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

Concept: Biological invasion and competitive exclusion.

The introduction of an alien species can turn invasive when it finds suitable environmental conditions and lacks natural predators or competitors in the new habitat. This often leads to the decline of indigenous species through predation, competition for resources, or disease transmission.

Example: The introduction of the Nile Perch (Lates niloticus) into Lake Victoria in East Africa in the 1950s serves as a classic illustration.

1. Introduction: The Nile Perch, a large predatory fish, was intentionally introduced to enhance the local fishing industry.
2. Invasion: In the absence of its natural predators and with an abundant food supply of smaller fish, the Nile Perch rapidly proliferated and established itself as the dominant predator. …

Concept and Intuition

An alien species (also known as an exotic, non-native, or introduced species) is a species living outside its native distributional range, which has arrived there by human activity, either deliberate or accidental. Not all alien species are harmful; many integrate into their new environments without significant negative impact.

However, an alien species becomes invasive when it establishes itself in the new environment and causes ecological or economic harm. This often happens because the new environment lacks the natural predators, parasites, or diseases that kept its population in check in its native habitat. Without these natural controls, the invasive species can reproduce rapidly and outcompete or prey upon indigenous species.

The decline of indigenous species occurs primarily through:

1. Competition: The invasive species might be more efficient at acquiring resources (food, water, light, space) than native species, leading to the native species' starvation or displacement.
2. Predation: The invasive species might directly prey on native species that have not evolved defenses against this new predator.
3. Disease Transmission: Invasive species can introduce novel pathogens or parasites to which native species have no immunity.
4. Habitat Alteration: Invasive species can physically change the environment, making it unsuitable for native species (e.g., altering soil chemistry, water flow, or light availability).

Illustration: The Nile Perch in Lake Victoria

The introduction of the Nile Perch (Lates niloticus) into Lake Victoria in East Africa is one of the most well-documented and devastating examples of an alien species becoming invasive and causing the decline of indigenous species.

1. Introduction of the Alien Species:

   In the 1950s and 1960s, the Nile Perch, a large, voracious predatory fish native to other African river systems (like the Nile River), was deliberately introduced into Lake Victoria by British colonial authorities and local fisheries departments. The primary motivation was to boost the commercial fishing industry, as Lake Victoria's native fish, primarily small cichlids, were not considered commercially valuable enough.

2. Characteristics of the Alien Species Enabling Invasion:

   The Nile Perch is a large, aggressive, and highly adaptable predator. In its native habitat, its population is regulated by various factors, including the availability of prey, competition from other large predators, and specific environmental conditions. However, in Lake Victoria, it found an ideal environment:

   • Abundant Prey: Lake Victoria was home to over 500 species of endemic cichlid fish, many of which were small and had not evolved defenses against such a large, efficient predator.
   • Lack of Natural Predators: The Nile Perch had no natural predators in Lake Victoria to control its population.
   • High Reproductive Rate: It could reproduce rapidly and grow to a large size, quickly dominating the aquatic ecosystem.

3. Impact on Indigenous Species:

   The Nile Perch, being an apex predator, began to consume the native cichlid fish species at an alarming rate. The cichlids, which had diversified over millions of years to fill various ecological niches (herbivores, detritivores, insectivores), were unprepared for this new, highly effective predator.

   • Within a few decades, the populations of hundreds of endemic cichlid species plummeted. Many species, particularly those from the genus Haplochromis, went extinct or became critically endangered. …