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Module IV – Threats to biodiversity (5 hrs)
Types of threats
Habitat loss, man- wildlife conflict (with case studies)
Invasive species
Over exploitation and human population
Climate change
Important factors leading to extinction of species and consequent loss of biodiversity are:
habitat loss and fragmentation, introduction of non-native species, overexploitation, soil, water
and atmospheric pollution, and intensive agriculture and forestry.
Habitat loss
Habitat loss in the terrestrial domain has been caused largely by the expansion of
agriculture: more than 30 per cent of land has been converted for agricultural production. Largescale commercial agriculture has adversely affected biodiversity, particularly agro-biodiversity.
Moreover, the growing demand for biofuels has taken a toll, with expanses of forests and natural
lands in South East Asia being converted into mono-crop plantations.
Direct habitat loss is a major threat to coastal ecosystems through aquaculture. Wetlands
in particular have faced a 50 per cent loss in the 20th century. Freshwater ecosystems are severely
affected by fragmentation. Benthic habitats have been degraded as a consequence of bottom
trawling and other destructive fishing methods
Invasive alien species
Invasive alien species threaten native biodiversity and are spreading through both
deliberate and unintentional introductions as a consequence of increasing levels of global travel
and trade. Poorly planned economic introductions, air transport, hull-fouling and ballast water
from ships, as well as trade in pets, garden plants and aquarium species, are significant pathways
for the dispersal of invasive species. Invasive alien species affect native species principally
through predation, competition and habitat modification.
They are found in nearly all countries and habitats, including marine ecosystems – for example
Pollutants such as pesticide and fertilizer effluents from agriculture and forestry, industry
including mining and oil or gas extraction, sewage plants, run-off from urban and suburban
areas, and oil spills, harm biodiversity directly through mortality and reduced reproductive
success, and also indirectly through habitat degradation. Inland wetlands and coastal marine
habitats face a major threat from waterborne pollutants. Meanwhile, atmospheric pollution in
terrestrial systems, particularly the deposition of eutrophying and acidifying compounds such as
nitrogen and sulphur is also important. Rates of nitrogen deposition increased sharply after 1940
but have levelled out since 1990, probably owing to an overall decrease in biomass burning,
though there is regional variation. Nevertheless, nitrogen deposition continues to be a significant
threat to biodiversity, especially for species that have adapted to low-nitrogen habitats
Overexploitation of wild species to meet consumer demand threatens biodiversity, with
unregulated overconsumption contributing to declines in terrestrial, marine and freshwater
ecosystems. In terrestrial systems, major exploited groups include plants for timber, food and
medicine; mammals for wild meat and recreational hunting; birds for food and the pet trade; and
amphibians for traditional medicine and food The threat to vertebrates from overexploitation is
particularly severe, driven, in particular, by demand for wildlife and wildlife products from East
In the marine realm, capture fisheries more than quadrupled their catch from the early
1950s to the mid-1990s. Since then, catches have stabilized or diminished, despite increased
fishing effort The proportion of marine fish stocks that are overexploited, depleted or recovering
from depletion rose from 10 per cent in 1974 to 32 per cent in 2008. Of the 133 local, regional
and global extinctions of marine species documented worldwide over the last 200 years, 55 per
cent were caused by overexploitation, while the remainder was driven by habitat loss and other
threats. The use of destructive fishing practices further amplifies the impacts of unsustainable
fishing on marine biodiversity and habitats.
Climate change
Climate change is an increasingly important threat to species and natural habitats. There
is widespread evidence that changes in phenology, including the timing of reproduction and
migration, physiology, behaviour, morphology, population density and distributions of many
different types of species are driven by climate change.
1. In the Arctic, tundra habitats are shrinking owing to tree-line advance
2. In the marine realm, climate change is causing widespread die-off of coral reefs through
rising temperatures and ocean acidification.
3. The Arctic ice cap is also shrinking rapidly, with likely impacts on ice-dependent species
4. For many wetlands, changes in rainfall and evaporation are expected to have major
impacts on water regimes, affecting both migratory and residential species.
5. Climate change will also act synergistically with other threats, such as the spread of
diseases and invasive alien species.
The ecological footprint
The ecological footprint is a resource accounting tool that measures how much
biologically productive land and sea area – crop and grazing land, forests, fishing grounds and
built-up land – is demanded by a given population or activity, and compares this to how much
land and sea is available. It has become an increasingly popular headline indicator of broad
human pressures on the environment. Ecological footprint analysis shows that the global demand
for biologically productive areas has approximately doubled since the 1960s (WWF 2010). In
2007, global society demanded more than 1.5 planets’ worth of productive biological capacity, a
deficit that can only be met through the depletion of stocks of renewable resources or the
accumulation of waste product, most importantly carbon dioxide (CO2) in the atmosphere.
Genetically modified organism (GMO)
A genetically modified organism (GMO) is defined by the Cartagena Protocol on
Biosafety as any living organism that possesses a novel combination of genetic material obtained
through the use of modern biotechnology (CBD 2000); generally by the transfer of genetic
material from one species to another. The vast proportion of GM crops has been modified to be
tolerant of broad-spectrum herbicides to allow more efficient weed control and/or to express a
toxin (Bt) that acts against the caterpillars of butterflies and moths that live and feed inside the
crop plants.
Genetic modification (GM) remains controversial, both a potential threat and an
opportunity for biodiversity conservation, depending on the context. The technology is widely
used in pharmaceuticals and crop production, but many consider it an unwarranted risk to the
environment and human health.
Environmental risks from GMOs
1. Loss of genetic diversity of agricultural species and their wild relatives through gene
flow, although this also occurs with non-GM crops.
2. Another concern is the effects on organisms that are not the target of the GM trait,
although Bt crops have few toxic effects on non-target species as the Bt toxins produced
are highly specific and only expressed in the plant itself.
3. GM crops tolerant of broad-spectrum herbicides such as glyphosate often result in fewer
weeds than conventional crops, and therefore make less food available to farmland birds.
4. In addition, species are evolving resistance to both glyphosate and Bt.
Habitat Loss and Fragmentation
The destruction of habitats is the primary reason for the loss of biodiversity. When people
cut down trees, fill a wetland, plough grassland or burn a forest, the natural habitat of a species is
changed or destroyed. These changes can kill or force out many plants, animals, and
microorganisms, as well as disrupt complex interactions among the species. A forest patch
surrounded by croplands, orchards, plantations, or urban areas is an example of fragmented
habitats. With the fragmentation of a large forest tract, species occupying deeper parts of forests
are the first to disappear. Overexploitation of a particular species reduces the size of its
population to an extent that it becomes vulnerable to extinction.
Disturbance and Pollution
Communities are affected by natural disturbances, such as fire, tree fall, and defoliation
by insects. Man-made disturbances differ from natural disturbances in intensity, rate and spatial
extent. For example, man by using fire more frequently may change species richness of a
community. Then, some human impacts are new, never before faced by biota, e.g. the vast
number of synthetic compounds, massive releases of radiation or spillover of oil in sea. These
impacts lead to a change in the habitat quality. Pollution may reduce and eliminate populations
of sensitive species. For example, pesticide linked decline of fish-eating birds and falcons. Lead
poisoning is another major cause of mortality of many species, such as ducks, swans and cranes,
as they ingest the spent shotgun pellets that fall into lakes and marshes. Eutrophication (nutrient
enrichment) of water bodies drastically reduces species diversity.
Introduction of Exotic Species
New species entering a geographical region are called exotic or alien species.
Introduction of such invasive species may cause disappearance of native species through
changed biotic interactions. Invasive species are considered second only to habitat destruction as
a major cause of extinction of species. Exotic species are having largeimpact especially in island
ecosystems, which harbour much of the world’s threatened biodiversity. A few examples are: (1)
Nile perch, an exotic predatory fish introduced into Lake Victoria (South Africa) threatens the
entire ecosystem of the lake by eliminating several native species of the small Cichlid fish
species that were endemic to this freshwater aquatic system. Water hyacinth clogs rivers and
lakes and threatens the survival of many aquatic species in lakes and river flood plains in several
tropical countries including India. Lantana camara has invaded many forest lands in different
parts of India, and strongly competes with the native species.
Extinction of Species
Extinction is a natural process. Species have disappeared and new ones have evolved to
take their place over the long geological history of the earth. It is useful to distinguish three types
of extinction processes.
Natural extinction: With the change in environmental conditions, some species disappear and
others, which are more adapted to changed conditions, take their place. This loss of species
which occurred in the geological past at a very slow rate is called natural or background
Mass extinction: There have been several periods in the earth’s geological history when large
number of species became extinct because of catastrophes. Mass extinctions occurred in millions
of years. Anthropogenic extinction: An increasing number of species is disappearing from the
face of the earth due to human activities. This man-made mass extinction represents a very
severe depletion of biodiversity, particularly because it is occurring within a short period of time.
The World Conservation Monitoring Centre has recorded that 533 animal (mostly
vertebrates) and 384 plant species (mostly flowering plants) have become extinct since the year
1600. More species have gone extinct from the islands than from the mainland or the oceans. The
current rate of extinction is 1000 to 10000 times higher than the background rate of extinction.
Some interesting observations about the current loss of species are: (1) From ten high-diversity
localities in tropical forests covering 300,000 km 2 , some 17,000 endemic plant species and
350,000 endemic animal species could be lost in near future. The tropical forests alone are losing
roughly 14000-40000 species per year (or 2-5 species per hour). The earth may lose up to 50% of
the species by the end of the 21 current rate of loss continues.
Susceptibility to extinction
The characteristics of species particularly susceptible to extinction are: large body size
(Bengal tiger, lion and elephant); small population size and low reproductive rate (Blue whale
and Giant panda). Feeding at high trophic levels in the food chain (Bengal tiger and Bald eagle),
fixed migratory routes and habit (Blue whale and Whooping crane) and localized and narrow
range of distribution (woodland caribou; many island species) also make the species susceptible
to extinction.