Causes of Biodiversity Loss — Revision Notes

⚡ 30-Second Revision

Evil Quartet: — Four major causes of biodiversity loss.

1. Habitat Loss & Fragmentation: Most important cause. Destruction/breaking up of habitats. 2. Over-exploitation: Unsustainable harvesting (e.g., Steller's sea cow, Passenger Pigeon). 3. Alien Species Invasions: Non-native species outcompete/prey (e.g., Nile Perch, Parthenium, Water Hyacinth). 4. Co-extinctions: Obligate dependency leads to domino effect (e.g., plant-pollinator).

Other Factors: — Pollution, Climate Change.

2-Minute Revision

Biodiversity loss is the reduction in the variety of life at genetic, species, and ecosystem levels, primarily driven by human activities. The four major causes, collectively known as the 'Evil Quartet', are crucial for NEET.

Habitat loss and fragmentation is the most significant, involving the destruction of natural habitats (e.g., deforestation for agriculture) and their division into smaller, isolated patches. This leads to reduced population sizes, increased 'edge effects', and hindered gene flow.

Over-exploitation refers to unsustainable harvesting of resources, exemplified by the extinction of the Steller's sea cow and Passenger Pigeon due to excessive hunting. Alien species invasions occur when non-native species are introduced and outcompete or prey on native ones, like the Nile Perch decimating cichlid fish in Lake Victoria, or invasive plants like Parthenium in India.

Lastly, co-extinctions describe the 'domino effect' where the extinction of one species leads to the extinction of another obligately dependent species, such as a specific pollinator and its plant.

Pollution and climate change are also significant contributing factors, exacerbating these primary threats.

5-Minute Revision

The crisis of biodiversity loss is fundamentally driven by human actions, categorized primarily into the 'Evil Quartet'. The most impactful of these is Habitat Loss and Fragmentation. This involves the outright destruction of natural habitats, such as clearing tropical rainforests for farming, and the subsequent breaking up of remaining habitats into smaller, isolated patches by roads or development.

Fragmentation reduces habitat area, isolates populations (leading to genetic drift and inbreeding), and increases 'edge effects' which negatively impact interior species. For instance, the Amazon rainforest's deforestation directly impacts countless species.

Next, Over-exploitation occurs when humans harvest resources faster than they can regenerate. Historical examples include the Steller's sea cow, hunted to extinction within decades, and the Passenger Pigeon, once abundant but wiped out by relentless hunting.

Modern overfishing and illegal wildlife trade continue this threat. Alien Species Invasions are another major cause, where non-native species, introduced intentionally or accidentally, outcompete or prey on native species.

The Nile Perch in Lake Victoria caused the extinction of over 200 cichlid fish species. In India, Parthenium, Lantana, and Water Hyacinth are notorious invasive plants. Finally, Co-extinctions highlight the interconnectedness of life; the extinction of one species can trigger the extinction of another obligately dependent species, like a specific pollinator and its plant.

Beyond the quartet, Pollution (e.g., plastic, pesticides, industrial waste) and Climate Change (e.g., rising temperatures, ocean acidification) are increasingly significant, altering habitats and pushing species to their limits.

A holistic understanding of these interconnected causes is vital for NEET.

Prelims Revision Notes

Causes of Biodiversity Loss (The Evil Quartet)

1
Habitat Loss and Fragmentation (Most Important Cause)

* Loss: Complete destruction of natural habitats (e.g., tropical rainforests for agriculture, urbanization). * Fragmentation: Breaking large habitats into smaller, isolated patches (e.g., by roads, dams). * Consequences: Reduced population size, increased 'edge effect' (altered microclimate, predation), barriers to dispersal and gene flow, increased vulnerability to local extinction. * Example: Deforestation of Amazon rainforest.

1
Over-exploitation

* Definition: Harvesting resources beyond their regenerative capacity. * Examples: * Steller's Sea Cow: Hunted to extinction within 27 years of discovery (18th century). * Passenger Pigeon: Once abundant, driven to extinction by relentless hunting (early 20th century). * Overfishing, over-logging, poaching (tigers, rhinos, elephants).

1
Alien Species Invasions

* Definition: Non-native (exotic) species introduced to a new area, becoming invasive and harming native biodiversity. * Mechanism: Outcompete native species, prey on them, introduce diseases, alter habitat.

* Examples: * Nile Perch in Lake Victoria: Led to extinction of >200 species of endemic cichlid fish. * Invasive Plants in India: Parthenium (carrot grass), Lantana camara, Eichhornia crassipes (water hyacinth).

* African Catfish (Clarias gariepinus): Threat to indigenous catfish in Indian rivers. * Goats on oceanic islands (overgrazing).

1
Co-extinctions

* Definition: Extinction of one species leads to the extinction of another obligately associated/dependent species. * Mechanism: 'Domino effect' in symbiotic relationships. * Examples: * Plant-pollinator mutualism (e.g., specific fig tree and fig wasp). * Host-parasite relationships (loss of host leads to loss of specific parasite).

Other Significant Causes

Pollution: — Industrial effluents, agricultural runoff (eutrophication), plastic waste, pesticides (biomagnification), air pollution (acid rain) degrade habitats and harm organisms.
Climate Change: — Rising global temperatures, altered precipitation, extreme weather, ocean acidification. Pushes species beyond adaptive limits, especially vulnerable ecosystems (coral reefs, polar regions).

Key Terms to Remember

Biodiversity Hotspots: — High endemism, high threat.
Endemic Species: — Restricted to a specific region, highly vulnerable.
Extinction Vortex: — Downward spiral of small populations.
Edge Effect: — Changes at habitat boundaries due to fragmentation.
Biomagnification: — Accumulation of toxins up the food chain.

Vyyuha Quick Recall

To remember the 'Evil Quartet' of biodiversity loss, think of H.O.A.C.:

Habitat Loss & Fragmentation
Over-exploitation
Alien Species Invasions
Co-extinctions

Imagine a 'HOAC' (like a hoard) of problems causing species to disappear!