Invasive Species: An Overview of Means of Introduction, Pathway of Invasion, Impacts, and Mitigation of Impacts

Alien invasive species refer to the organisms that have crossed natural ecological boundaries, established themselves in regions beyond their native habitats and cause significant economical and ecological damage.

Invasive species pose a major threat to global biodiversity. They cause significant economic damage to sectors like agriculture, forestry, and fisheries, and also impact human health.

The rapid growth in global trade, tourism, transportation, and travel over the last century has greatly increased the spread of these species by helping them overcome natural geographic barriers.

Meaning and Definition of Alien Invasive Species

An alien, introduced, exotic, non-native, or non-indigenous species is one that exists outside its original range because of human influence, whether through intentional or accidental means.

These non-native species can impact ecosystems in various ways. When they cause harm, they are classified as invasive species. Such introductions have occurred both deliberately and unintentionally. Deliberate introductions are often driven by economic, environmental, or social purposes.

For instance, in forestry, species like Pinus, Eucalyptus, and Acacia are valued for pulp, timber, and firewood, but they can also heavily strain local water supplies.

Alien invasive species refer to the organisms that have crossed natural ecological boundaries, established themselves in regions beyond their native habitats and cause significant economical and ecological damage.

Some introduced species thrive in new environments due to advantageous biological traits that enable them to reproduce and spread rapidly.

Invasive alien species often outcompete native plants and animals for essential resources like food, water, and space. Their ability to multiply quickly makes them particularly disruptive.

These biological invasions are a major element of human-driven global environmental change and are considered the second leading cause of biodiversity loss worldwide.

Alien species pose serious threats to ecosystems, leading to significant environmental, economic, and health-related consequences. They can negatively affect biodiversity by driving native species to decline or extinction through competition, predation, disease transmission, and disruption of ecosystem processes.

Invasive Alien Species (IAS) have far-reaching effects not only on natural ecosystems but also on human societies. They can lead to considerable economic losses in agriculture by reducing crop and livestock productivity, diminish native biodiversity, incur high management costs, and impact human health.

These non-native species are a major force behind ecosystem degradation, habitat loss, and species extinction—second only to habitat destruction. The movement of species around the globe is occurring at an unprecedented rate.

Means of Introduction Invasive Species

• Invasive plants must first colonize and then persist in their new environment. A small proportion of introduced species may become highly prolific and dominant within plant communities.
• For instance, Bromus tectorum (cheatgrass), an annual species accidentally introduced from Asia, has now spread extensively across global grasslands, occupying millions of hectares. Similarly, Heracleum mantegazzianium, initially cultivated as an ornamental plant and now commonly known as giant hogweed, has escaped cultivation and become a significant invasive species across three continents.
• Pinus radiata (Monterey pine), native to a limited region in the western United States, was widely planted for timber production but is now considered a weed in many areas. These cases exemplify the range of growth forms that invasive plants can assume.

• In the Indian subcontinent, particularly in the northwestern Himalayas, species such as Ageratum conyzoides, Parthenium hysterophorus, Lantana camara, and Eupatorium adenophorum (syn. Ageratina adenophora) are among the most problematic invaders, significantly reducing native biodiversity.
• Many invasive plant species were introduced intentionally for purposes such as forage, fiber, medicinal use, ornamental value, erosion control, or timber production.
• Unintentional introductions have occurred through means such as ship ballast, contaminated crop seeds, animal fur, or soil attached to nursery plants.

• Many of these invasive species are responsible for biodiversity loss also inflict severe economic damage. Some examples include the zebra mussel (Dreissena polymorpha), Lantana camara, kudzu (Pueraria lobata), Brazilian pepper (Schinus terebinthifolius), and various species of Rattus, all of which pose ecological and economic threats.
• Invasive species represent a wide range of taxonomic groups, with plants, mammals, and insects contributing disproportionately to the most damaging invasions.

• Globally, invasive alien species have driven thousands of native species to extinction or near-extinction, particularly on islands but also across continental ecosystems. Entire native ecological communities have been irreversibly altered or lost due to invasion.

Characteristics of Invasive Species

Invasive species, whether aquatic or terrestrial, typically exhibit a suite of biological and ecological traits that facilitate their successful establishment and proliferation in novel environments. These characteristics commonly include:

• Accelerated growth rates
• High reproductive capacity
• Strong dispersal mechanisms
• Absence of natural predators or population controls in the introduced range
• Tolerance to a broad spectrum of environmental conditions
• Generalist feeding behavior, enabling utilization of diverse food sources
• Capacity to adapt to variable and changing environmental conditions
• Competitive superiority over native species through more efficient exploitation of limited resources

Pathways of Invasion (Alien Invasive Species)

The successful establishment of invasive species typically follows a three-phase process: introduction, establishment, and spread. Each stage involves overcoming distinct ecological and biological barriers, and the progression through these stages determines the invasive potential of a species.

Introduction

• Non-native species may be introduced either intentionally, often for economic or aesthetic purposes—such as in agriculture, horticulture, forestry, or ornamental trade—or unintentionally through global trade and human mobility.
• Accidental introductions frequently occur via contaminated shipping containers, ballast water discharge from ships, importation of infested timber, produce carried by tourists, or soil associated with ornamental plants.
• Examples of introduction pathways include:
  • Long-distance dispersal, such as the migration of the brown planthopper (Nilaparvata lugens) in rice ecosystems;
  • Contaminated transport, for instance, Parthenium hysterophorus introduced into India with wheat imports;
  • Human-mediated activities, such as trade and travel;
  • Escape from cultivation, as seen with ornamental or aquarium plants like water lettuce and water fern.

• Although many introduced species fail to establish due to ecological or environmental constraints, a subset may adapt and proliferate, posing significant risks to native biodiversity, agriculture, human health, and ecosystem stability.

Establishment

• Following successful introduction, the next stage involves the development of a viable, self-sustaining population. The probability of establishment is strongly influenced by the species’ intrinsic growth rate, reproductive strategy, and competitive capabilities.
• For instance, agricultural weed species with heavier seeds tend to germinate more rapidly, facilitating more successful establishment than those with lighter seeds.
• Invasive alien plants (IAPs) frequently exhibit traits such as:
  • Production of numerous and easily dispersing seeds;
  • Rapid growth rates;
  • Efficient recourse utilization to native species.

• For example, Centaurea diffusa, a noxious weed in North America, has shown greater negative effects on the biomass of native North American grasses than on grasses from its native Eurasian range, indicating a strong invasive advantage in its introduced environment.

Spread

• The final stage involves the expansion of the invasive species beyond its initial point of introduction, which requires the ability to overcome geographical and ecological barriers within the new habitat.
• For successful spread, the species must not only have effective dispersal mechanisms but also possess the capacity to survive across a range of abiotic conditions (e.g., temperature, moisture, soil types) and biotic interactions (e.g., competition, predation, mutualism) in the invaded landscape.

Impacts of Alien Invasive Species

There are many examples of significant economic, social and environmental impacts caused by invasive alien species. The impacts of IASs include displacement of native species, change of soil chemical profile, rewarding pollinators better than the native species thereby reducing the reproductive success of local species, changing hydrological regimes, making the new habitat fire prone and limiting the photosynthetic efficiency of the local species by reducing light availability.

Impacts of Invasive species on Ecosystem, Ecosystem Services and Biodiversity

• Invasive alien species (IAS) are among the leading drivers of global biodiversity decline, surpassed only by habitat destruction.
• Their threat to native biodiversity exceeds that posed by pollution, overexploitation, and disease. IAS disrupt ecosystems by decreasing native species populations and modifying critical ecological processes.
• These disruptions include predation, competition for resources, hybridization, and the spread of pathogens and parasites.

• IAS often restructure ecosystems by dominating resource use and altering environmental conditions to suit themselves, thus disadvantaging native species.
• These changes can undermine vital ecosystem services like water purification, flood control, and soil fertility.
• Additionally, invasive species contribute to land degradation, reduce natural resource availability, and threaten ecosystem-derived livelihoods, including food, medicine, and building materials that support human well-being and income generation..

Impacts of Invasive species on Economic and Social Aspect

• Invasive alien species (IAS) impose substantial economic burdens globally, with estimated management and damage costs reaching billions annually in regions such as Europe and the United States.
• These costs arise from their ability to disrupt infrastructure, reduce agricultural productivity, and interfere with ecosystem services.
• Aquatic invasive alien species (IAS) significantly disrupt economic activities by obstructing navigation routes, clogging irrigation infrastructure, degrading water quality, and diminishing fishery productivity.
• These disruptions lead to substantial economic losses, including the devaluation of property and setbacks to regional development.
• Moreover, the financial burden is magnified by the difficulty in measuring the full extent of ecological damage—particularly the loss of biodiversity and the deterioration of vital ecosystem services, which often remain unaccounted for in traditional economic assessments.

• Some species serve as vectors for diseases, contributing to outbreaks of illnesses like malaria, yellow fever, and West Nile virus. Others, such as Heracleum mantegazzianum and Ambrosia artemisiifolia, trigger severe skin and respiratory conditions.
• Invasive aquatic species further exacerbate health risks by transporting pathogens in ballast water and accumulating harmful pollutants, such as PCBs and PAHs, which can be passed through the food chain, increasing health hazards for both humans and animals.

Impacts of Invasive species on Human Health and Livelihoods

• IAS contribute to landscape changes and ecosystem degradation, thereby reducing ecosystem services vital to human well-being.
• Species like the red palm weevil threaten urban greenery, while predators like the yellow-legged hornet devastate honeybee populations, affecting pollination.
• Additionally, species such as the zebra mussel damage industrial facilities, increasing maintenance costs.

• IAS also threaten food security and rural livelihoods, particularly in regions already vulnerable to climate stress. Invasive weeds and pests reduce crop yields, degrade pasture lands, and increase production costs.
• Many farmers must invest heavily in control measures or abandon land altogether, leading to job losses and economic displacement.
• Livestock production suffers when invasive plants colonize grazing lands, leading to overuse of marginal areas and resource-based conflicts among communities.

Impacts of Invasive species on Agricultural and Aquatic Ecosystem

• Agriculture is severely impacted by IAS through the introduction of weeds, insects, and pathogens—often via global trade.
• These invaders limit the availability of arable land, obstruct irrigation, and harm livestock directly or by contaminating grazing areas.
• Manual removal efforts are time-consuming and costly, diverting household labor from other essential activities.

• In aquatic systems, invasive weeds like Eichhornia crassipes, Salvinia molesta, and Pistia stratiotes degrade water quality, block fish migration, and reduce freshwater availability, impacting both agriculture and domestic use.
• These species can also promote mosquito breeding by creating stagnant water bodies, increasing vector-borne disease risks.

Impacts of Invasive species on Soil Dynamics and Ecosystem Functioning

• Invasive plants alter soil composition and microbial communities, often to their own benefit.
• They may perform nitrogen fixation, suppress native nitrogen-fixing species, release allelopathic compounds, or modify erosion patterns.
• These changes disrupt nutrient cycles and reduce the resilience and productivity of native ecosystems, creating feedback loops that further promote invasion.

Mitigation of Impacts of Alien Invasive Species

As invasive alien species (IAS) pose a global challenge, collaborative efforts at international, regional, and local levels are essential for creating unified approaches. Numerous international agreements address IAS concerns.

For example, the 1992 Convention on Biological Diversity (CBD) requires parties to take action to control and eradicate harmful alien species, while also preventing new ones from being introduced. In 2002, the CBD outlined specific ‘Guiding Principles’ to assist nations in prioritizing their strategies for managing IAS. Likewise, the 1979 Bern Convention emphasizes the need for strict regulation of non-native species introductions.

According to the CBD’s Guiding Principles, the primary approaches for addressing IAS are prevention, early detection, and swift response, all of which depend on a thorough understanding of how species establish and spread.

Certain habitats, like nutrient-rich coastal and river environments or human-altered areas, are more prone to invasions than harsh, nutrient-poor environments like mountains or dry grasslands.

Main steps for mitigating alien species:

Prevention of Introductions

• This approach is the most efficient and economical. Learning from past invasive species, like the zebra mussel in the Great Lakes, highlights the importance of intercepting invasive species early, which could prevent extensive ecological and economic losses.

Early Detection

• Identifying an invasive species early is critical for successful eradication or containment. Early detection, through species-specific or site-targeted surveys, focuses on high-risk entry points and areas with significant biodiversity value.

Eradication

• When prevention fails, eradication is the next preferred option, especially if the species is detected early. It requires a cost-benefit analysis and sufficient resources for success. Eradication methods include mechanical control, chemical treatments, habitat management, and targeted hunting.

Control

• When eradication is not feasible, controlling the species to reduce its impact is necessary. The goal is to reduce the population below a manageable level. Control methods include mechanical, chemical, and biological techniques, with integrated pest management often being the most effective approach. Biological control offers a sustainable and ecologically safe alternative, especially in conservation areas, and is becoming more common due to pesticide restrictions.

References

• Cox, George W. 2004. Alien Species and Evolution: The Evolutionary Ecology of Exotic Plants, Animals, Microbes, and Interacting Native Species. Washington, D.C.: Island Press.
• Ehrenfeld, J. G. 2010. Ecosystem consequences of biological invasions. Annual review of ecology, evolution, and systematics, 41, 59-80.
• Isard, Scott A., and Stuart H. Gage. 2001. Flow of Life in the Atmosphere: An Airscape Approach to Understanding Invasive Organisms. East Lansing, Mich: Michigan State University Press.
• ISSG. 2007. Global Invasive Species Database (GISD). Invasive Species Specialist Group of the IUCN Species Survival Commission.
• Lake J.C, Leishman M.R. 2004. Invasion success of exotic plants in natural ecosystems: the role of disturbance, plant attributes and freedom from herbivores. Biol Conserv.
• Reddy C.S, Bagyanarayana G, Reddy K.N, Raju V.S. 2008. Invasive Alien Flora of India. USGS, USA: National Biological Information Infrastructure.