Alien species invasion has been a threat to biodiversity. Justify with the help of a suitable example. List any other three causes responsible for such a loss.

The correct answer is:
Example of Alien species invasion:
• Nile Perch, introduced into Lake Victoria (in East Africa), led to the extinction of Cichlid fish (more than 200 species) in the lake.
• OR Introduction of African catfish (Clarias gariepinus), for aquaculture, posing a threat to indigenous catfish.
• OR Introduction of carrot grass (Parthenium) / Lantana / Water hyacinth (Eichhornia), which are invasive weeds that pose a threat to native species.

Other three causes of biodiversity loss:
(i) Habitat loss and fragmentation
(ii) Over-exploitation
(iii) Co-extinction

Justification of Alien Species Invasion as a Threat to Biodiversity:
When foreign (alien) species are introduced unintentionally or deliberately into a new geographical area, some of them turn invasive. Because they lack natural predators in the new environment, they multiply rapidly and outcompete the local native species for resources (like food, space, and light). This leads to a decline or complete extinction of indigenous species, severely disrupting the local ecological balance.

Examples:
1. Nile Perch in Lake Victoria: The introduction of the predatory Nile Perch into Lake Victoria in East Africa led to the ecological disaster of losing more than 200 species of native Cichlid fish, which were unique to the lake.
2. African Catfish: The illegal introduction of the African catfish Clarias gariepinus for aquaculture purposes poses a direct threat to the survival of native catfishes in Indian rivers.
3. Invasive Plants: Plants like carrot grass (Parthenium), Lantana, and water hyacinth (Eichhornia) act as aggressive weeds, choking out native vegetation and degrading natural habitats.

Other Three Major Causes of Biodiversity Loss (The Evil Quartet):
Besides alien species invasion, the other three primary drivers of biodiversity loss are:

1. Habitat Loss and Fragmentation: This is the most critical cause. Deforestation, urbanization, and pollution destroy the natural homes of species. When large habitats are broken into smaller fragments due to human activities, mammals and birds requiring large territories, as well as migratory animals, are badly affected, leading to population declines.

2. Over-exploitation: When humans overharvest natural resources faster than they can replenish (for food, medicine, or commercial gain), it leads to endangerment or extinction. Examples include the extinction of Steller's sea cow and the passenger pigeon.

3. Co-extinction: When a host species becomes extinct, any obligate parasite or symbiotic partner uniquely associated with it also faces mandatory extinction. For example, in a co-evolved plant-pollinator mutualism, the extinction of one partner inevitably leads to the demise of the other.