Causes Behind Biodiversity Loss

Background

When we look at nature, we see a delicate balance—species depend on habitats, food chains depend on species, and ecosystems depend on all of them. But today, several major forces (many of them man-made) are breaking this balance and causing large-scale biodiversity loss.

Let’s understand these causes step by step.

Natural Ecological Disturbances

Natural events such as:

• Weather extremes
• Wildfires
• Floods
• Volcanic eruptions

These can drastically alter ecosystems and eliminate some species locally. However, such disturbances are:

• Temporary, and
• Part of natural ecological cycles (succession)

Ecosystems have evolved to recover from these.

The real danger today comes from permanent, human-caused disruptions.

The “Evil Quartet” – Main Anthropogenic Causes

Permanent biodiversity loss is closely linked to:

1. Habitat Loss
2. Overexploitation
3. Alien (invasive) species
4. Secondary extinction

These four are often referred to as the Evil Quartet.

Additionally, WWF identifies 6 key threats:

• Agriculture
• Hunting
• Logging
• Pollution
• Invasive species
• Climate change

Habitat Loss and Fragmentation

This is the leading cause of species extinction globally.

What is Habitat Loss?

It means the area where a species can live, feed, reproduce is reduced or destroyed.

Most dramatic example: Tropical Rainforests

• Earlier → covered 14% of Earth’s land
• Today → only 6% remains

This is catastrophic because rainforests store immense biodiversity.

Examples of habitat destruction:

• Amazon rainforest cleared for soy cultivation and cattle ranching
• Wetlands filled for land development
• Forests cleared for:
  • Industries
  • Agriculture
  • Dams
  • Housing
  • Tourism/Sports

What is Habitat Fragmentation?

Imagine breaking a big forest into many tiny islands separated by human presence (roads, farms, towns).

A large continuous habitat → becomes → many smaller, isolated patches.

These patches:

• have fewer resources
• support fewer species
• increase risk of extinction

Fragmentation is often more harmful than total loss, because it disrupts movement and reproduction.

Consequences: Barrier Effect & Inbreeding

Fragmentation acts like a “wall” inside nature.

Inbreeding

When individuals of the same small, isolated population mate repeatedly, it causes:

• Reduction in genetic diversity
• Higher chances of genetic defects
• “Inbreeding depression” → population becomes weak, sick, and unable to survive

Why does this happen?

Genes come in dominant and recessive forms.
Recessive genes often carry inherited diseases.

When close relatives mate:

• both likely carry the same recessive genes
• offspring inherit recessive traits → leading to deformities or diseases

Examples:

• Koalas in Australia: inbreeding made them highly vulnerable to disease
• Indian tigers: Central India once had high genetic diversity, but highways now divide forests and isolate tiger populations

Fragmentation of Tiger Habitats in India

Some key examples:

• 55,000 km of roads pass through forests and protected areas
• NH-37: A 36 km flyover is planned over Kaziranga National Park
• NH-44: Cuts across corridors linking Kanha, Satpura, Pench, Bandhavgarh & Panna
• NH-6: Passes through tiger corridors around Melghat, Bor, Nagzira & Simlipal

These highways isolate tiger populations, increasing risks of inbreeding and local extinction.

Alien (Invasive) Species

An invasive alien species is:

• a non-native species
• that spreads rapidly
• disrupts the local ecosystem
• causes biodiversity and economic loss

Example:

Nile perch introduced into Lake Victoria (Africa) caused the extinction of over 200 species of cichlid fish.

Why are invasive species so dangerous?

• No natural predators
• Native species lack defenses
• They alter soil chemistry
• They outcompete native species
• They disrupt food chains
• They introduce diseases

Invasive species are the second biggest cause of biodiversity loss after habitat destruction.

Destruction of Sholas – A Case Study

Sholas = tropical montane forests of the Western Ghats, surrounded by high-altitude grasslands (1700–2600 m).

Threat:

Invasive alien species like:

• Eucalyptus
• Wattle (Acacia)
• Lantana
• Tea plantations
• Parthenium

are replacing Shola forests and grasslands.

But why is this happening?

Traditionally:

• Frost prevented Shola forests from spreading
• Frost-killed saplings prevented invasives from spreading

But now:

• Warmer nights
• Less frost due to climate change
  → Invasives like acacia are invading grasslands

Consequences:

• Water sources are drying
• More landslides
• Loss of habitat for Nilgiri Tahr (Endangered)
• Toda tribe (PVTG), which depends on Shola grasslands, is affected

How Do Invasive Species Enter?

Mostly through:

• Ships carrying organisms in ballast water
• Insects hiding in traded wood
• Ornamental plants escaping cultivation
• Intentional introduction to control pests
• Climate change enabling movement into new regions

Threats to Native Wildlife from Invasive Species

Direct Threats

• Predation (e.g., African Catfish)
• Outcompeting native species
• Spreading diseases
• Destroying progenies (eggs/young)

Indirect Threats

• Altering food webs
• Providing poor nutrition to wildlife
• Replacing diverse ecosystems with monocultures
• Changing soil chemistry
• Increasing wildfire intensity

In short, invasive species can remake an entire ecosystem into something unrecognizable.

Over-Exploitation – When “Need” Becomes “Greed”

Over-exploitation happens when humans start extracting resources faster than nature can replace them. For thousands of years, nature and humans coexisted. But in the last few centuries, our consumption patterns have crossed ecological limits.

Examples from the last 500 years

Species like:

• Steller’s Sea Cow
• Passenger Pigeon

were hunted so aggressively that they vanished forever.

Even today:

• Marine fish populations are collapsing due to over-harvesting.
• Whales were hunted nearly to extinction for oil.
• Trees are felled faster than forests can regenerate.
• Wildlife is hunted for ivory, skins, horns, and illegal trade.

Large mammals such as tigers, rhinos, and elephants were pushed to the brink due to uncontrolled poaching before legal protection came into force.

Species Extinction – How and Why Species Disappear

Species may go extinct due to two broad processes:

1. Deterministic Processes

These have a clear cause-and-effect relationship.

Examples:

• Glaciations
• Human activities such as deforestation, hunting, or pollution

These pressures systematically push species toward extinction.

2. Stochastic Processes (Random Events)

These are unpredictable “chance events” that affect survival.

Examples include:

• Sudden weather changes
• Diseases
• Natural disasters
• Sudden rise in predators or competitors

Stochastic and deterministic processes often combine, making extinction more likely—especially for species already under stress.

Which Species Are Most Vulnerable to Extinction?

Species with certain traits are especially at risk:

1. Rarity / Low Abundance

Small populations are fragile.

2. Poor Dispersal Ability

Species unable to migrate or move when habitats change.

3. High Tropic Status

Carnivores (tigers, eagles, sharks) sit at the top of food chains and have naturally low population sizes.

4. Low Adult Survival Rates

Species that produce few offspring or take long to mature (e.g., elephants) recover slowly.

Co-Extinctions – When One Species Falls, Others Follow

Ecosystems are interconnected like a chain. When one species disappears, others dependent on it may also vanish.

Examples:

• If a host fish becomes extinct, all its parasitic organisms also disappear.
• In a plant–pollinator mutualism, if either the plant or the pollinator goes extinct, the partner species cannot survive.

Co-extinction is one of the most silent and least studied forms of biodiversity loss.

Genetically Modified Organisms (GMOs)

A GMO or transgenic organism has artificially altered DNA through:

• Mutation
• Insertion
• Deletion of genes

Example:

• Bt cotton contains genes from the bacterium Bacillus thuringiensis.

Why are GMOs created?

To introduce new traits such as:

• Pest resistance
• Herbicide tolerance
• Higher productivity

However, GMOs bring ecological risks.

How GMOs Can Threaten Biodiversity

1. GMO species may become dominant

By outcompeting natural species.

2. Transgene escape

Pollen transfer from GMO crops to wild plants can:

• Alter soil microbes
• Create aggressive hybrids
• Change ecological balances

3. Herbicide resistance

Many GMO crops are engineered to withstand herbicides.

As a result:

• Herbicides are sprayed heavily
• Weeds die
• But so do other beneficial plants
• Food webs collapse
• Soil loses protection against erosion

Biodiversity declines silently when entire landscapes become GMO monocultures.

The Curious Case of Genetically Modified Mosquitoes

To combat diseases like dengue, Zika, and chikungunya, scientists experimented with GM mosquitoes.

Oxitec’s GM Male Aedes aegypti

• Engineered with a self-limiting gene
• Supposed to produce offspring that die before adulthood
• Released in Brazil (2013–2015) — ~500,000 per week

Unexpected outcome

Some offspring:

• Survived
• Reached adulthood
• Bred with native mosquitoes
• Introduced new genetic material into wild populations

This raises concerns similar to GM crops:

Genetic material introduced into the wild may have consequences we do not fully understand.

Trade in Wildlife – A Global Threat

Illegal wildlife trade affects:

• Species survival
• Ecosystems
• Human health

How it harms nature:

• Species are harvested unsustainably
• Habitats are destroyed to access animals
• Illegal markets rarely follow sanitary standards → spreads zoonotic diseases
• Loss of predators or keystone species disturbs entire ecosystems

Examples include trade in:

• Tiger bones
• Pangolin scales
• Exotic birds
• Turtles and tortoises

Pollution and Climate Change

(Slow but Severe Killers)

Climate Change

Alters:

• Habitat ranges
• Migration patterns
• Species reproduction cycles
• Distribution of plants, pathogens, and animals

Extreme weather events cause sudden population crashes.

Ocean Acidification

Due to excess atmospheric carbon dissolving in oceans.

This affects:

• Coral reefs
• Shell-forming organisms
• Entire marine food webs

Pollution

Includes:

• industrial waste
• agricultural chemicals
• plastics
• oil spills

These degrade habitats and kill countless species each year.