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Review Paper Concept of plant invasion Ecology as Prime Factor for Biodiversity Crisis: Introductory review Prabhat Kumar Rai Department of Environmental Science, Mizoram University, Tanhril, Aizawl-796004, Mizoram, INDIA Available online at: 
www.isca.in, www.isca.me
Received 22nd January 2015, revised 14th March 2015, accepted 2nd April 2015 AbstractBiodiversity is extremely precious resource issue in current civilization as it is inextricably linked with sustainable development. Plant invasion is the priority threat to global biodiversity and hence deleterious to both ecology and economy of any nation. Exotic invasive plant species poses serious threat to the native biodiversity. Invasive plants transmogrify the landscape ecology in a highly complex manner leading to a sort of ecological explosion. Global terrestrial as well as aquatic ecosystems are invaded by various invasive plant species. Invasive species are alien species whose introduction and spread threatens ecosystems, habitats or species with socio-cultural, economic and/or environmental harm, and harm to human health. Present review precisely introduce to invasion ecology and its interrelated interaction with other anthropogenic perturbations.Keywords: Invasion, biota, environmental problems, indigenous. Introduction In recent Anthropocene era, biodiversity extends to humankind multifaceted direct economic benefits and direct essential services through natural ecosystems and plays a prominent role in ecosystem function and stability1-8. In recent era of rapid industrialization and urbanization, anthropogenic perturbations are causing a biodiversity crisis, with species extinction rates up to 1000 times higher than background 9,1. Plant invasions have caused an unprecedented loss of the global native biodiversity. Henceforth, in the current era of Anthropocene, unravelling the facts that what makes the replacement of indigenous climax communities originating through succession by exotic invasive species1,10-14 is of extreme importance. Plant invasion is one of the major threats to global biodiversity, being ‘big five’ environmental issues of public concern1,15,16 and one of the six most serious environmental problems which may influence future economical and social development1,17-19. Earth Summit in Rio de Janeiro, 1992 regarded invasive species as one of the main reasons for the loss of biodiversity20,21. Through advent of communication, science and technology, various factors like transport, migration, and commerce, humans are continuing to disperse an ever-increasing array of species across previously insurmountable environmental barriers such as oceans, mountain ranges, rivers, and inhospitable climate zones22,1.  Humans have extensively altered the global environment, changing global biogeochemical cycles, transforming land and enhancing the mobility of biota23,1. Further, high natural resource extraction, short food chains, food web simplification, habitat homogeneity, landscape homogeneity, heavy use of herbicides, pesticides, and insecticides, large importation of non-solar energy, large importation of nutrient supplements, convergent soil characteristics, modified hydrological cycles, reduced biotic and physical disturbance regimes, global mobility of people, goods, and services are some characteristics of intentionally modified ecosystems24,1. Gallagher and Carpenter (1997)25 remarked on human-dominated ecosystems, the concept of a pristine ecosystem, untouched by human activity, “is collapsing in the wake of scientists’ realization that there are no places left on Earth that doesn’t fall under humanity’s shadow.”  Distribution factors: Initially trade and then transportation systems of various kinds have distributed invasive species around the world1,26,27 as Kaiser (1999)28 notes, “The world’s ecosystems will never revert to the pristine state they enjoyed before humans began to routinely crisscross the globe.” Even those considered “natural” almost inevitably contain invasive species, frequently in dominant roles. Moreover, modern technological systems continue to increase the scale of these impacts29. Recent advances in genetics and molecular biology also paved the way for an open debate on their impacts on ecology and global biodiversity. The histories of invasions and agriculture are intimately linked, with many crop and livestock pests being invasive species and vice versa30. Particularly, agricultural biotechnology, i.e. the insertion of genes into crops, has generated concern over the risk of producing new invasive species or exacerbating current weed problems1,31,30. Also, it has been demonstrated that the sale and transport of prohibited invasive plants and their misidentification present the greatest risk associated with the horticultural trade32.  
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In the light of aforesaid, it is clear that modern intensive agriculture paved the way for invasive plants through trade and transport all across the globe. Further, it leads to land use changes (conversion of forests/grasslands into agro ecosystems), habitat fragmentation as well as increase the level of persistent organic pollutants (POPs), hence, resulting in the increased level of CO/climate change. All these factors are directly or indirectly linked to biological invasions, in totality resulting in biodiversity loss (figure-1). Although invasion results from multifaceted interrelated mechanisms, changes in biota resulting from habitat conversion and land use change, reduces genetic and species diversity; and due to the introduction of exotic species, leading to a homogenization of the global biota33. Climate and land use change also transmogrify the invasion process15.  Land use systems: Land use change and habitat fragmentation may facilitate the invasion success, indirectly affecting the biodiversity. Land-use change is projected to have the largest global impact on biodiversity by the year 2100, followed by climate change, nitrogen deposition, species introductions and changing concentrations of atmospheric CO15. Land-use change is expected to be of particular importance in the tropics, climatic change is likely to be important at high latitudes, and a multitude of interacting causes will affect other biomes23,34-36. Land use and biological invasions have been the major drivers of grassland biodiversity change37. Invasive species bring changes in plant community composition which may be inextricably correlated with external land-use impacts38. Land use change resulting from agriculture may be the major driver for invasive plants since approximately 10 hectares of natural ecosystems would be converted to agriculture by 205039. Furthermore, the homogenization of Earth’s biota through the establishment and spread of alien species may also be attributed to increased trade and tourism linked with40-42. Figure-1 Paradigm of global interlinked ecological issues or concernsResource paucity (due to expanding population) Modern intensive agriculture (due to rapid industrialization/urbanization) 
Habitat fragmentation 
Biological invasions 
Land use/Land cover change 
Elevation in COconcentration 
Increase in POPs concentration 
Climate change 
Biodiversity Loss 
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Invasive Plants and the issues: It is worth to mention a few prominent invasive species e.g.Lantana camara, Mikania micrantha, Chromolaena odorata, Eupatorium adenophorum, Cytisus scoparius,  Mimosa invisa Parthenium hysterophorus and Prosopis juliflora amongterrestrial exotics, and Eichhornia crassipes and Pistia stratiotes among aquatics, have posed serious threat tothe native flora43,44. Henceforth, it will be better if we can provide a differential categorisation of invasion in terrestrial and aquatic ecosystems. There is a huge documentation of literature on ecosystem level impacts of plant invasion. The impacts may be further expanded on terrestrial (forest, riparian) ecosystems and aquatic (fresh water and marine) ecosystems. Ecosystem structures in terrestrial and aquatic ecosystems have many similarities; however, certain variations may have implications in context of invasion ecology. In terrestrial ecosystems, the plant-soil system involves a complex network of interactions and feedbacks and may function almost in isolation from the surrounding landscape45. Terrestrial ecosystems receive inputs from the atmosphere and from adjacent ecosystems. In contrast, aquatic systems have separate subsystems within the water column, at the water-sediment interface, and within the sediments. It is worth to mention that all these components can exchange materials with each other, as well as with other ecosystems and the atmosphere. Water movements play an important structuring role, and the dynamics of sediments are as important as the dynamics of nutrients, oxygen, and carbon in determining aquatic ecosystem function. Thus, it may be that invasive alter aquatic and terrestrial ecosystems through different pathways1,45. Invasion ecology is not very old field and attained pace from 20th century, and mostly the researches in its multifaceted disciplines attained pace during last fifty years46. Although the first paper on species invasions47 appeared in 1919, study of the phenomenon is often traced back to Darwin’s Beagle voyage, when he documented many European plants thriving as aliens in South America. He pointed out that escape from the parasites and diseases that attack them in their native range may contribute to the rapid spread of invading plants and animals48. In eighteenth century, plant invasion was generally being recognized as ecological phenomenon. However, in current scenario, invasive species are the second largest threat to global biodiversity just after habitat destruction and is the prime factor responsible for species extinction in most island states43,49. Global terrestrial as well as aquatic ecosystems are invaded by various invasive plant species50-52. In present era of ecological sciences, concept of plant invasion may be treated as a sort of ecological explosion53. Invasion of exotic invasive species is among the most important ecological perturbation experienced by natural ecosystems43. Such perturbation drives and regulates the dynamic ecosystems1,43,54,55
 
In order to understand plant community patterns, it has been suggested to investigate that plant-plant replacements, first by sampling long-term vegetation plots in order to map them, and then by manipulating mechanisms and tolerances in field experiments in order to understand what causes them56. Several reviews have enriched the ecological literatures with the several facets of invasion ecology, however, multifaceted issues in totality, associated with this ecological phenomenon were lacking. Present book aims to provide a critical review on mechanisms, impact and management of invasive species particularly in context of plants. However, at some places, biological invasions would be described in terms of animals also. Now, before going into details of the present review, it is pertinent to understand the generalized definitions and terms associated with invasion ecology. Davis57 in his book mentioned different views and terminology imposed in invasion ecology and unlikely to reach any consensus.  Definition: The Convention on Biological Diversity held in 1992, defines an invasive species as “an alien species whose introduction and spread threatens ecosystems, habitats or species with socio-cultural, economic and/or environmental harm, and/or harm to human health” 57-59. Whereas the scientific biological definition neglects the perspective of impacts and describes the naturalisation and unintended spread of unwanted organisms in areas where they have not previously occurred naturally60,59. Organisms immigrating to new localities and their descendants have been referred to as alien, adventive, exotic, introduced and non-indigenous22, 43. Species that have been transported by humans from one region to another are defined as alien or exotic61,62. Although there are several definitions of alien invasive species, however, the one given by GISP (2003)63 seems to be most relevant in totality i.e.: ‘Invasive alien species are non-native organisms that cause, or have the potential to cause, harm to the environment, economies, or human health’. Thus establishment and spread of these species threatens landscape in terms of economy as well as environment43,64. Established: A species with a self-sustaining population outside of its native range. Indigenous species: a species found within its native range. Invasive species: a nonindigenous species that spreads from the point of introduction and becomes abundant. Nonindigenous species: a species introduced to areas beyond its native range by human activity. Noninvasive species: A nonindigenous species that remains localized within its new environment. Transition: One step in the invasion sequence (e.g. transportation, release and establishment). 
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Conclusion  A principal result of the Industrial Revolution and subsequent technological development is the evolution of a planet where the dynamics of major natural systems are increasingly affected by human activity (Rai, 2013). Invasive species are a serious problem for the world,both ecologically and economically. The impact of invasive species on native species and ecosystems has been immense. Literatures revealed that 10% of the 260,000 vascular plant species is assumed to be potential invaders (Rai, 2013). Eighty conifer taxa (79 species and one hybrid; 13% of species) are known to be naturalized, and 36 species (6%) are ‘invasive’. Majority of research studies on biological invasions is reported from developed countries, with scanty information on plant invasions in the biodiversity-rich developing countries, including India, and these gaps have serious implications for global environmental policy making. Towards filling these knowledge gaps, categorization of invasive alien biota is the crucial starting point in developing countries With reference to developing countries like India having four biodiversity hotspots, developing the consolidated strategy in order to combat the problems of plant invasion is the need of the hour. The conservation, enhancement and sustainable and equitable use of biodiversity should be accorded high priority in all national environment protection programmes. In the recently held CABI (Commonwealth Agricultural Bureau International)-MSSRF Workshop on Alien Invasive species at Chennai, India the seven point action plan was developed to manage the invasive alien species with a focus on public awareness; research to fill gaps in our knowledge of the invasive alien species and measure the social, economic and ecological impact of invasive alien species, as well as to evaluate traditional ecological knowledge at the local level; Action at local, state level., national level, regional level and global level should be taken in this regard. Scientific literature on biological invasions in the developing world is currently scarce. India, a fast-globalizing country, faces a high risk of biological invasions. However, research and policy efforts on biological invasions in India are presently inadequate. In order to address the scale and impacts of this anthropogenic mixing of biotas, as well as an opportunity for basic biological insight, invasion biology has become a rapidly developing discipline with broad ecological and conservation implications. Preparing a comprehensive database of alien floras has been an essential tool for the understanding of plant invasions in different parts of world. Biodiversity threats are not evenly distributed, therefore, prioritization is essential to minimize biodiversity loss arising out of the invasion. Invasion papers were least likely to be cross-linked (6%) with other fields, whereas gap/patch dynamics papers were most likely to be cross-linked (15%). This tendency toward intellectual isolation may be impeding efforts to achieve more powerful generalizations in ecology by reducing the number of potentially productive exchanges among researchers.  Furthermore, there is need for greater vigil against alien invasive species, since with growing world trade in food grains and other agricultural commodities, there is an increasing possibility of introducing new pests, weeds and harmful micro-organisms. Recent methods based on analyses of genetic markers have provided tools for the retracing of invasion routes and the history of the invading populations from their geographic origin to their final spread in the invaded area. Further, it is worth to mention that future studies in invasion ecology must ensure the links between species traits, ecosystem stocks, ecosystem flows and ecosystem services in order to have appropriate impact assessment to suggest sustainable management approaches. Invasion biologists across the different countries developed different model frameworks for the invasion process in conjunction with different taxa and different environments leading to a plethora of confusing concepts. 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