Alien plant invasion: An Indian Perspective
By: *S. Tripathi, FNA
The importance of plant invasions in the transformation of landscapes did not escape the attention of Victorian biologists including Darwin and Wallace. Darwin (1872) pointed to the rapid spread of alien cardoon (Cynara cardunculus) and a tall thistle (Silybum marianum) in Argentina, and Wallace (1905) reported abundant growth and distribution of alien Rumex acetosella in New Zealand. In recent times, the problem of invasion by alien plant species has become a matter of great concern all over the world. The migration of plant species from one geographical region to another across natural barriers such as high mountains, seas, and oceans has been taking place since time immemorial, but the movement of plant species through natural dispersal agent 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 (Tripathi 2009). If a plant species arrives in a territory where the habitat conditions are similar to those of its native place, it germinates, survives, grows, reproduces, and produces self-sustaining populations in natural and semi-natural ecosystems in the course of time. Many exotic species may grow luxuriantly in the new environment, expand their range of distribution at a fast rate, and even pose a serious threat to the native species of the invaded area. These alien invasive species are characterized by rapid growth, high reproductive capacity, efficient dispersal mechanism, strong competitive ability, and ability to adapt physiologically to new environmental conditions. Thus, they are able to cope successfully with the biological and physical conditions of the invaded territory.
The problem of biological invasion has been recognized by the Scientific Committee on the Problems of the Environment (SCOPE) as central problem in the conservation of biological communities. 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. The problem of plant invasion has engaged the attention of ecologists, conservationists, and environmentalists all over the world over the past 4 to 5 decades, especially after the launch of the Global Invasive Species Programme (GISP) by SCOPE. However, in India, so far the problem of plant invasion has not been addressed adequately, although in India too, several exotic plants have invaded the high-value biodiversity areas and have adversely affected the natural and semi-natural ecosystems.
Alien Plant Invasion in India
The extent of distribution, rate of spread, and persistence of invasive alien species directly influence the native biodiversity of an invaded region, therefore, the trends in invasion by alien species have been identified as an important indicator of the loss of biodiversity. Biological invasion is one of the major threats to biodiversity, next only to habitat destruction. In northeast India invasion of plant species is triggered by human-induced habit fragmentation, land degradation, forest degradation, land-use and land-cover changes, Jhum cultivation and other kinds of anthropogenic stresses that impact natural ecosystems. Studies on population dynamics and growth of a number of exotic species such as Chromolaena odorata, Ageratina adenophora, Ageratina riparia, and Imperata cylindrica in relation to burning, age of “jhum” fallows, associated vegetation, varied density and light regimes, and soil conditions have clearly revealed that these weeds are particularly successful in disturbed habitats (Tripathi 1985). The facilitative effect of these drivers on plant invasion may presumably be mediated through the reduction in various kinds of biotic and physical resistances 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 an invading plant species in a host community could be a very challenging area of ecological study. Apart from causing depletion of native biodiversity, invasive alien species alter species composition of plant communities, affect physical, chemical and biological properties of soil, and affect the plant community development and ecosystem processes adversely, but reliable quantitative data available on these aspects are few and far between. 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 process need to be studied in detail. In India, a good number of high-value biodiversity sites have been invaded by invasive alien plants but unfortunately, studies on the biology of plant invasion are rather scanty. The biology and population dynamics of a number of exotic weeds were studied by the author and his collaborators in the department of botany at North-Eastern Hill University, Shillong, Meghalaya, from the 1980s onwards. Research on weed biology has also been conducted at several other universities and research organizations in India, but the plant invasion perspective is missing in most of these studies.
There is every likelihood that invasive alien species would adversely affect vegetation pattern and processes by progressively replacing existing indigenous flora in not only high value biodiversity areas such as the northeastern region of India and the Western Ghats but also in other parts of the country. Adhikari, Tiwari and Barik (2015) identified the hotspots of alien species invasion in India through Ecological Niche Modelling (ENM) using species occurrence data of IAS from the Global Biodiversity Information Facility (GBIF). About 49% of the total geographical land area of India was predicted to be prone to invasion at moderate to high levels of climatic suitability. According to this study, nineteen of 47 eco-regions of India harboured invasion hotspots. Most ecologically sensitive regions of India, including the “biodiversity hotspots” and coastal regions coincide with invasion hotspots, indicating their vulnerability to alien plant invasion. The intersection of anthropogenic biomass and ecoregions with the regions of ‘high’ climatic suitability was classified as hotspot of alien plant invasion.
ENM has also been proved to be a powerful tool in predicting the invasion potential of specific IAS under different climate change scenarios (Barik and Adhikari, 2011). Such knowledge base is crucial for guiding the formulation of an effective policy and management strategy for controlling the invasive alien species.
Important aspects related to plant invasion
Some of the interesting aspects and exciting points emerging as a result of analysis and synthesis of the scientific information gathered on the ecology of plant invasion (Tripathi 2009, 2013) are presented below:
Genetic changes are likely to occur in a species subsequent to invasion in a new region, and these changes may hold the key to its success in the invaded land. An invading species that colonizes a novel environment has to face a genetic challenge, because it has not experienced the selective pressures presented by the new environment. Despite this, most alien species become successful invaders, although they have to face challenges from the already well-adapted native species. Biologists need to find out the underlying mechanisms and processes that make the invading species so successful in their new environments.
Invasive alien species (e.g., Eupatorium odoratum) are intrinsically better competitors than the native species. And so they offer strong competition and pose a serious threat to native species in the invaded region (Yadav and Tripathi 1981). The native species of an area show a decline in resource use, and the invaders can increase their distribution and abundance at the expense of the resident species. This may cause a drastic reduction in the population size of several native species and some of them may even be eliminated from their natural habitats.
Most invasive plant species possess high phenotypic plasticity (Rai and Tripathi 1983) coupled with hybridization capacity and highly efficient reproductive strategies. Rai and Tripathi (1983) have reported that in case of
Galinsoga ciliata and Galinsoga parviflora the reproductive effort showed considerable plasticity, but it is maintained at a fairly high level even under the stressed ecological conditions. Their populations are characterized by the presence of at least three seedling cohorts, which emerge at different times, and these seedling cohorts differ in their half-life, survivorship, and seed output (Rai and Tripathi 1984). This adds to their level of plasticity and contributes to their adaptability, ecological success, and ability to invade new areas.
Many invasive plant species, for example,
Ageratina riparia or Eupatorium riparium (Rai and Tripathi 1984),
Parthenium hysterophorus, Chromolaena odorata, and Ageratina adenophora (Tripathi et al. 2012), release chemical compounds into the environment, which are not generally harmful to them, but those chemicals suppress the growth of other species growing in the close proximity of invasive species. This negative effect (often referred to as an 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 areas that they invade.
The herbivores and parasites or pathogens, the natural enemies of invasive species, that were regulating the population growth of invaders in their native place are absent in an invaded region. Invading species generally arrive in new environments without their co-evolved natural enemies from their natural habitats. This may provide invaders opportunities for luxuriant growth and more prolific reproduction, which allow them to outcompete 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 new environments.
The aforementioned 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 natural enemies in their native land. The soundness of these “invasion hypotheses” can be tested only when we apply a comparative biogeographical approach towards the problem of biological invasion and have sufficient relevant data on invasive alien plants from their native as well as invaded regions. In one of his earlier articles, Tripathi (1985) emphasized that a comparative study of population behavior, individual fitness, and reproductive strategies of invasive alien species in their countries of origin and in the invaded territory could be quite revealing and rewarding.
Genetic Diversity and Invasibility of Alien Plants
For any species to become successful in a new environment, it is essential that the species genetically adapts itself to its new environment. It may also be mentioned here that preserving genetic diversity is absolutely necessary for a species to continually adapt genetically to a changing environment. Therefore, investigating the genetic adaptability of invasive alien plant species in the new environments should also be an issue of focus for population ecologists and conservation biologists.
Greater genetic diversity may lead to greater phenotypic variation and adaptability. The greater overall hetero-zygosity may increase population fitness and reduce genetic bottlenecks that can limit the adaptive evolution of fitness-related traits. Therefore, invasive populations with greater genetic diversity may be at an evolutionary advantage, and there are a number of examples of invasive species that have adapted to evolve adaptive traits in their introduced range. Intra-specific genetic diversity of plant populations is an important factor shaping the diversity and structure of communities, and thus, it is also important for assessing the impact of invasive species on organisms of higher trophic levels. Genetic diversity influences invasion success, however, the increase in invasibility of alien plants vis-à-vis their genetic diversity needs to be tested empirically under varied ecological conditions of India.
Invasive Alien Plants and Ecosystem Processes
As mentioned earlier, invasive alien species cause depletion in native biodiversity the components of which are well adapted to their physical environment and interactions among themselves. As and when some of the native species are lost, the subtle interactions which contributed to the development of a distinct plant community and ecosystem structure are disturbed and this may affect ecosystem processes. The complex communities comprising both plant and animal species become simpler, food chain is shortened and food web complexity is reduced. Thus invasive alien species can cause changes in the food web architecture, which provides one of the mechanisms by which ecosystem processes are altered. There could be three pathways underlying the mechanism: (a) trophic cascades in which invasive species may reduce populations of consumers that transport nutrients between habitats reducing inputs and within-system cycling; (b) invasive alien plants producing such secondary metabolites which can adversely affect the species composition and abundance of detritivores, may alter the litter decomposition rates; (c) litter of invasive alien plants could be either less or more palatable to decomposers than the native species, and this may change the litter decomposition rates and nutrient cycling in the ecosystem.
The way forward
Like climate change the problem of alien plant invasion needs to be addressed very seriously. Indeed, the problem has been engaging the attention of ecologists all over the globe. The economic and ecological costs associated with the invasion of alien plants are indeed staggering. However, in India, so far the problem of alien plant invasion has not been adequately addressed.
There is a strong need for undertaking long-term research programs on different aspects of alien plant invasion in a network mode covering the entire length and breadth of India, as the problem of invasive alien plants is already quite alarming in this country and it is going to further worsen during the next few decades from now. In view of the enormity of the alien plant invasion problem in this country, the researches related to this subject must be accorded high priority. Aspects of alien plant invasion that need to be addressed on urgent basis are:
Monitoring the distribution, rates of invasion and population dynamics of invasive alien species under the climate change scenario.
Pathways of alien species invasion,
Identifying the hotspots of invasive alien species in India through Ecological Niche Modeling using species occurrence data, and predicting vulnerability of ecological habitats and ecosystems to alien species invasion,
Impact of invasive alien species on native biodiversity, plant community structure, natural succession and ecosystem processes,
Impact of alien plant invasion on the diversity and abundance of soil microbes and its implications for nutrient cycling in the ecosystem.
Bio-prospecting of invasive alien plants emphasizing particularly on the possibility of exploiting their secondary metabolites (e.g. allelochemicals) for producing natural pesticides and drugs.
In view of the various ecological implications of alien plant invasion, there is a need to launch a national website on biological invasion, set up regional biological centres across the country, and create a national biological invasion authority which should cover the entire gamut of problems associated with biological invasion. As the invasion by alien plant species is more conspicuous and serious in India, the proposed authority, initially, could lay a greater emphasis on invasive alien plants than the alien animals.
*Formerly, Professor of Botany at North-Eastern Hill University, Shillo ng (Meghalaya), and INSA Senior Scientist at CSIR-National Botanical Research Institute, Rana Pratap Marg, Lucknow, India, firstname.lastname@example.org