      2. Speciation : Natural Selection and Genetic Drift

      3. Isolating Mechanisms

      4. Natural Speciation

      5. Genetic Drift

    5. Extinction

      1. Definition

      2. Pseudoextinction

      3. Causes of Extinction

    6. IUCN Categories of Threat

      1. 2006 Release

      2. 2007 Release

      3. Categories

      4. Criticism

      5. Mass Extinctions

    7. Terrestrial Biodiversity

    8. Biodiversity Hotspots

      1. The Biodiversity Hotspots by Region

      2. Approaches to Biodiversity Conservation

    9. Biodiversity and Climate Change

    10. Let us sum up

    11. Check your progress and the key

    12. Assignments/ Activities

    13. References/ Further Readings

    1. INTRODUCTION

Biodiversity is the variation of life forms within a given ecosystem, biome or for the entire Earth. Biodiversity is often used as a measure of the health of biological systems.

Biodiversity found on earth today consists of many millions of distinct biological species,. The whole of the earth's biodiversity - including all organisms and their immense genetic variation, as well as their complex assemblages of communities and ecosystems, is the result of four billion years of evolutionary change. Humanity shares with all other species, a genetic heritage and numerous ecological linkages that form the matrix, within which human societies have developed a complex set of psychological, ethical and spiritual values, associated with biodiversity.

In this unit, we shall review all aspect of biological diversity in the world in general, and India in particular. The diversity of species on earth constitutes a natural heritage and life-support system for every country and all people. Humans have always depended on the biodiversity around them for food, fuel, shelter, and health. The 80% world's population relies on traditional medicines derived from natural bio-resources. Wild foods still account over 40 percent of consumption by ethnic communities and others. The life - giving services that are often taken for granted, maintenance of potable water, clean air, and fertile soil all flow from the every day functions.and activities of healthy ecosystems. Without biodiversity human would perish.

    1. OBJECTIVES

The main objectives of this unit are to analyse the biological wealth present on the earth, sustainable use of its components and their conservation. The major objectives of present study are :

  • To understand the biological diversity and its component;

  • To study the importance of biological diversity in functioning and stability of ecosystem;

  • To understand the importance of biological diversity for evolution and for maintaining life sustaining systems of the biosphere,

  • To study the conservation and sustainable use of terrestrial biological diversity,

  • To study speciation, extinction and IUCN categories of threat.

    1. BIOLOGICAL DIVERSITY

      1. Concept and Levels of Biological Diversity

The term biological diversity defined as "variation of life at all levels of biological organization". Another definition holds that biodiversity is a measure of the relative diversity among organisms present in different ecosystems. "Diversity" in this definition includes diversity within a species and among species, and comparative diversity among ecosystems.

A third definition that is often used by ecologists is the "totality of genes, species, and ecosystems of a region". An advantage of this definition is that it seems to describe most circumstances and present a unified view of the traditional three levels at which biodiversity has been identified:

  • Genetic diversity - diversity of genes within a species. There is a genetic variability among the populations and the individuals of the same species.

  • Species diversity - diversity among species in an ecosystem. "Biodiversity hotspots" are excellent examples of species diversity.

  • Ecosystem diversity - diversity at a higher level of organization, the ecosystem. Diversity of habitat in a given unit area. To do with the variety of ecosystems on Earth.

The 1992 United Nations Earth Summit in Rio de Janeiro defined "biodiversity" as "the variability among living organisms from all sources, including, 'inter alia', terrestrial, marine, and other aquatic ecosystems, and the ecological complexes of which they are part: this includes diversity within species, between species and of ecosystems". This is, in fact, the closest thing to a single legally accepted definition of biodiversity, since it is the definition adopted by the United Nations Convention on Biological Diversity.

If the gene is the fundamental unit of natural selection, according to E. O. Wilson, the real biodiversity is genetic diversity. For geneticists, biodiversity is the diversity of genes and organisms. They study processes such as mutations, gene exchanges, and genome dynamics that occur at the DNA level and generate evolution.

For ecologists, biodiversity is also the diversity of durable interactions among species. It not only applies to species, but also to their immediate environment (biotope) and their larger ecoregion. In each ecosystem, living organisms are part of a whole, interacting with not only other organisms, but also with the air, water, and soil that surround them..

      1. Measurement of Biodiversity

Biodiversity is a broad concept, so a variety of objective measures have been created in order to empirically measure biodiversity. Each measure of biodiversity relates to a particular use of the data.

Biodiversity is usually plotted as taxonomic richness of a geographic area, with some reference to a temporal scale. Whittaker described three common metrics used to measure species-level biodiversity, encompassing attention to species richness or species evenness:

Species richness - The least sophisticated indices of measurements of species diveristy. There are two main indices are avialable :

  1. Simpson's index (D) : Simpson (1949) gave the probability of any two individuals drawn at random from an infinitely large community belonging to the same species as :

D = Pi2

Where, Pi = the proportion of individuals in the ith species.

  1. Shannon-Weaver Index (Shannon and Weaver, 1949) : Shannon index takes into account the degree of evenness in species abundances. The value of the Shannon index obtained from empirical data usually falls between 1.5 and 3.5 and rarely surpasses 4 (Margalef, 1972). The Shannon Index is calculated from the equation:

Shannan-Weaver Index (H') =

Where; ni = Number of individual species

N = Total number of species

There are three other indices which are commonly used by ecologists:

  • Alpha (α) diversity refers to diversity within a particular area, community or ecosystem, and is measured by counting the number of taxa within the ecosystem (usually species)

  • Beta (β) diversity is species diversity between ecosystems; this involves comparing the number of taxa that are unique to each of the ecosystems.

  • Gamma (γ) diversity is a measure of the overall diversity for different ecosystems within a region.

The relationship is as follows :

γ = α + β + Q

where, Q = Total number of habitats or communities,

α = Average value of α diversities,

β = Average value of β diversities.

      1. The Current Status of Biodiversity

Nobody knows for sure exactly how many species exist, or how rapidly species are disappearing through extinction. About 1.75 million species out of an estimated total of 10-20 m. have been collected and named by systematizes, with the most undercounted species being found among bacteria, protoctista (microorganisms), insects and fungi. Though the total number of species is unknown, biologists and taxonomists have accomplished reasonably complete samples in specific regions such as Western Europe. Species inventories show that some ecosystems are richer in terms of biodiversity than others. Groombridge and Jenkins (2000) go so far as to say, "The single most important fact about biological diversity is that it is not evenly distributed over the planet."

As a soft guide, however, the numbers of identified species as of 2007 can be broken down as follows:

  1. 287,655 plants, including:

    • 15,000 mosses,

    • 13,025 ferns,

    • 980 gymnosperms,

    • 199,350 dicotyledons,

    • 59,300 monocotyledons;

  1. 74,000-120,000 fungi;

  2. 10,000 lichens;

  3. 1,250,000 animals, including:

    • 1,190,200 invertebrates:

      • 950,000 insects,

      • 70,000 mollusks,

      • 40,000 crustaceans,

      • 130,200 others;

    • 58,808 vertebrates:

      • 29,300 fish,

      • 5,743 amphibians,

      • 8,240 reptiles,

      • 10,234 birds, (9799 extant as of 2006)

      • 5,416 mammals.

Insects make up the vast majority of animal species. However the total number of species for some phyla may be much higher:

  1. 10-30 million insects;

  2. 5-10 million bacteria;

  3. 1.5 million fungi;

  4. ~1 million mites

      1. Distribution of Biodiversity

Biodiversity is not distributed uniformly across the globe. It is consistently richer in the tropics and in other localized regions such as the California Floristic Province. As one approaches Polar Regions one generally finds fewer species. Flora and fauna diversity depends on climate, altitude, soils and the presence of other species. Generally, species diversity per unit area tends to increase with decreasing latitude, with highest diversity found in the tropics. Thus, in terms of natural land cover classes, tropical forests have the highest densities of biodiversity per unit area; desert, tundra, and boreal forests have the lowest. Topographical variations in the landscape lead to higher species diversity, and some highly localized ecosystems, such as wetlands, are also species-rich. Recognition that some areas possess higher levels of biodiversity, and especially endemics (plants or animals that are only found in localized areas), has fueled interest in the identification of biogeographical areas of species richness, and therefore of high conservation value.

Earth is endowed with immensely rich varieties of forms, which are roughly estimated as 20 million. Of these estimated species only 8% (i.e. 1.75 million) have been identified. Amongst 1.75 million identified described organisms, producers constitute fairly negligible proportion (4%), decomposers 15% and consumers 81%. When comparing this proportion to the biomass generated by the three groups of organisms, the significance of the group of producers becomes readily apparent, as they show highest biomass i.e. (90%).

In our country, out of total identified species (microorganisms, plants, and animals), producers, consumers, and decomposers constitute 19.6%, 58.4% and 22.0%, respectively. The country is also rich in endemic species. The endemic plants comprise of 4950 angiosperms and 200 pteridophytes. The endemic animal species comprise of 37 mammal, 50 birds, 152 reptiles, 85 amphibians, 78 fishes and 635 invertebrates.

In the year 2006 large numbers of the Earth's species were formally classified as rare or endangered or threatened species; moreover, many scientists have estimated that there are millions more species actually endangered which have not yet been formally recognized. About 40 percent of the 40,177 species assessed using the IUCN Red List criteria, are now listed as threatened species with extinction - a total of 16,119 species.

      1. Evolution of Biodiversity

Biodiversity found on Earth today is the result of 4 billion years of evolution. The origin of life has not been definitely established by science, however some evidence suggests that life may already have been well-established a few hundred million years after the formation of the Earth. Until approximately 600 million years ago, all life consisted of bacteria and similar single-celled organisms.

The history of biodiversity during the Phanerozoic (the last 540 million years), starts with rapid growth during the Cambrian explosion—a period during which nearly every phylum of multicellular organisms first appeared. Over the next 400 million years or so, global diversity showed little overall trend, but was marked by periodic, massive losses of diversity classified as mass extinction events.


Fig. 3.1 : Apparent marine fossil diversity during the Phanerozoic

The apparent biodiversity shown in the fossil record (Fig. 3.1) suggests that the last few million years include the period of greatest biodiversity in the Earth's history. However, not all scientists support this view, since there is considerable uncertainty as to how strongly the fossil record is biased by the greater availability and preservation of recent geologic sections. Some scientists argue that corrected for sampling artifacts, modern biodiversity is not much different from biodiversity 300 million years ago. Estimates of the present global macroscopic species diversity vary from 2 million to 100 million species, with a best estimate of somewhere near 13-14 million, the vast majority of them arthropods.

Most biologists agree however that the period since the emergence of humans is part of a new mass extinction, the Holocene extinction event, caused primarily by the impact humans are having on the environment. It has been argued that the present rate of extinction is sufficient to eliminate most species on the planet Earth within 100 years.

New species are regularly discovered (on average between 5-10,000 new species each year, most of them insects) and many, though discovered, are not yet classified (estimates are that nearly 90% of all arthropods are not yet classified). Most of the terrestrial diversity is found in tropical forests.

      1. Role of Biodiversity in Ecosystem Function and Stability

There are a multitude of anthropocentric benefits of biodiversity in ecosystem function and stability. Biodiversity is central to an ecocentric philosophy. It is important to understand the reasons for believing in conservation of biodiversity. One way to identify the reasons why we believe in it is to look at what we get from biological diversity and the things that we loose as a result of species extinction, which has taken place over the last 600 years. Mass extinction is the direct result of human activity and not of natural phenomena which is the perception of many modern day thinkers.

Biodiversity provides many ecosystem services that are often not readily visible. It plays a part in regulating the chemistry of our atmosphere and water supply. Biodiversity is directly involved in recycling nutrients and providing fertile soils. Experiments with controlled environments have shown that humans cannot easily build ecosystems to support human needs; for example insect pollination cannot be mimicked by human-made construction, and that activity alone represents tens of billions of dollars in ecosystem services per annum to humankind.

There are many benefits that are obtained from natural ecosystem processes. Some ecosystem services that benefit to society are air quality, climate (both global CO2 sequestration and regional and local), water purification, disease control, biological pest control, pollination and prevention of erosion. Along with those come non- material benefits that are obtained from ecosystems which are spiritual and aesthetic values, knowledge systems and the value of education that we obtain today. However, the public remains unaware of the crisis in sustaining biodiversity. Biodiversity takes a look into the importance to life and provides modern audiences with a clear understanding of the current threat to life on Earth.

In Agriculture

For some foodcrops and other economic crops, wild varieties of the domesticated species can be reintroduced to form a better variety than the previous (domesticated) species. The economic impact is gigantic, for even crops as common as the potato (which was bred through only one variety, brought back from the Inca), a lot more can come from these species. Wild varieties of the potato will all suffer enormously through the effects of climate change. A report by the Consultative Group on International Agricultural Research (CGIAR) describes the huge economic loss. Rice, which has been improved for thousands of years by humans, can through the same process regain some of its nutritional value that has been lost since.

Crop diversity is also necessary to help the system recover when the dominant crop type is attacked by a disease:

  1. The Irish potato blight of 1846, which was a major factor in the deaths of a million people and migration of another million, was the result of planting only two potato varieties, both of which were vulnerable.

  2. When the rice grassy stunt virus struck rice fields from Indonesia to India in the 1970s, 6273 varieties were tested. Only one was luckily found to be resistant, a relatively feeble Indian variety, known to science only since 1966, with the desired trait. It was hybridised with other varieties and now widely grown.

  3. In 1970, coffee rust attacked coffee plantations in Sri Lanka, Brazil, and Central America. A resistant variety was found in Ethiopia, coffee's presumed homeland, which mitigated the rust epidemic.

Monoculture, the lack of biodiversity, was a contributing factor to several agricultural disasters in history, including the Irish Potato Famine, the European wine industry collapse in the late 1800s, and the US Southern Corn Leaf Blight epidemic of 1970.

Higher biodiversity also controls the spread of certain diseases as pathogens will need adapt to infect different species.

Biodiversity provides food for humans. Although about 80 percent of our food supply comes from just 20 kinds of plants, humans use at least 40,000 species of plants and animals a day. Many people around the world depend on these species for their food, shelter, and clothing. There is untapped potential for increasing the range of food products suitable for human consumption, provided that the high present extinction rate can be stopped.

Science and medicine

A significant proportion of drugs are derived, directly or indirectly, from biological sources; in most cases these medicines can not presently be synthesized in a laboratory setting. About 40% of the pharmaceuticals using natural compounds found in plants, animals, and microorganisms. Moreover, only a small proportion of the total diversity of plants has been thoroughly investigated for potential sources of new drugs. Many drugs are also derived from microorganisms.

Through the field of bionics, considerable technological advancement has occurred which would not have without a rich biodiversity. ..

Industrial materials

A wide range of industrial materials are derived directly from biological resources. These include building materials, fibers, dyes, resins, gums, adhesives, rubber and oil. There is enormous potential for further research into sustainably utilizing materials from a wider diversity of organisms.

Leisure, cultural and aesthetic value

Many people derive value from biodiversity through leisure activities such as hiking in the countryside, birdwatching or natural history study. Biodiversity has inspired musicians, painters, sculptors, writers and other artists. Many cultural groups view themselves as an integral part of the natural world and show respect for other living organisms.

Popular activities such as gardening, caring for aquariums and collecting butterflies are all strongly dependent on biodiversity. The number of species involved in such pursuits is in the tens of thousands, though the great majority do not enter mainstream commercialism.

The relationships between the original natural areas of these often 'exotic' animals and plants and commercial collectors, suppliers, breeders, propagators and those who promote their understanding and enjoyment are complex and poorly understood. It seems clear, however, that the general public responds well to exposure to rare and unusual organisms-- they recognize their inherent value at some level, even if they would not want the responsibility of caring for them. A family outing to the botanical garden or zoo is as much an aesthetic or cultural experience as it is an educational one.

Philosophically it could be argued that biodiversity has intrinsic aesthetic and/ or spiritual value to mankind in and of itself. This idea can be used as a counterweight to the rather notion that tropical forests and other ecological realms are only worthy of conservation because they may contain medicines or useful products.

    1. SPECIATION

Speciation is the process by which new species of organisms arise. Earth is inhabited by millions of different organisms, all of which likely arose from one early life-form that came into existence about 3.5 billion years ago. It is the task of taxonomists to decide which out of the multitude of different types of organisms should be considered species. The wide range in the characteristics of individuals within groups makes defining a species more difficult. Indeed, the definition of species itself is open to debate.

      1. Concepts of Species

In the broadest sense, a species can be defined as a group of individuals that is "distinct" from another group of individuals. Several different views have been put forward about what constitutes an appropriate level of difference. Principal among these views are the biological-species concept and the morphological-species concept.

The biological-species concept delimits species based on breeding. Members of a single species are those that interbreed to produce fertile offspring or have the potential to do so. The morphological-species concept (from the ancient Greek root "morphos," meaning form) is based on classifying species by a difference in their form or function. According to this concept, members of the same species share similar characteristics. Species that are designated by this criterion are known as a morphological species.

Organisms within a species do not necessarily look identical. For example, the domestic dog is considered to be one species, even though there is a huge range in size and appearance among the different breeds. For naturally occurring populations of organisms that we are much less familiar with, it is much more difficult to recognize the significance of any character differences observed. Therefore deciding what characteristics should be used, as criteria to designate a species can be difficult.

      1. Speciation : Natural Selection and Genetic Drift

Before the development of the modern theory of evolution, a widely held idea regarding the diversity of life was the "typological" or "essentialist" view. This view held that a species at its core had an unchanging perfect "type" and that any variations on this perfect type were imperfections due to environmental conditions. Charles Darwin (1809-1882) and Alfred Russel Wallace (1823-1913) independently developed the theory of evolution by natural selection, now commonly known as Darwinian evolution.

The theory of Darwinian evolution is based on two main ideas. The first is that heritable traits that confer an advantage to the individual that carries them will become more widespread in a population through natural selection because organisms with these favorable traits will produce more offspring. Since different environments favor different traits, Darwin saw that the process of natural selection would, over time, make two originally similar groups become different from one another, ultimately creating two species from one. This led to the second major idea, which is that all species arise from earlier species, therefore sharing a common ancestor.

When so much change occurs between different groups that they are morphologically distinct or no longer able to interbreed, they may be considered different species; this process is known as speciation. A species as a whole can transform over time into a new species (vertical evolution) or split into more separate populations, each of which may develop into new species (adaptive radiation).

Modern population geneticists recognize that natural selection is not the only factor causing genetic change in a population over time. Genetic drift is the random change in the genetic composition of a small population over time, due to an unequal genetic contribution by individuals to succeeding generations. It is thought that genetic drift can result in new species, especially in small isolated populations.

      1. Isolating Mechanisms

Whether natural selection and genetic drift lead to new species depends on whether there is restricted gene flow between different groups. Gene flow is the movement of genes between separate populations by migration of individuals. If two populations remain in contact, gene flow will prevent them from becoming separate species (though they may both develop into a new species through vertical evolution).

Gene flow is restricted through geographic effects such as mountain ranges and oceans, leading to geographic isolation. Gene flow can also be prevented by biological factors known as isolating mechanisms. Biological isolating mechanisms include differences in behavior (especially mating behavior), and differences in habitat use, both of which lead to a decrease in mating between individuals from different groups.

When geographic separation plays a role in speciation, this is known as allopatric speciation, from the Greek roots allo, meaning separate, and "patric," meaning country. In allopatric speciation, natural selection and genetic drift can act together.

For example, imagine a mudslide that causes a river to back up into a valley, separating a population of rodents into two, one restricted to the shady side of the river, the other to the sunny side. Because coat thickness is a genetically inherited trait, eventually, through natural selection, the population of animals on the cooler side may develop thicker coats. After many generations of separation, the two groups may look quite different and may have evolved different behaviors as well, to allow them to survive better in their respective habitats. Genetic drift may occur especially if either or both populations remain small. Eventually these two populations may be so different as to warrant designation as different species.

It is also possible for new species to form from a single population without any geographic separation. This is known as "ecological" or "sympatric" (from the Greek root sym, meaning same) speciation, and it results in ecological differences between morphologically similar species inhabiting the same area. Sympatric speciation can occur in flowering plants in a single generation, due to the formation of a polyploid. Polyploidy is the complete duplication of an organism's genome, for example from n chromosomes to 4n. Even higher multiples of n are possible. This increase in a plant's DNA content makes it reproductively incompatible with other individuals of its former species.

Formation of new and distinct species, whereby a single evolutionary line splits into two or more genetically independent ones. One of the fundamental processes of evolution, speciation may occur in many ways. Investigators formerly found evidence for speciation in the fossil record by tracing sequential changes in the structure and form of organisms. Genetic studies now show that such changes do not always accompany speciation, since many apparently identical groups are in fact reproductively isolated (i.e., they can no longer produce viable offspring through interbreeding). Polyploidy is a means by which the beginnings of new species are created in just two or three generations.

Speciation is the evolutionary process by which new biological species arise. There are four modes of natural speciation, based on the extent to which speciating populations are geographically isolated from one another: allopatric, peripatric, parapatric and sympatric. Speciation may also be induced artificially, through animal husbandry or laboratory experiments. Observed examples of each kind of speciation are provided throughout.

      1. Natural speciation

All forms of natural speciation have taken place over the course of evolution, though it still remains a subject of debate as to the relative importance of each mechanism in driving biodiversity.

There is debate as to the rate at which speciation events occur over geologic time. While some evolutionary biologists claim that speciation events have remained relatively constant over time, some paleontologists such as Niles Eldredge and Stephen Jay Gould have argued that species usually remain unchanged over long stretches of time, and that speciation occurs only over relatively brief intervals, a view known as punctuated equilibrium.
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