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Vol. 15 No. 3 - July 2009

Alien Plant Invasion: A Hot Ecological Issue

By: R. S. Tripathi*

Migration of species from one geographical region to another across the natural barriers such as high mountains, seas and oceans has been taking place since time immemorial. The movement of species through natural dispersal agents has been rather slow. However, with globalization there has been a phenomenal increase in trade, tourism, travel and other human activities, and this has caused both intentional and unintentional introduction of species from one country to another at a pace that was never witnessed before. If a plant species arrives in a territory where the habitat conditions are similar to its native place, it germinates, survives, grows, reproduces and produces self-sustaining populations in areas of natural and semi-natural vegetation in course of time. Many exotic species may grow luxuriantly in the new environment and can expand their range of distribution at a fast rate, and may even pose a serious threat to the native species. Such introduced or exotic species are referred to as invasive alien species. Invasive plant species are characterized by rapid growth, high reproductive capacity, efficient dispersal mechanism, high competitive ability and wide ecological amplitude. They have unique ability to adapt physiologically to new environmental conditions. Most of the invasive plant species possess high phenotypic plasticity coupled with hybridization capacity and highly efficient reproductive strategies. These attributes of invasive species contribute to their ecological success and capability to invade new areas. It may be noted that all the alien/exotic species that enter a new territory may not be invasive.

The problem of biological invasion has been recognized by SCOPE (Scientific Committee on the Problems of Environment) as a central problem in the conservation of biological communities. The invasive alien plants have serious ecological implications for the conservation of native biodiversity, maintenance of plant community structure, plant succession and ecosystem processes in the areas invaded by them. Keeping in view the crucial role played by invasive alien species in the conservation of biological diversity and its sustainable use, the Convention on Biological Diversity (CBD) proclaimed invasive alien species as the theme of this year's International Day for Biological Diversity. The United Nations have declared 22 May the International Day for Biological Diversity and the year 2010 the International Year of Biodiversity. The problem of plant invasion, has engaged the attention of ecologists, conservationists and environmentalists all over the world during the past 3-4 decades especially after the launch of the Global Invasive Species Programme (GISP) by the SCOPE during eighties. As of now, the problem of plant invasion has become a matter of grave concern all over the world. However, in India, so far, the problem of plant invasion has not been addressed as adequately as it should have been done, although several exotic plants have invaded the high-value biodiversity areas and have adversely affected the natural and semi-natural vegetation/ecosystems.

Biological invasion and climate change are the two hottest topics of ecology these days. The extent of distribution, rate of spread, and persistence of invasive alien species directly influence the native biodiversity of the invaded region and therefore, the trends in invasion by alien species has been identified as an important indicator of the loss of biodiversity. The 'Convention on Biological Diversity's framework for monitoring progress towards its “2010 Target” which commits CBD to achieve by 2010, a significant reduction in the current rate of biodiversity loss, regards biological invasion as one of the two major threats to biodiversity. It has been reported by a number of researchers that the invasion of plant species in the new environments is triggered by man-induced habitat fragmentation, land degradation, forest degradation, land use and land cover changes, fire regime and other kinds of anthropogenic stresses that impact natural ecosystems. The facilitative effect of these drivers on plant invasion may presumably be mediated through the reduction in biotic and physical resistance of various kinds that would have been offered by the undisturbed host plant community. The quantification of the extent of influence exercised by different kinds of environmental resistances to the invading plant species in the host community could be a very challenging area of ecological study. Apart from causing depletion of native biodiversity, invasive alien species also alter species composition, affect soil physical, chemical and biological properties, and affect community development and ecosystem processes adversely, but no reliable quantitative data is available on any of these aspects. The effects of invasive alien species on the distribution, abundance and population dynamics of native plant species in natural ecosystems, hydrology, soil biology, and ecosystem processes need to be studied in detail. In India, a good number of high-value biodiversity sites have been invaded by several invasive alien plants, but unfortunately, the studies on the biology of plant invasion have been only scanty. The biology and population dynamics of a number of exotic weeds have been studied by the author and his collaborators at the department of Botany, North-Eastern Hill University, Shillong during 1980- 2000. Besides, researches on weed biology have also been conducted at some other universities and research organizations in India, but the plant invasion perspective has been missing in most of these studies.

Although the problem related to the invasion of alien plant species has engaged the attention of ecologists, foresters, agricultural scientists and government agencies over the past few decades, nothing tangible has been done to eradicate and manage the ever-increasing populations of invading plant species that have already established in the invaded areas and are extending their range of distribution due to man-induced habitat fragmentation and other kinds of anthropogenic stresses. The economic and ecological costs associated with the invasion of these alien plant species are indeed staggering. Some of the interesting aspects, and the exciting points that emerge as a result of analysis and synthesis of the scientific information gathered on relevant aspects of plant invasion, are presented below.

  • The genetic changes are likely to occur in alien species subsequent to invasion in a new region, and these changes may hold key to their success in the invaded land. An invading species that has colonized a novel environment has to face a genetic challenge, because it has not experienced the selective pressures presented by the new environment. Despite this, alien species become successful invaders although they have to face challenges from the already well adapted native species. The biologists need to find out the under lying mechanisms and processes that make the invading species so successful in their new environment.

  • Some invasive alien species being intrinsically better competitors, offer strong competition to native species in the invaded region. The native species show a decline in resource use and invaders can increase their distribution and abundance at the expense of the resident species of the area. This may cause a drastic reduction in population size of several native species and some of them may even be eliminated from their natural habitats.

  • Many invasive plant species release chemical compounds into the environment , which are not generally harmful to them, but those chemicals suppress the growth of other plant species growing in close proximity of such invasive species. This negative effect (often referred to as allelopathic effect) of invaders on the native species confers a tremendous competitive advantage on the former. The 'chemical release hypothesis' offers a plausible explanation for the spectacular success of invasive plant species in the new areas that they invade.

  • The herbivores and parasites or pathogens, the natural enemies of the invasive species, that were regulating their population growth in their native place are absent in the invaded region. Invading species generally arrive in their new environments without their co-evolved natural enemies from their natural habitats that they had occupied in their native place. This may provide invaders opportunities for luxuriant growth and more prolific reproduction, which allows them to out-compete native species, and expand their range of distribution. This is the basis of the so-called 'enemy release' or 'escape' hypothesis which is employed to explain the spectacular success of invasive alien plants in their new environments.

The above hypotheses or approaches explain why and how alien species become more successful in the invaded land compared to their native place. It may be mentioned that the majority of studies on invasive alien species have been conducted in the invaded territory, and surprisingly, we do not have any quantification regarding their abundance, competitive success, aggressiveness and response to the natural enemies in their native land. The soundness of these “invasion hypo-theses” can be tested only when we have comparative bio-geographical approach towards the problem of biological invasion and have sufficient relevant data from the native as well as invaded regions.

For any species to become successful, it is essential that it genetically adapts itself to its new environments. It may also be mentioned here that preserving genetic diversity is absolutely necessary for species to continually adapt genetically in a changing environment. Therefore, investigating the genetic adaptability of invasive alien plant species in the new environments should also be an issue of focus among population ecologists and conservation biologists.

Besides impacting native biodiversity through direct competitive suppression, the invasive alien species having strong allelopathic potential may al so influence plant diversity by converting a complex plant community into a much simpler one, which is characterized by the dominance of only a few species. In extreme situations where the impact is severe, the single species-dominance may also result. This kind of effect of invasive species has serious implications for food chain length and complexity of food web, which are pre-requisites for ecosystem stability and smooth functioning of ecosystems. Unfortunately, the researches related to impact of invasive plant species on ecosystem processes are scarce. Ecological impact of plant invasion is a serious cause of concern to all of us. The ecological consequences of plant invasion are many-fold and we need to have zero-tolerance towards invasive alien species. The issue of alien plant invasion needs to be addressed very seriously like the problem of climate change.

Indeed, the problem has engaged the attention of ecologists during the past few decades, but the coordinated rigorous effort involving multinational research teams has been missing. In India where the problem of alien plant invasion is quite severe, the Union Government has recently taken some initiative to address the issue in a befitting manner. The state governments should also follow suit. Some of the aspects/points related to alien plant invasion in India on which we need to focus our attention without any further delay, are as follows.

  • The status of alien plant invasion in India.

  • Identification of the worst alien species that have invaded the natural ecosystems in different bio-geo-graphic regions of India, their distribution, rates of invasion, and their population dynamics.

  • Invasive alien species and their impact on native biodiversity, plant community composition and eco-system processes.

  • Attributes and ecological strategies of the worst invasive alien species and their pathways of invasion.

  • Identification of the habitats and ecosystems that are most vulnerable to plant invasion.

  • Exploring the causes of spectacular success of the invasive plants in the natural ecosystems of India.

  • Plant invasion as related to various kinds of anthropogenic disturbances, fire regimes, species richness and species composition and habitat characteristics of the host plant communities.

  • Plant invasion in relation to elevated concentration of carbon dioxide, climate change and other global changes.

  • Genetic adaptability of exotic species in contrasting ecological habitats.

  • Impact of invasive alien plants on physical, chemical and biological properties of soil of the areas which they invade.

  • Effects of invasive alien plants on ecosystem processes.

  • Strategies and action plans for effective management of some of the worst invasive plant species at the local, eco-region and national levels.

There is a need to launch a coordinated/network research program at the national level in India encompassing the points outlined above. We need to develop a policy framework for tackling the problem of plant invasion in India. Special emphasis has to be laid on some of the worst invasive alien weeds that have been spreading very fast and have become established and naturalized in several parts of India. Notable among such alien species are Ageratum conyzoides L., Eichhornia crassipes Solms., Eupatorium odoratum L. (syn. Chromolaena odorata (L.) King & Robinson), E. adenophorum Spreng. (syn. Ageritina adenophora (Spreng.) King & Robinson), E. riparium Regel. (Ageritina riparia (Regel.) King & Robinson, Galinsoga ciliata Blake, Lantana camara L., Mikania micrantha H.B.&.K., Parthenium hysterophorus L. and Prosopis juliflora DC.

There is a need to launch a national website and create a National Authority that should cover the entire gamut of the problems associated with biological invasion. The proposed Authority could monitor the invasion and spread of alien species, educate people about the adverse impact of invasive alien species on native flora and integrity of natural ecosystems, and on human and animal health. The Authority could also formulate strategy and action plan for preventing alien invasion, and for the control and eradication of invasive exotic species, and suggest suitable mitigation measures where the preventive and control measures fail to yield the desired results.

 

*INSA Senior Scientist, National Botanical Research Institute, Lucknow-226 001

E-mail: tripathirs@yahoo.co.uk

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Financial support from the Indian National Science Academy, New Delhi under the INSA Senior Scientist Scheme is gratefully acknowledged.


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


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