Home  EnviroNews  International Conferences  Picture Gallery  Sponsor  Contact  Search  Site Map

Vol. 13 No. 3 - July 2007

Invasive species: The concept, invasion process, and
impact and management of invaders

 By: Purnima Raizada*

Invasion is considered as the second most important threat to biodiversity after habitat destruction. Alien species, which locally become dominant and invade natural communities, are referred to as invasive species. Further, IUCN also defines alien invasive species, as a species that becomes established in natural or semi-natural ecosystem or habitat, is an agent of change and threatens biological diversity. Invasive species are so much important in the present scenario that, article 8(h) of the Biodiversity Convention asks for measures ‘‘to prevent the introduction, control or even eradication of those alien species which threaten ecosystems, habitats or species’’. These invaders could be plant, animal or microbial species. Invasive species are either accidentally introduced or they are introduced by man to fulfill his needs. After introduction, they can expand their population and create mono-specific thickets. In this way these species can affect ecosystem processes, biodiversity patterns and community structure. Some important invasive species across the world are Salvinia molesta, Eucalyptus species, Hakea species, Lantana camara, Caesalpinia decapetala, Chromolaena odoratum, Eichhornia crassipes and Solanum mauritianum in Africa; Chrysanthemoides monilifera, S. mauritianum, Brassica tournefortii, Asparagus densiflorus, L. camara, Ardisia elliptica and Berberis thunbergii in Australia; Fallopia japonica, Impatiens glandulifera, Rhododendron ponticum, Gleditsia triacanthos Crassula helmsii, Acer pseudoplatanus and Ailanthus altissima in Europe; Centaurea diffuas, Bromus tectorum, Hydrilla verticillata, Melaleuca quinquenervia, Mimosa pudica,  Tamarix spp., Panicum repens, E. crassipes, Centaurea solstitialis, Phragmites australis and Imperata cylindrica  in North American continent. Some invasive species found in Indian subcontinent, their country of origin, habitats invaded and impact on ecosystems are presented in  table 1.

Factors conferring invasiveness

Factors which play a key role in the successful establishment and survival of these species are: unrestrained vegetative spread, escape from biotic constraints, prolific seed production, highly successful seed dispersal, germination, and colonization, adaptive morphological and ecological characters, superior propagule characteristics favoring greater mobility, and ability to supplant native flora either competing for resources or exerting allelopathic effects.

Invasion Process

There are three major phases of plant invasion: introduction, colonization and naturalization. When an ecosystem is disturbed either by natural processes or due to some anthropogenic factors it provides a kind of invasion window to the alien propagule. Gradually, it overcomes the environmental, reproductive and dispersal barriers and expands its population. Environmental factors like resources availability favoring establishment of alien propagule are believed to be the most important at introduction phase because introduced propagule has to compete with the established flora that is already well adapted to the site. Figure 1 provides an insight into the invasion process.

Impacts of invaders

Invasive species modify all the major ecosystem processes in the way, which suits them best. Alteration in litter dynamics is the first and foremost impact observed in the ecosystem, which an invader invades. Gradually other ecosystem processes depending on litter dynamics viz. soil biota, nutrient dynamics and biogeochemical cycles are also modified. Later, geomorphology and hydrology of the area are also changed as invasion proceeds. During the course of establishment these invasive species also interfere with native species recruitment either by allelopathic suppression or by competing with seedlings for resources. The invasive species are also known to alter fire regimes.

Management strategy against invaders

Invasive species cause degradation of ecosystem. As an invasive species is established in an ecosystem it interferes with the growth and recruitment of other native species. Further, once an equilibrium among species has occurred, it further disturbs homeostasis of ecosystem and weakens it. Accidental introduction is not easily checked but intentional introduction of alien species should be done with prior assessment so that they may not harm the ecological integrity of an ecosystem. For management of these species basic strategy of prevention, eradication and control should be adopted as explained in figure. While opting for biological control methods we should take care that the biological agents may not create a problem in later years.

Figure: Management strategy against invasive species.

Table 1: Some important invasive species in India

This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.