Invasive Species

Invasive species are organisms introduced into new environments where they cause ecological, environmental or economic harm. Although the movement of species across regions has occurred naturally throughout evolutionary history, human activity has significantly accelerated these processes, particularly since the Age of Discovery and through expanding global trade networks. Many invasive species are introduced accidentally, while others are brought intentionally for agriculture, landscaping or biological control. Once established, these species may threaten biodiversity, disrupt ecological processes and impose substantial management costs.

Terminology and Conceptual Background

The term invasive species is commonly used to describe a subset of non-native or alien species that pose measurable risks to native species, habitats or human interests. However, the definition remains contested due to its subjective nature and interdisciplinary usage. Some researchers include only harmful non-native species, whereas others broaden the concept to encompass native species that become destructive following human-induced changes to their food webs or habitats. Examples include native predators or grazers whose populations expand rapidly after disruption of natural controls.
Debates also arise around what constitutes a “native” species. For instance, horses evolved in North America but went extinct there before being reintroduced by Europeans in the 15th century, creating ambiguity regarding their classification. The scientific field examining these issues is generally known as invasion ecology or invasion biology, although it lacks formal standardisation. Foundational work such as Charles Elton’s The Ecology of Invasion by Animals and Plants (1958) shaped early thinking by proposing broad principles of invasion processes.
In response to inconsistent terminology, some scholars have proposed classification systems based on biogeographical criteria rather than taxonomy or human-centred evaluations. These approaches assess the success of populations within ecosystems without automatically assuming harmfulness or focusing solely on introduced species.
Governmental definitions often narrow the scope. For example, in the United States, Executive Order 13112 defines an invasive species specifically as an alien species whose introduction causes, or is likely to cause, economic, environmental or human health damage.

Causes of Biological Invasions

Biological invasions typically begin when species are transported beyond their native range, either intentionally or accidentally. Before becoming invasive, species must survive at low population densities, establish reproductive populations and subsequently expand. Multiple introductions often facilitate success, a phenomenon described as high propagule pressure. Frequent movement of goods and vehicles—such as ships, aircraft and road traffic—provides repeated opportunities for establishment.

Ecosystem-Based Mechanisms

In any ecosystem, resource availability plays a central role in determining how easily an invasive species can establish itself. Stable ecosystems tend to maintain equilibrium, but disturbances such as wildfires, land clearing or nutrient enrichment may create conditions favourable to invaders. Fast-growing or highly adaptable species can outcompete native organisms for limiting resources such as nitrogen and phosphorus.
Species occupy ecological niches that reflect their roles in native ecosystems. Invaders may exploit unoccupied niches or generate new interactions through their presence. For example, edge effects created by agricultural expansion produce novel habitat boundaries that may favour invasive species better adapted to disturbed conditions.
Debates exist regarding whether diverse ecosystems are more or less resistant to invasion. Elton proposed that highly diverse communities offer fewer available niches, reducing susceptibility. Later studies suggested the opposite at larger spatial scales, showing that species-rich ecosystems may also support numerous invaders due to increased resource heterogeneity and weaker biotic interactions. These differing conclusions emphasise the importance of scale in invasion studies.
Island ecosystems, such as Guam, New Zealand and Madagascar, often experience severe impacts because native species evolved with limited predators and competitors. The introduction of organisms like the brown tree snake on Guam, Polynesian rats in New Zealand or invasive shrubs and aquatic plants in Madagascar has resulted in pronounced biodiversity loss. Aquatic invaders such as Eichhornia crassipes, which forms dense floating mats, have harmed water quality and increased management costs in many regions.
Geomorphological impacts also occur. Invasive plants may stabilise soil or alter erosion patterns through bioconstruction and bioprotection. For instance, kudzu, introduced in the United States to reduce erosion, has reshaped landscapes by growing rapidly over native vegetation. Invasive animals contribute through bioturbation and bioerosion, as seen in the case of the Chinese mitten crab, which destabilises riverbanks through burrowing.
Native species can act invasively when ecological relationships are disrupted. The purple sea urchin off the coast of northern California has expanded dramatically due to the reduction of its natural predator, the sea otter, leading to widespread kelp forest decline.

Species-Based Mechanisms

Certain biological traits may predispose species to invasiveness. These characteristics include rapid growth, high reproductive rates, broad diets, tolerance to variable environmental conditions and effective dispersal abilities. Some invasive species also employ aggressive competitive behaviours, chemical defences or mutualisms with other introduced organisms.
Studies examining trait patterns have produced mixed results. One influential analysis suggested that a high percentage of invasive species could be identified by their traits alone, implying that certain combinations of characteristics enhance invasion success. However, subsequent research indicated that many non-invasive species possess similar traits and that invasiveness cannot reliably be predicted by traits in isolation. This discrepancy reflects the complexity of ecological interactions and the importance of context-specific factors.
Notable invasive species include plants such as kudzu, Heracleum mantegazzianum, Reynoutria japonica and Centaurea solstitialis, as well as animals such as European rabbits, feral cats and carp. Some of these species are listed among the world’s most harmful invasive organisms due to their extensive ecological and economic impacts.

Impacts and Global Patterns

Invasive species can profoundly alter ecosystems by reducing biodiversity, changing nutrient cycles, modifying habitat structure and initiating trophic cascades. Agricultural productivity may decline, native species may face increased predation or competition and ecosystems may become less resilient to further disturbance.
Examples illustrate these varied effects. In New Zealand, the arrival of early Polynesian settlers brought dogs and rats that decimated endemic bird species. In Madagascar, a combination of deforestation and introduction of non-native plants and animals has contributed to long-term habitat degradation. Invasive plant species such as Lantana camara, now established across more than 60 countries, pose ongoing management challenges and have prompted extensive governmental interventions.
Some invasive species incur additional consequences through their interactions with geomorphological processes. For example, invasive plants may accelerate sediment accumulation, while burrowing animals disrupt soil structures or erosion patterns. These changes can affect waterways, agricultural lands and built environments.

Integrated Understanding of Invasiveness

Biological invasions result from a complex interplay of ecological, evolutionary and anthropogenic forces. Traits of the invading species, conditions in the recipient ecosystem and the nature of human activity all influence outcomes. Although many invasions yield negative consequences, not all introduced species become invasive, and some may integrate without causing significant harm.
Continued research in invasion ecology seeks to standardise terminology, develop predictive frameworks and improve management strategies. Given ongoing globalisation, climate change and land-use modification, the threat posed by invasive species is expected to persist, necessitating coordinated international responses and robust ecological understanding.