1. Background

“Loss of habitat constitutes the greatest threat to the existence of biodiversity

The second worst threat is the biological invasion of exotic species
(Convention for Biological diversity, 1992)”

An exotic plant, also referred to as alien, introduced, foreign, non-indigenous or non-native, is one that has been introduced by humans intentionally or otherwise through human agency or accidentally from one region to another. An exotic plant that has escaped from its original ecosystem and is reproducing on its own in the regional flora is considered a naturalized species. Those naturalized exotics that become so successful as to spread in the flora and displace native biota or threatens valued environmental, agricultural or personal resources by the damage it causes are considered invasive.

Humans have been transporting animals and plants from one part of the world to another for thousands of years, sometimes deliberately for social or personal gain and sometimes accidentally. In most cases, such introductions are unsuccessful, but when they do become established as an invasive exotic species (defined by IUCN (2000) as “an alien species which becomes established in natural or semi-natural ecosystems or habitat, is an agent of change, and threatens native biological diversity”), the consequences can be catastrophic. According to the Convention for Biological Diversity, invasive exotic species are the second largest cause of biodiversity loss in the world and impose high costs to agriculture, forestry, and aquatic ecosystems. In fact, introduced species are a greater threat to native biodiversity than pollution, harvest, and disease combined.

The global extent and rapid increase in invasive species is homogenising the world’s flora and fauna (Mooney & Hobbs, 2000) and is recognized as a primary cause of global biodiversity loss. Bio-invasion may be considered as a form of biological pollution and significant component on global change and one of the major causes of species extinction (Mooney and Drake, 1987; Drake et al., 1989).

Foresters, taxonomists and ecologists are now well aware of the problems caused by the invasion of exotic species into natural areas and the associated negative effects on global patterns of native biodiversity. Once established, some exotic species have the ability to displace or replace native plant and animal species, disrupt nutrient and fire cycles, and cause changes in the pattern of plant succession. Studies are underway to better understand the impacts of these species on native ecosystems.

Many invasive plants continue to be admired by people who may not be aware of their weedy nature. Others are recognized as weeds but property owners fail to do their part in preventing their spread. Some species do not even become invasive until they are neglected for a long time. Invasive plants are not all equally invasive. Some only colonize small areas and do not do so aggressively. Others may spread and come to dominate large areas in just a few years. The loss due to invasive species in United States estimated to be ~$125-150 billion each year and 25% of US agriculture gross national product lost due to foreign pests and weeds (McNeely et al. 2001). Exotic plants can spread rapidly because of our mobile society and the intentional transportation of ornamental and forage plants (Randall and Marinelli, 1997).

1.1 Characteristic features of Invasive species: [top of page]

Invasive species possess characteristic features like “pioneer species” in varied landscapes, tolerant of a wide range of soil and weather conditions, generalist in distribution, produces copious amounts of seed that disperse easily, grows aggressive root systems, short generation time, high dispersal rates, long flowering and fruiting periods, broad native range, abundant in native range. Preliminary data from one interesting study shows that invasive species are likely to have relatively small amounts of DNA in their cell nuclei. Apparently, the cells in these plants are able to divide and multiply more quickly and consequently the entire plant can grow more rapidly than species with higher cellular DNA content. This gives them a leg up in disturbed sites.

1.2 Global trade and Species introductions: intentions and accidents

The trade-based global economy stimulates the cultivation of economically important species. But it also stimulates the accidental spread of same species or others. International law regulating the unintentional introduction of harmful exotic species through trade is weak. There are two major conventions with provisions on exotics. One is the International Plant Protection Convention (IPPC), which presently addresses crop pests only. The IPPC could be expanded in scope to explicitly protect native (non-agricultural) plant life from introduced pests. The other major international agreement addressing exotic species, the Convention on Biological Diversity, lacks teeth (Jenkins, 1999). Article 8(h) addresses alien species by calling for the parties to: “as far as possible and as appropriate: … Prevent the introduction of, control or eradicate those exotic species which threaten ecosystems, habitats or species”.

The following exotic species are commonly cultivated for food / forage / timber / ornamental / other economic purposes in our country.

a. Introduced plants from Australia:
Acacia dealbata, Acacia decurrens, Acacia holosericea, Acacia longifolia, Acacia mangium, Acacia melanoxylon, Araucaria bidwilli, Araucaria columnaris, Araucaria cunnighamii, Araucaria excelsa, Brassaia actinophylla, Callistemon lanceolatus, Callitris cupressiformis, Callitris glauca, Castenospermum australe, Casuarina equisetifolia, Eucalyptus alba, Eucalyptus camaldulensis, Eucalyptus citriodora, Eucalyptus globulus, Eucalyptus grandis, Eucalyptus paniculata, Eucalyptus punctata, Eucalyptus saligna, Eucalyptus tereticornis, Eucalyptus torelliana, Eucalyptus viminalis, Grevillea banksii, Grevillea robusta, Melaleuca leucodendron, Stenocarpus sinuatus, Thuja occidentalis, Thuja orientalis.

b. Introduced species from Western Asia and Europe:
Allium cepa, Allium sativum, Althaea rosea, Asparagus officinalis, Avena sativa, Brassica oleracea, Cicer arietinum, Coriandrum sativum, Ficus carica, Lactuca sativa, Lens culinaris, Medicago sativa, Morus nigra, Nerium oleander, Papaver somniferum, Pisum sativum, Spinacea oleracea.

c. Introduced species from the Mediterranean Area and Africa:
Aloe variegata, Antirrhinum majus, Asparagus plumosus, Avena fatua, Briza maxima, Calendula bicolor, Capsella bursa-pastoris, Carthamus tinctorius, Cassia didymobotrya, Catharanthus roseus, Chenopodium album, Chenopodium murale, Chrysanthemum coronarium, Cichorium endivia ssp. divaricatum, Convolvulus arvensis, Cyperus flabelliformis, Foeniculum vulgare, Haworthia coarctata, Hordeum vulgare, Ipomoea batatas, Ipomoea cairica, Lathyrus sativus, Nasturtium officinale, Nerium indicum, Nigella damascena, Oxalis purpurea, Pelargonium inquinans x zonale, Pelargonium x hybridum, Pennisetum typhoides, Plantago major, Poa annua, Raphanus sativus, Ricinus communis, Salvia officinalis, Sansevieria trifasciata, Senecio vulgaris, Sisymbrium officinale, Spergula arvensis, Stellaria media, Taraxacum officinale, Verbena officinalis, Verbena supina, Veronica arvensis, Vicia benghalensis.

d. Introduced species from Europe/ America/Mexico/West Indies/Brazil:
Agave americana, Agave sisalana, Allamanda cathartica, Allamanda violacea, Anacardium occidentale, Arundo donax, Angelonia biflora, Annona muricata, Annona reticulata, Annona squamosa, Arachis hypogaea, Asclepias physocarpa, Averrhoa bilimbi, Averrhoa carambola, Bauhinia blakeana, Begonia manicata, Beta vulgaris, Bignonia alliacea, Bignonia megapotamica, Bixa orellana, Bixa orellana, Bougainvillea glabra, Bougainvillea spectabilis, Brassica rapa, Browallia viscosa, Brunfelsia americana, Caesalpinia coriaria, Caesalpinia pulcherrima, Caladium bicolor, Capsicum annuum, Capsicum frutescens, Carica papaya, Cassia grandis, Catesbaea spinosa, Ceiba pentandra, Centaurea cyanus, Cereus caesius, Cestrum diurnum, Cestrum nocturnum, Chenopodium ambrosioides, Chorisia speciosa, Citharexylum subserratum, Cleome spinosa, Clitoria ternatea, Cocos nucifera, Colvillea racemosa, Conyza bonariensis, Coreopsis lanceolata, Coreopsis stillmanii, Coriandrum sativum, Cosmos bipinnatus, Couroupita guianensis, Crescentia cujete, Cuphea lanceolata, Dahlia variabilis, Datura stramonium, Daucus carota, Delonix regia, Delphinium ajacis, Dianthera secunda, Dianthus barbatus, Dieffenbachia seguine, Duranta repens, Eruca sativa, Eschscholtzia californica, Euphorbia pulcherrima, Fragaria vesca, Furcraea foetida, Gaillardia pulchella, Godetia amoena, Gossypium herbaceum, Gossypium hirsutum, Graptophyllum pictum, Guazuma tomentosa, Gustavia augusta, Hamelia patens, Helianthus annuus, Hibiscus elatus, Hibiscus liliiflorus, Hibiscus sabdariffa, Iberis amara, Ipomoea indica, Iresine herbstii, Jacaranda mimosaefolia, Jacquemontia pentantha, Jacquinia ruscifolia, Jatropha panduraefolia, Jatropha multifida, Kigelia pinnata, Lathyrus aphaca, Lychnis coeli-rosa, Lycopersicon esculentum, Magnolia grandiflora, Malpighia coccigera, Malpighia glabra, Malva parviflora, Malvaviscus arboreus, Manihot esculenta, Manilkara achras, Medicago lupulina, Medicago polymorpha, Mentha piperita, Millettia peguensis, Monstera deliciosa, Morus nigra, Musa acuminata, Nicotiana alata, Nicotiana tabacum, Ochroma pyramidale, Oenothera rosea, Opuntia elatior, Opuntia imbricata, Opuntia microdasys, Opuntia vulgaris, Oxalis latifolia, Papaver rhoeas, Parkinsonia aculeata, Passiflora edulis, Passiflora suberosa, Pavonia hastata, Persea americana, Petrea volubilis, Petunia axillaris, Phoenix dactylifera, Physalis peruviana, Pinus caribaea, Pinus gregii, Pinus merkusii, Pinus michoacana, Pinus occidentalis, Pinus oocarpa, Pinus patula, Pinus pringlei, Pinus pseudostrobus, Pinus rudis, Pinus taiwanensis, Pisonia grandis, Pithecellobium dulce, Plumeria alba, Plumeria rubra, Portlandia grandiflora, Portulaca grandiflora, Portulacaria afra, Prunus domestica, Psidium guajava, Punica granatum, Pyrostegia venusta, Rauvolfia tetraphylla, Ravenia spectabilis, Reseda odorata, Rhoeo discolor, Rivina humilis, Russelia equisetiformis, Salvia splendens, Solanum grandiflorum, Solanum tuberosum, Solidago canadensis, Spergula arvensis, Stellaria media, Stigmaphyllon ciliatum, Tabebuia chrysantha, Tabebuia pentaphylla, Tabebuia rosea, Tagetes erecta, Talinum calycinum, Tamarindus indica, Tecoma gaudichaudii, Tecoma stans, Tecomaria capensis, Tecomella undulata, Theobroma cacao, Thevetia peruviana, Thunbergia alata, Thunbergia erecta, Thyrallis glauca, Tithonia rotundifolia, Tropaeolum majus, Vaccaria pyramidata, Verbena peruviana, Vernonia elaeagnifolia, Vicia hirsuta, Viola odorata, Viola tricolor, Yucca gloriosa, Zebrina pendula, Zea mays, Zephyranthes grandiflora, Zinnia elegans.

1.3 Monitoring: [top of page]

Monitoring is required in the invasive species management to determine: location/early detection of problematic species, whether a species is likely to become a problem in an area, whether a species is responding to management efforts (if taken?), the impact of exotic plant control methods on native species. Monitoring of Invasion can be done through qualitative approach like species inventory (seasonally) and quantitative approach using phytosociological methods and mapping using ground-based methods (via map overlays or GPS), remotely-sensed images (aerial photos, high resolution multi-spectral digital data).

1.4 Control: [top of page]

Current control methods for invasive exotics are expensive, lengthy, and risky because total eradication is required to prevent reestablishment. Effective site-eradication procedures require multi-year treatments, continued monitoring, and follow-up treatments. All infestations on adjacent lands must be treated to prevent reinvasion. Unfortunately, infestations common along railway tracks, roads, and utility right-of-ways are rarely treated for eradication, fostering widespread immigration to adjacent lands.

Common methods for control of invasive weed species are: Mechanical, Chemical and Biological control.

Mechanical Control: It is one of the common method employed for control of invasive species. Mechanical control involves ploughs, scythes, mowers, hoes, cultivators, rotary weeders etc. Using these tools, the weeds are physically lifted from the soil, cut off or buried. But it is laborious and needs lot of man power.

Chemical Control: This method has proved costly and partly successful in several cases. Using of chemicals is not desirable due to pollution that they cause.

Biological Control: In most cases of exotic pest invasions, biological control has proven the most effective and environmentally sound approach to their management. Introducing a natural enemy (e.g. water hyacinth weevil Neochetina spp. for Eicchornia; Lantanophaga pussilidactyla insect for Lantana camara, Zygogramma bicolorata insect and Cassia uniflora plant species for Parthenium hysterophorus) for eradication of invasive species is a current focus of interest for biological conservationists. But biological control may not be evenly effective over all areas infested by the invasive species.

Past research studies for developing eradication methods were often limited in duration (only one or two years) and habitats (one site). Appropriate long-term support and funding has been lacking. Control projects for invasive species, offer a logical, long-term solution but none have been seriously attempted in India. The high investments and long-term research required for control programs have been made only for western rangeland exotic plant species, and more recently for tropical exotics in Florida.

1. Crucial aspects of exotic plant ecology that influence control strategies are as follows:
2. Invasive exotics continue to spread because natural predators were not imported from the plant's home range and native predators are too weak;
   After an exotic plant is introduced there is a "lag phase" of decades to centuries before an exponential spread phase (Baskin 2002). Thus, some species that currently appear non-invasive may eventually begin to spread rapidly.
3. Invasive exotic plants can prevent or retard natural succession and reforestation by forming dense infestations.
4. Invasion by exotics continues to decrease biological diversity and detract from their primary mission;
5. The partial shade tolerance of some exotic species (Cassia pumila) allows them to become established under developed forest canopies;
6. Alternanthera philoxeroides, Eicchornia crassipes and Pistia stratiotes are invading riparian/aquatic habitats to the exclusion of native species and regeneration;
7. The initial spread of exotics were observed along highway and utility right-of-ways, "disturbed habitats," and riparian systems, greatly facilitates migration into extensive forest areas; and
8. Because many "disturbed habitats" occur in cities, exotic plants can present severe problems for urban forestry programs, which is made more difficult by exotic species mixtures.
9. Allelopathic effect by species like Prosopis juliflora, Hyptis suaveolens, Lantana camara retard the growth of other species.
10. The invasions potentially lead to an increase in species richness, as invasive species are added to the existing species pool. But also leads to extinction of native species, resulting in decrease of species richness. The negative interactions is primarily the competition with natives for food and sustenance, which may not allow coexistence.

1.6 Research Issues, Legal and Institutional Needs: [top of page]

There is no question that understanding and dealing with the invasive problem is an enormous challenge. To meet this need SCOPE (Scientific Committee on Problems of Environment) is calling for a focused, coordinated, and broadly based approach to the invasive species problem that would engage the large, and global concerned with these issues. It is recognized that research initiatives in India are now needed to improve the understanding of the ecology of the invasive process, the knowledge on predictive powers on which species are likely to become invasive and under what conditions, characteristics of invasive species, impacts of global climate change on invasive species.

It is realized that a legal and institutional approach to the country’s biosecurity threat is a prerequisite to long-term success against introduction of invasive species. The government should strengthen its quarantine authority through new legislations which prohibits the introduction of alien species without an approved consent or permit. Also unauthorized introduction of wild and domesticated animals and plants into new areas, between states and within the country should be reviewed and monitored by several government departments, viz. Forest, Agriculture, Environment, Veterinary and Public Health.

We need to develop techniques to make rapid assessment of the status and movement of invaders and of their potential ecosystem impacts. At present our knowledge about the status of invaders is generally of two states only – it is present (which is derived from flora lists) or it is firmly established and doing devastating damage (often learned from the popular newspapers). Information is needed between these two extremes. In order to acquire this information a rapid sapling approach is needed that would produce a quantitative assessment of the status of invading species, that could be repeated at intervals, to provide a clear focus on emerging problems, helping to alleviate the crisis management approach to invaders. Efforts are needed to utilize developing technologies for tracking invasives including remote sensing and GIS. Further, we need to develop global maps of the distributions of the most abundant and most devastating invasive species, as well as the most sensitive ecosystem types (Mooney, 1999).

2. India [top of page]

India has a characteristic geographic location at the junction of the three major biogeographic realms, namely, the Indo-Malayan, the Eurasian and the Afro-tropical (60 45' to 370 6'N latitude and 680 7' to 970 25' E longitude) with a land frontier of about 15,200 km and a coastline of 7,516 km. Its geographical area is 3287.263 sq. km (3, 287 million Hectares). It is the seventh largest country in the world and the second largest in Asia. It is considered to be one of the twelve centres of origin and diversity of several plant species in the world.

Physiographically, the Indian mainland may be divided into three distinct regions: the Himalaya, the Indo-Gangetic Plain, and the Peninsular India. The Himalaya form a mountain chain along the extreme northern margins of India, extending east-west about 2,500 km in length, varying in width between 240 to 340 km, occupying an area about 500.000 sq.km in India, and rising from the low lying Indian plains to the highest peaks at Kanchenjunga (8,586 m) in Sikkim. The Indo-Gangetic plain comprises alluvial low lands lying parallel to the south of the Himalaya (ca. 2,400 km). This region is agriculturally more productive. The Peninsular India (Deccan, Eastern Ghats, Western Ghats) lies south to the Indo-Gangetic plain, the two being separated roughly by the boundary of Tropic of Cancer.

Climate: India is mainly a tropical country but due to great altitudinal variations, almost all climatic conditions from hot deserts to cold deserts exist. There are four seasons in the year: (1) Spring (January-March), (2) Summer (April-June), (3) South-West monsoon (July-September), and (4) Winter season (October-December). The south-west or the summer monsoon is the main source of rainfall in the country providing 80% of the precipitation, though some precipitation is received in winter months too.

Flora: According to World Conservation Monitoring Centre (WCMC), 1,604,000 species have been described at the global level. Thus India accounts for 8 % of the global biodiversity existing in only 2.4% land area of the world. According to Nayar (1989) the number of flowering plant species endemic to the present political boundaries of this country is 4900 out of a total of 15000, i.e. 33 %. Hajra & Mudgal (1997) report 5400 endemics in 17000 angiospermous species of India, which comes to 31.76 %. India is an important center of agri-biodiversity having contributed 167 species to the world agriculture and homeland for 320 species of wild relatives of crops.

India's rich vegetation wealth and diversity is undoubtedly due to the immense variety of the climatic and altitudinal variations coupled with varied ecological habitats. There are almost rainless areas to the highest rainfall area in the world. The altitude varies from the sea level to the highest mountain ranges of the world. The habitat types vary from the humid tropical Western Ghats to the hot desert of Rajasthan; from cold desert of Ladakh and icy mountains of the Himalayas to the long, warm coast line stretches of Peninsular India. The extreme diversity of the habitats has resulted in such luxuriance and variety of flora and fauna that almost all types of forests, ranging from scrub forest to the tropical evergreen rain forest, coastal mangrove to the temperate and alpine flora occur in this region. Champion and Seth (1968) have recognized 16 major forest types comprising 221 minor types in India. Of these, the Tropical moist deciduous forest forms the major percentage (37%) of forest cover in India. Tropical dry deciduous forest forms 28.6% and the remaining ones are scattered in minor percentage. Presently, India has 678,333 sq.kms (20.64 %) of forest cover (FSI, 2003).

The flora of India shows close affinity with the flora of Indo-Malayan and Indo-Chinese region. This fact prompted Hooker (1904) to the erroneous conclusion that India has no flora as a separate entity but is an admixture of the floras from adjacent countries. According to Nayar (1977) 35% of Indian flora has south-east Asian and Malayan, 8% temperate, 1% steppe, 2% African, and 5% Mediterranean- Iranian elements. The cultivated and naturalized aliens constitute only 18%. Chatterjee (1939) reported 6,850 species as endemic to India for the first time. The subsequent phytogeographers after critical analysis of flora have convincingly concluded that India has a flora of its own and as many as 5400 (33%) taxa of Indian flora are endemic to present Indian boundaries.

It is very difficult to choose exotic species, from all over the country, that really are “invasive” or "worse" than any others. Species and their interactions with ecosystems are very complex. After an extensive review of literature on global invasive species and of India and their spread based on history and field observations, a list of 173 species was prepared. Of these, some species may have invaded only a restricted region, but have a huge probability of expanding, and causing great damage. Some species are ready colonizers in disturbed areas and may have less impact on natural populations. Other species may already be globally widespread and causing cumulative but less visible damage.

In addition to negative impact on indigenous flora and economy, some exotic plants were very much useful to local people. A prominent examples are Prosopis juliflora, Borassus flabellifer and Leucaena leucocephala. Among the reasons for the admiration expressed for these plants are: adaptability, rapid growth and multiple uses. Prosopis juliflora used for coal extraction, firewood, erosion control, fencing. Borassus flabellifer is an economically important species, introduced to India in ancient times. The cut flower stalks yield sugar and toddy, the fruits are roasted and eaten, leaves are used for thatching. Leucaena leucocephala used for firewood, erosion control, fodder, green manures, providing shade and shelter for other crops. Several exotic weedy plants like Argemone mexicana, Cassia tora, Cleome viscosa, Chamaesyce hirta, Croton bonplandianum, Eclipta prostrata, Ipomoea carnea, Malachra capitata, Mimosa pudica, Physalis angulata, Tridax procumbens were used in native medicine. Alternanthera philoxeroides and Portulaca oleracea were used as leafy vegetables.