If an alien from a distant galaxy were to visit our planet
Earth, the first thing that would amaze and baffle him
would most probably be the enormous diversity of life
that he would encounter. Even for humans, the rich variety
of living organisms with which they share this planet never
ceases to astonish and fascinate us. The common man
would find it hard to believe that there are more than
20,000 species of ants, 3,00,000 species of beetles, 28,000
species of fishes and nearly 20,000 species of orchids.
Ecologists and evolutionary biologists have been trying
to understand the significance of such diversity by asking
important questions– Why are there so many species?
Did such great diversity exist throughout earth’s history?
How did this diversification come about? How and why
is this diversity important to the biosphere? Would it
function any differently if the diversity was much less?
How do humans benefit from the diversity of life?
In our biosphere immense diversity (or heterogeneity)
exists not only at the species level but at all levels of
biological organisation ranging from macromolecules
within cells to biomes. Biodiversity is the term popularised
by the sociobiologist Edward Wilson to describe the
15.1 Biodiversity
15.2 Biodiversity Conservation
combined diversity at all the levels of biological organisation.
The most important of them are–
(i) Genetic diversity: A single species might show high diversity at
the genetic level over its distributional range. The genetic variation
shown by the medicinal plant Rauwolfia vomitoria growing in
different Himalayan ranges might be in terms of the potency and
concentration of the active chemical (reserpine) that the plant
produces. India has more than 50,000 genetically different strains
of rice, and 1,000 varieties of mango.
(ii) Species diversity: The diversity at the species level, for example,
the Western Ghats have a greater amphibian species diversity than
the Eastern Ghats.
(iii) Ecological diversity: At the ecosystem level, India, for instance,
with its deserts, rain forests, mangroves, coral reefs, wetlands,
estuaries, and alpine meadows has a greater ecosystem diversity
than a Scandinavian country like Norway.
It has taken millions of years of evolution, to accumulate this rich
diversity in nature, but we could lose all that wealth in less than two
centuries if the present rates of species losses continue. Biodiversity and
its conservation are now vital environmental issues of international concern
as more and more people around the world begin to realise the critical
importance of biodiversity for our survival and well- being on this planet.
15.1.1 How Many Species are there on Earth and How
Many in India?
Since there are published records of all the species discovered and named,
we know how many species in all have been recorded so far, but it is not
easy to answer the question of how many species there are on earth.
According to the International Union for Conservation of Nature and
Natural Resources (IUCN) (2004), the total number of plant and animal
species described so far is slightly more than 1.5 million, but we have no
clear idea of how many species are yet to be discovered and described.
Estimates vary widely and many of them are only educated guesses. For
many taxonomic groups, species inventories are more complete in
temperate than in tropical countries. Considering that an overwhelmingly
large proportion of the species waiting to be discovered are in the tropics,
biologists make a statistical comparison of the temperate-tropical species
richness of an exhaustively studied group of insects and extrapolate this
ratio to other groups of animals and plants to come up with a gross
estimate of the total number of species on earth. Some extreme estimates
range from 20 to 50 million, but a more conservative and scientifically
sound estimate made by Robert May places the global species diversity
at about 7 million.
Let us look at some interesting aspects about earth’s biodiversity based
on the currently available species inventories. More than 70 per cent of
all the species recorded are animals, while plants (including algae, fungi,
bryophytes, gymnosperms and angiosperms) comprise no more than 22
per cent of the total. Among animals, insects are the most species-rich
taxonomic group, making up more than 70 per cent of the total. That
means, out of every 10 animals on this planet, 7 are insects. Again, how
do we explain this enormous diversification of insects? The number of
fungi species in the world is more than the combined total of the species
of fishes, amphibians, reptiles and mammals. In Figure 15.1, biodiversity
is depicted showing species number of major taxa.
Figure 15.1 Representing global biodiversity: proportionate number of
species of major taxa of plants, invertebrates and vertebrates
It should be noted that these estimates do not give any figures for
prokaryotes. Biologists are not sure about how many prokaryotic species
there might be. The problem is that conventional taxonomic methods are
not suitable for identifying microbial species and many species are simply
not culturable under laboratory conditions. If we accept biochemical or
molecular criteria for delineating species for this group, then their diversity
alone might run into millions.
Although India has only 2.4 per cent of the world’s land area, its share
of the global species diversity is an impressive 8.1 per cent. That is what
makes our country one of the 12 mega diversity countries of the world.
Nearly 45,000 species of plants and twice as many of animals have been
recorded from India. How many living species are actually there waiting
to be discovered and named? If we accept May’s global estimates, only
22 per cent of the total species have been recorded so far. Applying this
proportion to India’s diversity figures, we estimate that there are probably
more than 1,00,000 plant species and more than 3,00,000 animal species
yet to be discovered and described. Would we ever be able to complete
the inventory of the biological wealth of our country? Consider the immense
trained manpower (taxonomists) and the time required to complete the
job. The situation appears more hopeless when we realise that a large
fraction of these species faces the threat of becoming extinct even before
we discover them. Nature’s biological library is burning even before we
catalogued the titles of all the books stocked there.
15.1.2 Patterns of Biodiversity
(i) Latitudinal gradients: The diversity of plants and animals is
not uniform throughout the world but shows a rather uneven
distribution. For many group of animals or plants, there are
interesting patterns in diversity, the most well- known being the
latitudinal gradient in diversity. In general, species diversity
decreases as we move away from the equator towards the poles.
With very few exceptions, tropics (latitudinal range of 23.5° N to
23.5° S) harbour more species than temperate or polar areas.
Colombia located near the equator has nearly 1,400 species of birds
while New York at 41° N has 105 species and Greenland at 71° N
only 56 species. India, with much of its land area in the tropical
latitudes, has more than 1,200 species of birds. A forest in a tropical
region like Equador has up to 10 times as many species of vascular
plants as a forest of equal area in a temperate region like the Midwest
of the USA. The largely tropical Amazonian rain forest in South
America has the greatest biodiversity on earth- it is home to more
than 40,000 species of plants, 3,000 of fishes, 1,300 of birds, 427
of mammals, 427 of amphibians, 378 of reptiles and of more than
1,25,000 invertebrates. Scientists estimate that in these rain forests
there might be at least two million insect species waiting to be
discovered and named.
What is so special about tropics that might account for their greater
biological diversity? Ecologists and evolutionary biologists have
proposed various hypotheses; some important ones are (a) Speciation
is generally a function of time, unlike temperate regions subjected
to frequent glaciations in the past, tropical latitudes have remained
relatively undisturbed for millions of years and thus, had a long
evolutionary time for species diversification, (b) Tropical environments,
unlike temperate ones, are less seasonal, relatively more constant
and predictable. Such constant environments promote niche
specialisation and lead to a greater species diversity and (c) There
is more solar energy available in the tropics, which contributes to
higher productivity; this in turn might contribute indirectly to greater
(ii) Species-Area relationships: During his pioneering and extensive
explorations in the wilderness of South American jungles, the great
German naturalist and geographer Alexander von Humboldt
observed that within a region species
richness increased with increasing
explored area, but only up to a limit. In
fact, the relation between species richness
and area for a wide variety of taxa
(angiosperm plants, birds, bats,
freshwater fishes) turns out to be a
rectangular hyperbola (Figure15.2). On
a logarithmic scale, the relationship is a
straight line described by the equation
log S = log C + Z log A
S= Species richness A= Area
Z = slope of the line (regression
C = Y-intercept
Ecologists have discovered that the
value of Z lies in the range of 0.1 to 0.2,
regardless of the taxonomic group or the
region (whether it is the plants in Britain,
birds in California or molluscs in New York state, the slopes of the regression
line are amazingly similar). But, if you analyse the species-area
relationships among very large areas like the entire continents, you will
find that the slope of the line to be much steeper (Z values in the range
of 0.6 to 1.2). For example, for frugivorous (fruit-eating) birds and
mammals in the tropical forests of different continents, the slope is found
to be 1.15. What do steeper slopes mean in this context?
15.1.3 The importance of Species Diversity to the Ecosystem
Does the number of species in a community really matter to the functioning
of the ecosystem?This is a question for which ecologists have not been
able to give a definitive answer. For many decades, ecologists believed
that communities with more species, generally, tend to be more stable
than those with less species. What exactly is stability for a biological
Figure 15.2 Showing species area relationship.
Note that on log scale the relationship
becomes linear
community? A stable community should not show too much variation
in productivity from year to year; it must be either resistant or resilient to
occasional disturbances (natural or man-made), and it must also be
resistant to invasions by alien species. We don’t know how these attributes
are linked to species richness in a community, but David Tilman’s
long-term ecosystem experiments using outdoor plots provide some
tentative answers. Tilman found that plots with more species showed
less year-to-year variation in total biomass. He also showed that in his
experiments, increased diversity contributed to higher productivity.
Although, we may not understand completely how species richness
contributes to the well-being of an ecosystem, we know enough to realise
that rich biodiversity is not only essential for ecosystem health but
imperative for the very survival of the human race on this planet. At a
time when we are losing species at an alarming pace, one might ask–
Does it really matter to us if a few species become extinct? Would Western
Ghats ecosystems be less functional if one of its tree frog species is lost
forever? How is our quality of life affected if, say, instead of 20,000 we
have only 15,000 species of ants on earth?
There are no direct answers to such näive questions but we can develop
a proper perspective through an analogy (the ‘rivet popper hypothesis’)
used by Stanford ecologist Paul Ehrlich. In an airplane (ecosystem) all
parts are joined together using thousands of rivets (species). If every
passenger travelling in it starts popping a rivet to take home (causing a
species to become extinct), it may not affect flight safety (proper functioning
of the ecosystem) initially, but as more and more rivets are removed, the
plane becomes dangerously weak over a period of time. Furthermore,
which rivet is removed may also be critical. Loss of rivets on the wings
(key species that drive major ecosystem functions) is obviously a more
serious threat to flight safety than loss of a few rivets on the seats or
windows inside the plane.
15.1.4 Loss of Biodiversity
While it is doubtful if any new species are being added (through speciation)
into the earth’s treasury of species, there is no doubt about their continuing
losses. The biological wealth of our planet has been declining rapidly
and the accusing finger is clearly pointing to human activities. The
colonisation of tropical Pacific Islands by humans is said to have led to
the extinction of more than 2,000 species of native birds. The IUCN Red
List (2004) documents the extinction of 784 species (including 338
vertebrates, 359 invertebrates and 87 plants) in the last 500 years. Some
examples of recent extinctions include the dodo (Mauritius), quagga
(Africa), thylacine (Australia), Steller’s Sea Cow (Russia) and three
subspecies (Bali, Javan, Caspian) of tiger. The last twenty years alone
have witnessed the disappearance of 27 species. Careful analysis of records