Concept and Levels of Biological Diversity

BIOLOGICAL DIVERSITY- 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..

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 : 

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 = P_i²

Where, P_i = the proportion of individuals in the i^th species.

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.  

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:

o      15,000 mosses,

o      13,025 ferns,

o      980 gymnosperms,

o      199,350 dicotyledons,

o      59,300 monocotyledons;

2.                  74,000-120,000 fungi;

3.                  10,000 lichens;

4.                  1,250,000 animals, including:

o      1,190,200 invertebrates:

§    950,000 insects,

§    70,000 mollusks,

§    40,000 crustaceans,

§    130,200 others;

o      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:

5.                  10-30 million insects;

6.                  5-10 million bacteria;

7.                  1.5 million fungi;

8.                  ~1 million mites

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.

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.

The apparent biodiversity shown in the fossil record  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.