Biological Invasions (Alien Plant Species – Acacia mangium and Eucalyptus)

Biological invasion refers to the process by which species are introduced—intentionally or accidentally—into regions outside their natural range, where they establish, proliferate, and spread, often to the detriment of native biodiversity, ecosystems, and human interests. Invasive alien species are now recognised as one of the leading drivers of global environmental change and biodiversity loss. Among plants, certain fast-growing tree species such as Acacia mangium and Eucalyptus have been widely introduced for forestry, timber, and soil reclamation, but have subsequently become ecologically invasive in several tropical and subtropical regions, including parts of India.

Concept and Mechanism of Biological Invasion

When non-native plants are introduced to a new environment, they may become invasive if they:

1. Adapt successfully to local climatic and soil conditions.
2. Reproduce and spread rapidly without natural predators, pathogens, or competitors.
3. Outcompete native flora, altering ecological processes such as nutrient cycling, hydrology, and fire regimes.

Invasion typically progresses through stages of introduction, establishment, naturalisation, and spread. Human activities such as afforestation, agriculture, urban landscaping, and global trade accelerate these processes by providing suitable niches and dispersal pathways.

Ecological and Economic Impacts of Invasive Trees

While fast-growing alien trees are valued for timber, pulp, and soil restoration, their uncontrolled spread can cause:

• Suppression of native vegetation due to shading and competition for resources.
• Alteration of soil chemistry, especially through allelopathy (release of chemical inhibitors).
• Reduction in water availability due to high transpiration rates.
• Changes in fire frequency and intensity from flammable litter accumulation.
• Decline in biodiversity, as native plant and animal species lose habitat.

Two of the most notable examples of such ecological invaders are Acacia mangium and Eucalyptus species.

Acacia mangium

Taxonomy and Origin:

• Family: Fabaceae (Leguminosae)
• Native Range: Northern Australia, Papua New Guinea, and eastern Indonesia

A. mangium is a fast-growing evergreen tree, reaching heights of 25–30 metres. It was introduced to tropical regions of Asia, Africa, and South America for reforestation, soil rehabilitation, nitrogen fixation, and pulpwood production due to its ability to thrive on degraded lands.
Characteristics Promoting Invasiveness:

• Rapid growth rate and early maturity.
• Nitrogen-fixing ability, enabling survival in nutrient-poor soils.
• Abundant seed production and prolonged seed viability.
• High tolerance to drought, poor soil, and fire.

Ecological Impacts:

1. Suppression of Native Species: Its dense canopy and heavy leaf litter prevent sunlight from reaching the forest floor, inhibiting germination of native plants.
2. Soil Alteration: Nitrogen fixation alters nutrient cycles, encouraging weedy growth and destabilising ecosystems adapted to nutrient-poor conditions.
3. Hydrological Effects: Deep root systems lead to increased water uptake, reducing groundwater recharge and stream flow, particularly in dry regions.
4. Fire and Carbon Dynamics: The accumulation of dry leaf litter heightens fire risk, and post-fire regeneration is often dominated by A. mangium, further excluding native flora.

Case Studies: In India, Acacia mangium has been widely planted in the Western Ghats, Andaman Islands, and northeastern states for timber and afforestation. However, it has spread into adjacent natural forests, outcompeting native tree species such as Dipterocarpus and Shorea. Similar invasions have been documented in Sri Lanka, Malaysia, and Brazil, where it forms monocultures detrimental to local biodiversity.

Eucalyptus (General Overview)

Taxonomy and Origin:

• Family: Myrtaceae
• Native Range: Australia, Tasmania, and nearby islands

The genus Eucalyptus comprises over 700 species, several of which have been introduced globally since the 19th century for timber, fuelwood, oil extraction, and soil conservation. In India, species like Eucalyptus globulus, Eucalyptus tereticornis, and Eucalyptus camaldulensis were promoted during the British colonial period and later by social forestry programmes in the 1970s and 1980s.
Characteristics Promoting Invasiveness:

• Extremely fast growth and short rotation period.
• Efficient water use and deep rooting, allowing survival in semi-arid environments.
• Allelopathic properties, releasing chemical compounds that inhibit the growth of surrounding vegetation.
• High adaptability to diverse soil and climate types.

Ecological Impacts:

1. Depletion of Groundwater: Eucalyptus trees consume large quantities of water, leading to lowered water tables and reduced moisture in surrounding soils. Studies have shown significant hydrological depletion in regions with dense Eucalyptus plantations.
2. Loss of Biodiversity: The thick leaf litter, coupled with allelopathic chemicals like cineole and phenolics, suppresses understory growth, resulting in monocultures devoid of native herbaceous species.
3. Alteration of Soil Chemistry: Accumulated leaf litter acidifies the soil and reduces organic matter content, hindering the regeneration of native species.
4. Fire Hazards: The trees contain highly flammable oils, increasing the risk and intensity of forest fires.
5. Negative Effects on Wildlife: Native fauna, especially insects, birds, and small mammals, find little food or nesting material in Eucalyptus-dominated landscapes, leading to faunal displacement.

Distribution and Spread: In India, extensive Eucalyptus plantations exist in Tamil Nadu, Karnataka, Andhra Pradesh, Madhya Pradesh, and Himachal Pradesh. In regions like the Nilgiri Hills and Western Ghats, the species has escaped cultivation and encroached upon native grasslands and shola forests, causing severe ecological imbalance.

Comparative Analysis of Acacia mangium and Eucalyptus

Both species demonstrate how well-intentioned forestry introductions can lead to long-term ecological disruption when not managed sustainably.

Management and Control Strategies

1. Early Detection and Monitoring: Regular ecological surveys to identify new invasions and assess spread.
2. Mechanical and Chemical Control: Physical removal of seedlings and saplings before establishment, combined with controlled herbicide application where necessary.
3. Restoration of Native Vegetation: Replanting indigenous tree species and grasses to restore ecological balance and prevent re-invasion.
4. Biological Control: Exploration of natural pests or pathogens from the native range that can limit population growth, though this must be approached cautiously to avoid unintended effects.
5. Regulation of Plantations: Limiting large-scale monoculture plantations and promoting mixed-species forestry models using native trees.
6. Public Awareness and Policy Intervention: Educating local communities and forestry departments about the ecological costs of invasive trees. Implementing policies to control planting near sensitive ecosystems such as national parks, wetlands, and grasslands.

Ecological Lessons and Sustainable Approach

The introduction of Acacia mangium and Eucalyptus illustrates the paradox of biological introductions — species beneficial for short-term economic or environmental gains can have long-term ecological costs. Their spread exemplifies how alien species disrupt native ecosystems, altering hydrology, soil chemistry, and species composition.
Future reforestation and afforestation programmes must adopt a precautionary and ecosystem-based approach, prioritising native species, assessing potential invasiveness before introduction, and integrating ecological restoration with livelihood goals.