Q.Analyse the effects of 'Alien species invasion' on the biodiversity of a given area. Provide two examples.

Concept: Alien (invasive) species invasion – the introduction of non-native organisms that establish, spread, and disrupt native ecosystems.

Key effects on biodiversity:

1. Competition and displacement: Invasive species often outcompete native species for resources (food, space, light), leading to population declines or local extinctions of indigenous flora and fauna.

2. Predation pressure: Alien predators may lack natural enemies in the new habitat and prey heavily on native species that have no evolved defenses, causing dramatic population crashes.

3. Habitat alteration: Some invaders physically modify ecosystems (e.g., changing soil chemistry, fire regimes, or water availability), making conditions unsuitable for native species.

4. Disease transmission: Introduced species can carry novel pathogens or parasites to which native species have no immunity, triggering epidemics.

Examples: …

Why alien invasions matter

When a species arrives in a new habitat where it did not evolve—whether through human activity or natural dispersal—it enters an ecosystem without its natural predators, parasites, or competitors. This freedom often allows explosive population growth. The invader then interacts with native species that have no evolutionary history with it, no defenses, no behavioral adaptations. The result is ecological disruption at multiple levels: individual species decline, community structure collapses, and ecosystem functions break down.

Biodiversity loss from invasions happens through several mechanisms, and understanding each reveals why some invasions prove catastrophic while others remain benign.

Mechanisms of impact

1. Competition for resources

Invasive species often outcompete natives for food, space, light, or nutrients. Superior competitive ability—faster growth, higher reproductive rate, broader diet—allows the invader to monopolize resources. Native species, adapted to historical resource availability, find themselves starved out.

2. Predation pressure

When an alien predator enters a naive prey community, the prey lack escape behaviors or defensive traits. Predation rates skyrocket. Island ecosystems, with their endemic species evolved in predator-free environments, suffer especially severe losses.

3. Habitat modification

Some invaders physically alter the environment. Aggressive plants change soil chemistry, fire regimes, or water availability. Ecosystem engineers like beavers or certain grasses reshape entire landscapes, making habitats unsuitable for natives.

4. Disease and parasite transmission

Alien species carry novel pathogens to which native species have no immunity. Epidemics sweep through populations with no resistance, causing mass mortality.

5. Genetic pollution

Hybridization between invasive and native species dilutes unique genetic lineages, erasing millions of years of evolutionary distinctiveness.

Example 1: Nile perch in Lake Victoria

Lake Victoria, Africa's largest lake, once harbored over 500 endemic cichlid fish species—a spectacular example of adaptive radiation. In the 1950s, colonial administrators introduced the Nile perch (Lates niloticus), a large predatory fish reaching 2 meters and 200 kg, to establish a commercial fishery.

The cascade of destruction:

• Direct predation: The Nile perch is a voracious generalist predator. It consumed cichlids at unsustainable rates. Within decades, over 200 cichlid species went extinct—one of the largest vertebrate extinction events in recorded history.

• Trophic disruption: Cichlids had filled diverse ecological roles—algae grazers, detritivores, insectivores, scale-eaters. Their loss destabilized the entire food web. Algal blooms increased as herbivorous cichlids disappeared. Oxygen depletion followed.

• Ecosystem simplification: A complex, species-rich ecosystem collapsed into a simpler one dominated by a few species. Functional diversity—the variety of ecological roles—plummeted even more dramatically than species diversity.

• Socioeconomic impact: While the perch fishery generated revenue, it required expensive processing (sun-drying was insufficient for oily perch flesh). Traditional fishing communities lost access to diverse, easily preserved cichlid catches. The ecological cost far exceeded economic gains.

The Lake Victoria catastrophe demonstrates how a single invasive predator can trigger extinction cascades, erasing irreplaceable evolutionary heritage.

Example 2: Lantana camara in India

Lantana camara, a flowering shrub native to Central and South America, was introduced to India as an ornamental plant in the early 19th century. It escaped cultivation and now infests over 13 million hectares across the country, particularly in forests, grasslands, and protected areas.

Mechanisms of invasion:

• Aggressive growth: Lantana forms dense, impenetrable thickets 2–3 meters tall. Its rapid growth and prolific seed production (a single plant produces thousands of seeds annually) allow it to colonize disturbed areas quickly.

• Allelopathy: Lantana releases chemical compounds into the soil that inhibit germination and growth of other plants. This biochemical warfare suppresses native vegetation, creating monocultures.

• Competitive exclusion: By monopolizing light, water, and nutrients, Lantana outcompetes native grasses, herbs, and tree seedlings. Forest understories transform from diverse plant communities into Lantana monocultures.

Biodiversity impacts: …