Causes of biodiversity
The major causes for biodiversity decline are:
Habitat loss, fragmentation and destruction (affects about 73% of
Pollution and pollutants (smog, pesticides, herbicides, oil
Introduction of alien/exotic species
Over exploitation of resources (poaching, indiscriminate cutting
of trees, over fishing, hunting, mining)
Intensive agriculture and aquacultural practices
Hybridization between native and non-native species and loss of
Natural disasters (Tsunami, forest fire, earth quake, volcanoes)
Industrialization, Urbanization, infrastructure development,
Transport – Road and Shipping activity, communication towers, dam construction,
unregulated tourism and monoculture are common area of specific threats
Development of human society is inevitable.
Natural habitats are destroyed for the purpose of settlement, agriculture,
mining, industries and construction of highways. As a result species are forced
to adapt to the changes in the environment or move to other places. If not,
they become victim to predation, starvation, disease and eventually die or
results in human animal conflict.Over population, urbanization,
industrialization and agricultural advancements require additional land, water
and raw materials every year. This is made possible only through fragmentation
or destruction of natural habitats by filling wetlands, ploughing grasslands,
cutting down trees, forest, desilting rivers, constructing transport ways,
caving mountains, extracting, ores, changing the course of rivers and filling
The most dramatic example of habitat loss comes
from the tropical rainforests 14% of the earth’s land surface once covered by
these tropical forests, is not more than 6% now. The Amazon rainforest, a vast
area, harbouring millions of species, also called “Lungs of the planet”is
destroyed and being replaced for agriculture and human settlements. 90% of New
Zealand’s wetlands have been destroyed and cleared for cultivating soya beans
and raising grass for beef cattle. Kodaikanal and Nilgiri hills of Tamil Nadu
have been destroyed rapidly for human occupancy. Loss of habitat results in
annihilation of plants, microorganisms and forcing out animals from their
Habitat fragmentation is the process where a
large, continuous area of habitat is both, reduced in area and divided into two
or more fragments. Fragmentation of habitats like forest land into crop lands,
orchard lands, plantations, urban areas, industrial estates, transport and
transit systems has resulted in the destruction of complex interactions amongst
species, (food chain and webs) destruction of species in the cleared regions,
annihilation of species restricted to these habitats (endemic) and decreased
biodiversity in the habitat fragments. Animals requiring large territories such
as mammals and birds are severely affected. The elephant corridors and
migratory routes are highly vulnerable. The dwindling of many well-known birds
(sparrows) and animals can be attributed to this.
We depend on nature for our basic needs such as
food and shelter. However, when the need becomes greed, it leads to over
exploitation of natural resources. Excessive exploitation of a species, reduces
the size of its population to such a level that it becomes vulnerable to
extinction. Dodo, passenger pigeon and Steller’s sea cow have become extinct in
the last 200-300 years due to over exploitation by humans. Overfishing due to
population pressure leads to many marine fish (populations) declining around
Exotic species (non-native; alien) are organisms
often introduced unintentionally or deliberately for commercial purpose, as
biological control agents and other uses. They often become invasive and drive
away the local species and is considered as the second major cause for
extinction of species. Exotic species have proved harmful to both aquatic and
Tilapia fish (Jilabi kendai) (Oreochromis
mosambicus) introduced from east coast of South Africa in 1952
for its high productivity into Kerala’s inland waters, became invasive, due to
which the native species such as Puntius dubius and Labeo
kontius face local extinction. Amazon sailfin catfish is responsible
for destroying the fish population in the wetlands of Kolkata. The introduction
of the Nile Perch, a predatory fish into Lake Victoria in East Africa led to
the extinction of an ecologically unique assemblage of more than 200 nature
species of cichlid fish in the lake.
African apple snail (Achatina fulica) is
the most invasive among all alien fauna in India. This mollusc was first
reported in the Andaman and Nicobar Islands. It is now found across the country
and threatens the habitat of several native species. Moreover it is becoming a
vicious pest in vegetable farms.
Exotic earthworms compete for food with native
varieties and deplete their population in soil. Papaya Mealy Bug (Paracoccus
marginatus) is native of Mexico and Central America, is believed to have
destroyed huge crops of papaya in Assam, West Bengal and TamilNadu.
Industrialization is a major contributor to
climate change and a major threat to biodiversity. Energy drives our
industries, which is provided by burning of fossil fuels. This increases the
emission of CO2, a GHG, leading to climate change. Due to large scale
deforestation, the emitted CO2 cannot be absorbed fully, and its concentration
in the air increases. Climate change increases land and ocean temperature,
changes precipitation patterns and raises the sea level. This inturn results in
melting of glaciers, water inundation, less predictability of weather patterns,
extreme weather conditions, outbreak of squalor diseases, migration of animals
and loss of trees in forest. Thus, climate change is an imminent danger to the
existing biodiversity (Fig. 12.4).
In shifting cultivation, plots of natural tree
vegetation are burnt away and the cleared patches are farmed for 2- 3 seasons,
after which their fertility reduces to a point where crop production is no
longer profitable. The farmer then abandons this patch and cuts down a new
patch of forest trees elsewhere for crop production.This system is practiced in
north-eastern regions of India. When vast areas are cleared and burnt, it
results in loss of forest cover, pollution and discharge of CO2 which in turn
attributes to loss of habitat and climate change which has an impact on the
faunal diversity of that regions.
Coextinction of a species is the loss of a
species as a consequence of the extinction of another. (Eg., orchid bees and
forest trees by cross pollination). Extinction of one will automatically cause
extinction of the other. Another example for co-extinction is the connection
between Calvaria tree and the extinct bird of Mauritius Island, the Dodo. The
Calvaria tree is dependent on the Dodo bird for completion of its life cycle.
The mutualistic association is that the tough horny endocarp of the seeds of
Calvaria tree are made permeable by the actions of the large stones in bird's
gizzard and digestive juices thereby facilitating easier germination. The
extinction of the Dodo bird led to the imminent danger of the Calvaria tree
Pollutants and pollution are a major cause for
biodiversity loss. Excessive use of fertilisers, pesticides and heavy metals
have polluted the land, ground and surface water bodies. There is a tendency of
pesticide biomagnification which results in high concentrations at higher
trophic levels which has resulted in drastic decline in the population of fish
eating birds and falcons. Run off from fertilizer rich fields causes nutrient
enrichment of water bodies leading to eutrophication. Mercury, arsenic,
cadmium, chromium poisoning has led to depletion of biotic resources in
vulnerable ecosystems. Death of vulture population is attributed to the
veterinary medicine Diclofenac, which is responsible for the thinning of the
Spread of agriculture is sometimes at the cost
of wetlands, grasslands and forests. Intensive agriculture is based on a few
high yielding varieties. As a result, there is reduction in the genetic
diversity. It also increases vulnerability of the crop plants to sudden attack
by pathogens and pests. There are only few varieties of traditional paddy
strains today due to use to hybrid varieties in Tamil Nadu.
There is a tendency to grow economically
important and viable trees like Teak, Sandal, Oak, Sal in forests resulting in
loss of other forest trees.
These include spontaneous jungle fires, tree
fall, land slide, defoliation by insects or locust attack.
Species have been evolving and dying out
(extinction) ever since the origin of life. However, species are now becoming
extinct at a faster rate. This is destabilizing the ecological stability and
the distribution of biological diversity on earth. Human activities greatly
contribute to the loss of biodiversity. Natural resources such as land, water
and organisms are indiscriminately exploited by human beings.
According to the Convention of Biological
Diversity, direct and indirect human activities have a detrimental effect on
biodiversity. Direct human activities like change in local land use, species
introduction or removal, harvesting, pollution and climate change contribute a
greater pressure on loss of biodiversity. Indirect human drivers include
demographic, economic, technological, cultural and religious factors.
Even though new species are being discovered,
there is little hope for adding new species through speciation into the
biodiversity treasure. Monsoon failure, global warming, depletion in ozone
layer, landslides in hilly states, pollution are a few indirect effects of
human activities which results in the loss biodiversity. IUCN Red List (2004)
documents the extinction of 784 species in the 500 years.
It is estimated that the current rate of
biodiversity loss is 100 to 1000 times higher than the naturally occurring
extinction rate and is still expected to grow in the future. This loss of
biodiversity has a immense impact on plant animal and human life. The negative
effects include dramatic influence on the food web. Even reduction in one
species can adversely affect the entire food chain which further leads to an
overall reduction in biodiversity. Reduced biodiversity leads to immediate
danger for food security by reducing ecosystem services.
Hotspots are areas characterized with high
concentration of endemic species experiencing unusual rapid rate of habitat
modification loss. Norman Myers defined hot spots as “regions that harbour a
great diversity of endemic species and at the same time, have been
significantly impacted and altered by human activities.”
A hotspot is a region that supports at least
1500 endemic vascular plant species (0.5% of the global total) has lost more
than 70% of its original vegetation. There are 35 biodiversity hotspots in the
world. India is home to four biodiversity hotspots (as per ENVIS). They are
Himalaya (the entire Indian Himalayan region)
Indo-Burma: includes entire North-eastern India, except Assam and
Andaman group of Islands (and Myanmar, Thailand, Vietnam, Laos, Cambodia and
Sundalands: includes Nicobar group of Islands (and Indonesia,
Malaysia, Singapore, Brunei, Philippines)
A species that has been categorized as very
likely to become extinct is an Endangered species. Endangered (EN), as
categorized by the International Union for Conservation of Nature (IUCN) Red
List, is the second most severe conservation status for wild populations in the
IUCN's scheme after Critically Endangered (CR).
In 1998 there were1102 animal and 1197 plant
species in the IUCN Red List. In 2012, the list features 3079 animal and 2655
plant species as endangered (EN) worldwide.
Species is considered extinct when none of its
members are alive anywhere in the world. If individuals of a species remain
alive only in captivity or other human controlled conditions, the species is
said to be extinct in the wild. In both of these situations, the species would
be considered globally extinct. A species in considered to be locally extinct
when it is no longer found in an area it once inhabited but is still found
elsewhere in the wild.
In the 450 million years of life on Earth, there
had been 5 mass extinctions, which had eliminated at least 50% of the species
of flora and fauna on the globe.
The extinction of species is mainly due to
drastic environmental changes and population characteristics.
There are three types of Extinctions
i. Natural extinction is a slow process of
replacement of existing species with better adapted species due to changes in
environmental conditions, evolutionary changes, predators and diseases. A small
population can get extinct sooner than the large population due to inbreeding
depression (less adaptivity and variation).
ii. Mass extinction: The earth has
experienced quite a few mass extinctions due to environmental
catastrophes. A mass extinction occurred about 225 million years ago during the
Permian, where 90% of shallow water marine invertebrates disappeared.
iii. Anthropogenic extinctions These are abetted
by human activities like hunting, habitat destruction, over exploitation,
urbanization and industrialization. Some examples of extinctions are Dodo of Mauritius
and Steller’s sea cow of Russia. Amphibians seem to be at higher risk of
extinction because of habitat destruction.
The most serious aspect of the loss of
biodiversity is the extinction of species. The unique information contained in
its genetic material (DNA) and the niche it possesses are lost forever.
One more species goes extinct...
George the tree snail (Achatinella apexfulva)
died on January 1, 2019, at the age of 14. He was the last snail of his species,
and is emblematic of the loss of native Hawaiian molluscs.