Introduction

Invasive plants are the ones that have been deliberately or unintentionally brought to regions other than their natural habitats and cause serious implications to economy, environment, vegetation and humans in the invaded areas. These plants expand rapidly and threaten the local vegetation by competing for water, space and nutrients. The invasive species modify the light, temperature and solar radiation availability in the invaded area. The access to food, shelter, and relaxing sites are modified for many animals as well. This can result in variations in local vegetation, rate of recurrence of fires, nutrient cycling, water accessibility and soil structure. However, a few cases have reported benefits of invasive plants. ^{1, 2, 3} For instance, they can offer fire wood to local people or supplement resources for native animal species. However, these advantages do not outperform the negative impacts. Intrusive plants affect local species through complex interactions and routes. Lest these aspects are appropriately comprehended, it is hard to foresee what kind of effect invasive plants will have.

The global annual damage from invasive alien species (IAS) in different parts of the world has been assessed $30 billion in USA⁴, €12 billion in Europe⁵, and US $14 billion in China⁶. In perspective of these environmental and financial effects of IAS, a resolution on biodiversity in Nagoya, Japan (2010) implemented the tactical proposal for biological diversity 2011-20 for IAS identification and recognition of their pathways were prioritized and measures were encouraged to counter their introduction and establishment.

As in many other countries, alien plants⁷ have also plagued India and its magnitude will further intensify because of expanding Indian economy. Actually, the size of stock imports is a critical determining factor of the extent of IAS⁸ in addition to the frequency of introduction of foreign species. Being a rich mega-biodiverse land, the implications of IAS on the preservation and sustainable usage of biodiversity would be quite articulated in India. Henceforth, there is an immense requirement to construct an integrated structure for the forecast, anticipation, fast reaction and control of alien plant intrusions in India. The present review addresses characteristics of invasive species, recognizes 173 IAS in India and gives the impact of few important alien plants (Lantana camera L., Parthenium hysterophorus L., Nicotiana plumbaginifolia Viv. and geratum conyzoides L.) on local vegetation.

Distinctive characteristics of Invasive species

Invasive species have trademark characteristics in diverse soil types and different environmental setups. These are generalist in distribution, avoided by grazing animals, abundance in poor soils as the interspecific competition has been removed due to selective grazing of other native grasses, short life cycle, high reproductive input, high dispersal rates, produces small and light weighted seeds that could be dispersed easily by winds providing them with the advantages of exploring more long habitats⁹, broad and abundant native range. IAS usually contain comparatively little amounts of genetic material in their cell nuclei¹⁰. Evidently, their cells undergo quick mitosis and duplicate rapidly and subsequently the whole plant can develop briskly as compared to other plants with greater genomic content. It provides a massive advantage to IAS in distressed landscapes.

Invasive plants in India

The World Conservation Monitoring Centre (WCMC) recognizes 1,604,000 plant species worldwide and India shares 8% of it.  A total of 15000 angiospermic plants are found in India, out of which 4900 are native, i.e. only 33%¹¹. Hajra and Mudgal¹² have reported 5400 endemics out of a total of 17000 angiospermous plants of India. India is an essential focal point of the world agriculture as it has given 167 species to the mankind and has been home to 320 wild crop species. The current review has focused on 173 IAS in India (Table 1). These species are the etremee stubborn invasive plants of a diverse array of families as Mimosacceae, Asteraceae, Amaranthaceae, Papilionaceae, Solanaceae, Convolvulaceae and others.

Table 1: Invasive alien plants in India.

Selected Problematic Invasive Weeds in India

Lantana camera L.

Lantana camera L. is a member of Verbenaceae family well known for its invasive nature and is considered among the world’s top 10 worst weeds. In India, it was introduced at the beginning of 19^th century. Since its invasion to India, it has created a major problem to native flora of many states. L. camera has flourished in plains of India as well as is Himalayan regions¹³. Due to its allelopathic action it causes a serious threat to native vegetation. It contains several allelochemicals viz: phenolics, methylcoumarin, umbelloferone and salicylic acid. Among all these allelochemicals, salicylic acid is the most phytotoxic¹⁴. A recent report claims the occurrence of toxic allelochemicals in Lantana¹⁴. L. camera shows a significant effect on seed germination and growth of native vegetation^{15, 16, 17}.

Parthenium hysterophorus L.

Parthenium hysterophorus L. is a member of Asteraceae family, also known as congress grass is considered among 100 most invasive species in the world. In more than 40 countries, it is regarded as an obnoxious weed of field crops¹⁸. It has entered India accidently through conjugation with imported food from the USA between the period of 1950s to 1960s¹⁹. Its presence was first detected in Pune, Maharashtra (1955). After that it was delineated taxonomically by Rao in 1956¹⁹. It has the potential of quick colonization with several other characteristics, such as wide adaptability, less number of natural niche enemies, drought tolerance, prolonged viability of seeds, high production of seeds, seeds with small size and light weight, long dispersal of seeds through air, water, birds and other carriers, strong competition and allelopathy makes it a successful invader of crop fields¹⁸.  Besides that, it is also capable of causing human health problems like skin allergies, irritation of eyes with mild fever. It is unappetizing and harmful to farm animals. Various allelochemicals have been extracted from the roots, stem, leaves, inflorescence, spores and seeds. These include soluble phenolics and sesquiterpene lactones. The occurrence of plant growth suppressors in P.hysteriphorus was first reported by Rajan²⁰ and Kanchan²¹.  Along with Parthenin, other inhibitors such as caffeic acid and p-coumaric acid were also found in the stem²¹ of this weed. The leaf leachates of P. hysterophorus show inhibitory effects on several agricultural and tree crops^{22, 23}.

Nicotiana plumbaginifolia Viv.

Nicotiana plumbaginifolia Viv. (Solanaceae family) a native of Cuba, South America and Caribbean is reported as a weed of wastelands and cultivated lands. It is considered as an agricultural weed as well²⁴. It is invasive in India and is regarded as an aggressive colonizer²⁵. This herbaceous plant also has some medicinal values²⁶. The allelochemicals isolated from its leaves are hexamethoxy 4′,5′ methylenedioxyflavone, exoticin, dimethylenedioxy 3,5,3′trimethoxyflavone and hexamethoxy 6,7 methylenedioxyflavone. Mushtaq et al.^{25, 28} found the plant has serious growth inhibitory properties on other plants. Mushtaq and Siddiqui²⁹ have reported the negative growth effects of other members of Solanaceae family as well.

Ageratum Conyzoides L.

Ageratum conyzoides L. is a member of Asteraceae family is reported as a noxious weed in agricultural fields and a colonizer of open degraded lands. It is native to South and Central America³⁰. It also serves as a host to pathogens including nematodes thus, affects crops³¹. Soon after its invasion on land, it suppresses the local vegetation causing inadequacy of fodder. Its invasion to forests critically threatens many of essential medicinal and aromatic plants. A. conyzoides was identified in India before 1882³². Ageratum has been reported to suppress growth and yield of native crop species through the release of various phenolics and volatile oils³³. Besides its effect on crops, it also affects human health due to its strong pungent odor. Nausea, allergy and giddiness are some of the health problems caused by this weed in humans.

Role of floods in plants invaions.

Researchers believe that the prevalence of invasive plants is one of the most important environmental problems in Kerala India. Particularly after the floods, the prevalence of invasive plants in this area has increased. Seasonal flooding has a direct effect on plant invasion to new environments³⁴. Many plant species inhabiting mountain regions enter the plains and agricultural ecosystems through flood flows, and some of these species have a high potential for conversion into invasive weed species. These plants due to their specific biological, morphological and physiological characteristics and high adaptability to climatic conditions, has potential to rapidly grow and reproduction in new environments and compete with crops as invasive weeds³⁵. Shafroth et al.³⁶ evaluated the importance of flood events on the establishment of weeds and found that Saltcedar (Tamarix ramosissima) have potential to establish in new lands after floods. Floods can be considered as a seed carrier of plant species³⁷ which often transmits the seeds of invasive species to new environments depending on their biological status³⁸. Cuda et al.³⁹ studied the effects of floods on weed invasion in Europe regions and reported Impatiens glandulifera as an invasive weed due to floods and spreading through river corridors.

Management

Generally, the management of prolific invasive plant species is one of the most important agricultural and environmental challenges in India. These plants have potential for threaten ecosystems, capture the ecological niches in agricultural systems, and cause many problems in agricultural production. Therefore, it is very important to prevent the establishment and effective management of these plants. In order to prevent the entrance and establishment of invasive plants in new environments, all processes related to the import of seeds and reproductive organs of new plants, planting and transfer of plants in new areas should be carefully monitored and prevented from introduction of suspected plant species. Where preventive strategies are not successful, the rapid detection of areas occupied by these plants and complete eradication in the early stages of their establishment is very important. In the case of established invasive plants, preventing the distribution and transmission of seeds and reproductive organs can lead to manage the development of plant invasion. Srivastava et al.¹⁶ reported 149 invasive plants from North-Eastern Uttar Pradesh, India, and concluded that early identification and reporting of infestation and spread of these plants is very important for their effective management. Biological control method is one of the most effective methods for managing invasive plant species. The most important advantage of the biological control method for invasive plant species is that, after the establishment of biological control agents, a sustainable and long-term management of invasive species is achievable. Wheeler et al⁴⁰ reported biological control as a successful method for management of invasive weed Schinus terebinthifolia.

Conclusions

Invasive species form the second most serious threat to biodiversity after habitat destruction. Some invasive weed species make a dramatic impact on the processes of ecosystems and biodiversity when they invade. Invasion of an alien species is habitat-dependent. Generally the habitats that are nutrient-rich witness more conquest as compared to poor habitats. High reproductive potential, short life cycle and production of many seeds/propagules are some characteristic traits associated with successful invaders. Our analysis summarises the evidences for the impact of alien plant species on many ecological variables. Recently, the negative effects of alien species on resident crops and the mechanisms for these effects have attracted the focus of researchers. The decrease in local plant species due to invasive species and their development was the main focus of this review.

From the available literature it becomes evident that invasive species have both positive as well as negative impacts. This dual cost nature of many invasive species creates conflict of interests among stakeholders. Therefore, during the management of invasive alien species a uniform management policy cannot be adopted. The invasion of many weed species harmed Indian native flora e.g. Lantana camara invasion to India proved dreadful. Currently this invasive weed occupies 13 million hectares of land and further continues to expand its establishment because of its high invasive potential. The control of these invasive weeds may help in protection and restoration of biodiversity. Because of the fast economic globalization and increased trade with other countries, India has become a good recipient for alien invasive species. There is a need to inventorize the alien flora of India so that the research on invasion biology can be geographically understood. These inventories will act as scientific baselines to understand the pattern, impact and management of invasive plant species in India.

Acknowledgement

Authors are thankful to the Aligarh Muslim University, Aligarh, India; for their support in the collaboration of this project.

Conflict of interest

We declare there is no conflict of interest among us.

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