Illustrate with the help of an example how introduction of an alien species turns invasive and causes decline of an indigenous species.

The correct answer/options illustrating how the introduction of an alien species turns invasive and causes the decline or extinction of indigenous species include:
1. The introduction of the Nile perch into Lake Victoria, which eventually led to the extinction of an ecologically unique assemblage of more than 200 species of cichlid fish in the lake.
2. The introduction of the African catfish (Clarias gariepinus) for aquaculture purposes, which poses a substantial threat to the indigenous catfishes in our rivers.
3. Invasive weed species like Parthenium (carrot grass), Lantana, and water hyacinth (Eichhornia crassipes), which cause environmental damage and threaten the survival of native species.

Step-by-Step Explanation:

1. Understanding Alien Species Invasion:
When species from another geographical area (alien, exotic, or non-native species) are introduced unintentionally or deliberately into a new habitat for economic or other uses, some of them turn invasive. They start growing or reproducing rapidly due to the absence of their natural predators, competitors, or pathogens in the new environment.

2. Nile Perch in Lake Victoria:
Lake Victoria in East Africa was home to a highly diverse group of native cichlid fish. When the Nile perch—a large predatory fish—was introduced into the lake, it lacked natural predators to control its population. The Nile perch preyed heavily on the cichlids, leading to the extinction of more than 200 species of cichlid fish, disrupting the entire lake ecosystem.

3. African Catfish (Clarias gariepinus):
The African catfish was introduced for aquaculture (fish farming) to increase yield. However, individuals escaped into local river systems. Because they are hardy and aggressive predators, they compete with and prey upon native catfish species, threatening their survival.

4. Invasive Plants (Parthenium, Lantana, Water Hyacinth):
Invasive plants like Parthenium (carrot grass), Lantana, and water hyacinth grow aggressively. Water hyacinth, for instance, clogs water bodies, depleting oxygen and blocking sunlight, which suffocates and kills native aquatic plants and animals.