Until 2000, studies on invasive alien plants were scarce in South Asia. Only after 2001 did the number of studies start to increase rapidly (Fig. 1), and this trend still holds for all categories of research. Most research (41% of studies) was focused on the inventories of invasive plants, followed by studies on impacts (18%) and distribution of alien species (16%) in South-Asian countries (Fig. 2). Among all countries in South Asia, India was the first to start research on alien trees and shrubs in 1983.

Figure 1.

Numbers of research articles dealing with plant invasions in South-Asian countries in five-year periods and their cumulative number over the period of 1981–2022. See Suppl. material 1 for the articles on which the figure is based.

Figure 2.

Number of articles addressing different research topics in South-Asian countries over time. See Suppl. material 1 for the articles on which the figure is based.

The majority of the studies addressing the consequences of plant invasions were focused on the impacts of invaders on native plant diversity (46% of the total number of articles dealing with impact), followed by studies on soils (17%), biodiversity on other trophic levels (15%), agriculture (14%), socioeconomic impacts (6%), and human health (3%). Except for soil studies, there is very little research regarding the impacts on ecosystems, including hydrology.

We recorded 392 alien plant species that are invasive in South Asia. India harboured the highest number of invasive plant species (145), followed by Bhutan (101), Sri Lanka (94), Pakistan (73), Bangladesh (61), Maldives (38), Nepal (28), and Afghanistan (26). The numbers of naturalized species followed a similar pattern, with India (471), Pakistan (439), and Sri Lanka (401) harbouring the most. The ranking of countries shifted if species numbers per log area were taken as a measure, with India appearing the richest in invasive and Sri Lanka, Pakistan, and India in naturalized species (Table 1). Although India, which is the largest country in the studied regions in terms of area, harboured the highest numbers of invasive and naturalized plant species, Maldives had the highest percentages of these species in its flora. Due to the rich flora of South Asia, the percentage of naturalized plant species across the whole region was rather low, only 3.9% of the total flora. Afghanistan is the third largest country (after India and Pakistan), but the number of naturalized and invasive species recorded there was the lowest; however, this may be due to a lack of research. The total numbers of naturalized, invasive, and native species reported from the reviewed countries are shown in Table 1.

The relationships between the number of naturalized and native species (Fig. 3A) and the number of invasive and naturalized species (Fig. 3B) were not significant. The numbers of naturalized and invasive species on the mainland significantly increased with the increasing area of the country (Fig. 3C, D).

Figure 3.

Relationships of alien plant species numbers in the South-Asian region for mainland states A naturalized species relationship with native species (mainland: r=0.77, t=0.59, p=0.12, df=5) B invasive species relationship with naturalized species (mainland: r=0.67, t=-0.11, p=0.91, df=5) C species area relationship for naturalized species (mainland: r=0.77, t=2.83, p=0.04, df=5) D species area relationship for invasive species (mainland: r=0.92, t=1.00, p=0.008, df=5).

Forty-one invasive species occur in at least three South-Asian countries; we considered such species as widespread. Lantana camara and Pontederia crassipes are the most widely distributed, occurring in all eight South-Asian countries. Parthenium hysterophorus occurs in seven countries, Chromolaena odorata and Mimosa pudica in six countries, Ageratum conyzoides, Argemone mexicana, Leucaena leucocephala, Mikania micrantha, and Ricinus communis in five countries. Of the 41 widespread species, six are listed among 100 of the world’s worst invasive species (see Table 2 for distribution of the most widespread invasive species in South-Asian countries).

The 20 most problematic invasive plants in South Asia occurred in a range of habitat types (Table 3). The highest numbers were found in disturbed habitats (13 species), followed by riverine habitats and grassland (11 each), forests (10), wetlands (8), and woodland (7). Forest edges, ponds, shrubland (3 each), and ditches (2) harbour the least problematic invasives. Lantana camara is a species that is widespread in the greatest number of habitats, i.e. disturbed sites, forests, forest edges, riverine habitats, pastures, and woodland (Fig. 4).

Figure 4.

The occurrence of the most problematic invasive species in different habitat types. The presence of the species in a habitat is indicated by a blue cell. The classification of habitats of particular species is based on Weber (2017).

All the most problematic invasive plants in South Asia affect native species diversity (Table 3). In addition, eight species are reported to reduce the productivity of agricultural fields and alter soil properties, hence directly affecting the economy (Fig. 5). Clidemia hirta, Lantana camara, Leucaena leucocephala, Mikania micrantha, Parthenium hysterophorus, and Prosopis juliflora were reported to affect livestock and their products. Species like Alternanthera philoxeroides, Gymnocoronis spilanthoides, Myriophyllum aquaticum, and Ulex europaeus are responsible for hydrological changes that subsequently affect aquatic ecosystems. Only two invasive plants (Chromolaena odorata and Parthenium hysterophorus) are reported to have an impact on human health (Fig. 5).

Figure 5.

The impact of the most problematic invasive plants in South Asia classified into impact categories. The recorded impacts are indicated by blue cells. The information on impacts was taken from Weber (2017), CABI (2022) and Global Invasive Species Database (2022). See Table 3 for detailed description of impacts of particular species.

Only 17% of research papers focused on the management of invasive plants in South Asia, most of them on a single species (i.e. Ageratina adenophora, Chromolaena odorata, Lantana camara, Mikania micrantha, Parthenium hysterophorus, Pontederia crassipes; Sullivan et al. 2017; Raj et al. 2018; Poudel et al. 2019; Sharma et al. 2022). Physical or mechanical removal was the most widely used management method. Manual slashing, use of tractors, plowing, hand pulling, sickle weeding, repeated cutting, and burning were commonly applied. The physical methods were labour-intensive and effective only in a small area. Therefore, chemical methods, i.e. herbicides, were also used to limit the spread of invasive species (Tables 4, 5).

Biological control was used less often than physical methods. Biological control programs were implemented only in India and Pakistan. Due to open and porous international boundaries between India and other South-Asian countries, some of the biological agents like Zygogramma bicolorata have naturally reached Nepal, Bhutan, Pakistan, and Bangladesh. Although some biological control agents have established in South Asia, their impacts were not strong (Shrestha et al. 2022). Removing invasive species before flowering, sowing competitive species after their removal, raising awareness among people about the negative impacts, and identifying the factors responsible for the spread of invasive species are other ways to manage invasive species (Table 5).

In South Asia, research on the impacts of invasive species started only after 2001. Most of the studies have focused on the impacts of single invasive species such as Pontederia crassipes, Lantana camara, Mikania micrantha, and Parthenium hysterophorus (Kohli et al. 2006; Ahmed et al. 2007; Murphy et al. 2013; Rawat et al. 2019; Bhatta et al. 2020). The impacts of invasive plant species on native vegetation are reported from South Asia, and studies showed that invasive plants commonly reduced the richness, diversity, and evenness of native species (Thapa et al. 2016; Bhatta et al. 2020; Kumar and Garkoti 2021) and changed the species composition. However, studies focusing on impacts on socioeconomy, agriculture, health, and hydrology are not sufficiently represented. The particular findings from South Asia, reported in detail below, correspond to the global analyses by Vilà et al. (2011) and Pyšek et al. (2012).

Plant invasions have serious impacts on the environment of Asia, including natural habitats. In forests, dense patches of invasive plants inhibit seedling growth by blocking sunlight and stimulating the growth of other alien plants (Dogra et al. 2009a, b; Rupasinghe and Gunaratne 2017). In the Himalayas, invasive species like Ageratina adenophora and Lantana camara are problematic in pine forests and riparian forests because they enhance the soil nutrient cycling in invaded microsites and spread rapidly (Parveen et al. 2011; Kumar et al. 2021). In Indian forests, L. camara has posed a threat by replacing native understorey vegetation and hindering tree regeneration (Kohli et al. 2006). Similarly, in Nepal’s Bardia National Park, the invasion of Lantana camara has been responsible for over 50% reduction in native plant richness and diversity (Bhatta et al. 2020). Invasive species richness was reported to be inversely proportional to the tree canopy (Thapa et al. 2020); therefore, maintaining a closed tree canopy can prevent the invasion problem.

Invasion in grasslands suppressed palatable grasses and decreased their regeneration, threatening wildlife and making their habitat unsuitable (Akter and Zuberi 2009; Sullivan et al. 2017; Chhogyel and Kumar 2018). Parthenium hysterophorus is highly problematic in the grasslands of Nepal, India, and Pakistan (Javaid and Riaz 2012; Shrestha et al. 2015; Rokaya et al. 2020). The presence of invasive Centaurea iberica in the mountain grasslands of India suppressed native plant species diversity and changed their species composition (Reshi et al. 2008).

Thickets of invasive plants prevent the exchange of sunlight and heat, leading to poor oxygenation and the presence of carbonic and bicarbonic acids (Nguyen et al. 2015; Pandey et al. 2020). Pontederia crassipes is one of the world’s worst invasive weeds, which alters the physicochemical properties of water (Basaula et al. 2021) and competes with native hydrophytes for oxygen (Rashid et al. 2014). It damages aquatic ecosystems and deteriorates their aesthetic value (Pathak et al. 2021) and was reported to alter hydrological regimes and replace aquatic flora (Gupta and Yadav 2020; Pathak et al. 2021). Moreover, plant invasions in wetlands negatively affect crop production by hampering irrigation systems, blocking fishing areas, declining fish production, setting barriers to boating, and altering the water cycle (Keller et al. 2018; Pathak et al. 2021).

Plant invasions decrease agricultural productivity by reducing nutrient levels in the soil (Yakandawala and Yakandawala 2011; Paini et al. 2016; Chhogyel and Kumar 2018; Chhogyel et al. 2021). The fluctuation in agricultural production affects national economies and threatens food security (Kohli et al. 2006). Economic costs due to invasive species in agriculture are estimated in some countries, such as India and Pakistan (Diagne et al. 2020), but the estimates are still missing for other countries, for example Bangladesh (Mukul et al. 2020). The increased impact of Phenacoccus solenopsis on the cotton yield of India caused a loss of about US$ 1.217 billion and is forecasted to increase in coming years (InvaCost; Diagne et al. 2020). Ageratum conyzoides, Ageratum houstonianum, and Parthenium hysterophorus cause problems in the agricultural fields of South Asia (Kohli et al. 2006; Shrestha et al. 2019), and their impacts are reported in Nepal and Pakistan (Javaid and Riaz 2012; Shrestha et al. 2015; Rokaya et al. 2020). Ageratum conyzoides invading agricultural fields has caused a decline in crop productivity (Kohli et al. 2006; Shrestha et al. 2019; Shah et al. 2020). The impacts of invasive species are more pronounced in developing countries because local people depend more on agriculture, fisheries, and forestry (Mungi et al. 2018; Shah et al. 2020).

Some invasive plants produce allelopathic substances that affect plant diversity as well as soil microbial diversity by leaching allelochemicals into the soil (Inderjit et al. 2011; Thapa et al. 2020). Research has shown that invaded soils have high microbial biomass and rapid litter decomposition, which increases the availability of nutrients and, as a result, invasive species grow rapidly (Ahmad et al. 2019; Zhao et al. 2019; Kumar et al. 2021). These toxic chemicals help invasive species establish and spread rapidly (Kumar and Garkoti 2022).

Besides declining native plant diversity and changing ecosystem properties, invasive plants cause several diseases to humans and livestock in South Asia (Kumar and Prasad 2014; Rashid et al. 2014; Negi 2016). Parthenium hysterophorus is known to have negative impacts on human health, causing skin allergy, rhinitis, and irritation to the eyes (Kohli et al. 2006; Adkins and Shabbir 2014; Shrestha et al. 2015; Chhogyel et al. 2021). Due to direct exposure to invasive plants, health problems are also greater in developing countries. Several other species, like Ageratum houstonianum and Mimosa diplotricha, negatively affect human health and livestock conditions (Shrestha et al. 2019; Sharma et al. 2020), but there is very little research in this respect. On the other hand, some invasive plant species are used in traditional medicine as antimicrobial, antiseptic, and blood coagulants (Negi 2016).

Despite the recent increase in the number of published studies, research on the management of invasive plants in South Asia is still insufficient. Chemical, physical, and mechanical removal of invasive species are the most common practices in South-Asian countries (Raj et al. 2018). There are attempts to manage invasive species by physical removal with the participation of the local people (Sullivan and York 2021), which is labour intensive. For instance, the management of Pontederia crassipes by utilizing its biomass for various purposes has been adopted but was unsuccessful because of the absence of continuous funding (Patel 2012).

Experiences from other parts of the world show that control of invasive plants by physical and chemical methods is expensive and needs continuous long-term effort. Great Britain spent about ~£90 million annually on chemicals for controlling invasive weeds in agricultural land (Williams et al. 2010). On the other hand, biological control is the most effective and sustainable method to control invasive species because once established, it perpetuates itself and does not need continuous financial inputs for management (Clewley et al. 2012). Most of the countries which are successful in the eradication of invasive plants have adopted biological methods. For example, Azolla filiculoides have been controlled for over a decade in South Africa by a North American frond-feeding weevil, Stenoplemus rufinasus (Hill et al. 2008). In Australia, nine insects and two fungal pathogens are used as biological control agents against Parthenium hysterophorus (Dhileepan et al. 2019). Unfortunately, biological control is in the early stage and poorly developed in South Asia due to the high initial cost and long time required for screening. However, some biological control agents for Ageratina adenophora, Chromolaena odorata, Lantana camara, Mikania micrantha, Parthenium hysterophorus, and Pontederia crassipes were introduced to South Asia (Dhileepan and Senaratne 2009; Poudel et al. 2020; Shrestha et al. 2022). In Papua New Guinea, a gall fly Cecidochares connexa was found to successfully control the populations of invasive Chromolaena odorata (Day et al. 2013). In South Africa and some neighbouring countries, the flowering galling mite Aceria lantanae reduced the flower production of Lantana camara by up to 97% (Simelane et al. 2021).

Most alien species were introduced to South Asia for ornamental purposes, soil improvement, or as a fodder crop for animal husbandry; some were introduced as contaminants (Tiwari et al. 2005; Ekanayake et al. 2020). For instance, Lantana camara and Pontederia crassipes were introduced to botanical gardens in India as ornamentals (Kohli et al. 2006). Similarly, Spermacoce alata seeds entered Nepal along with the seeds of forage plants distributed to farmers (Shrestha 2016). There is abundant evidence showing that disturbance increases resource availability, making a plant community susceptible to invasion (Davis et al. 2000; Dogra et al. 2009b). Forest edges, agricultural land, grasslands, fallow land, roadside vegetation, and wetlands are susceptible to invasion as they feature higher levels of disturbance (Biswas et al. 2007; Shrestha and Dangol 2014; Rupasinghe and Gunaratne 2017). Moreover, lack of natural enemies, physical disturbance, and open forest canopies are also among the causes of the success of invasive plants (Mandal and Joshi 2014). Passenger air travel is considered one of the introduction vectors in South Asia (Early et al. 2016). Identifying the major drivers and pathways of plant invasions is important for their management.

Species like Lantana camara are very widespread and difficult to eradicate by mechanical, chemical, and biological methods (Love et al. 2009). In South Asia, the eradication of L. camara is nearly impossible, but the negative impacts could be reduced through management. Additionally, efforts should be made to prevent invasions in new areas. In Pakistan, chemicals like glyphosate and metribuzin are effective in controlling Parthenium hysterophorus when treated in a rosette stage (Khan et al. 2012). Herbicide treatment and competitive plants are also used in Pakistan to manage this species (Adnan et al. 2021). The chemical method is effective but not recommended because of its detrimental effects on other biota (Love et al. 2009; Rana et al. 2017). Allelopathic evaluation of invasive plants is important for the biological control of P. hysterophorus (Shinwari et al. 2013). Biological control using Zygogramma bicolorata has successfully retarded the growth of invasive P. hysterophorus by defoliating the plants (Shrestha et al. 2011; Shabbir et al. 2015; Weyl et al. 2021a, b). In addition to this, winter rust, Puccinia abrupta var. partheniicola is also reported to control P. hysterophorus by damaging leaf tissues (Iqbal et al. 2020; Maharjan et al. 2020; Weyl et al. 2021a). Australia has deliberately released this biological control, but countries like Nepal, India, Pakistan, and China have reported this rust to occur without intentional introduction (Iqbal et al. 2020). Laboratory experiments with Listronotus setosipennis in Pakistan have shown that this weevil is specific to P. hysterophorus (Weyl et al. 2021a). Similarly, Procecidochares utilis causes stem galling and suppresses the growth of Ageratina adenophora (Poudel et al. 2019). However, its effectiveness is low in the Himalayan region (Poudel et al. 2019). The main benefit of biological control methods is that they perpetuate by themselves but need rigorous research on host-ranging tests before releasing them in nature (Paterson et al. 2021). Countries have implemented different ways of eradication and management of invasive species, but biological control is still in its early stages in South Asia.

Australia and New Zealand have successfully managed some of the problematic invasive alien species that are also widespread in South Asia by focusing on prevention (Raj et al. 2018). In Australia, every dollar spent on the prevention of invasion benefits $25.60–38.30 (Sinden et al. 2004). Countries of Asia should adopt integrated methods of biological and chemical control, along with making use of competition with native plants, to effectively manage already established invasive plants (Shabbir 2014; Shabbir et al. 2015); identifying competitive native species and actively planting them can help in effective management (Khan et al. 2014; Balami and Thapa 2017). Moreover, the identification of dispersal pathways, high biosecurity, local community participation, and awareness among locals play a vital role in limiting the spread of invasive species (Kannan et al. 2016; Shrestha 2019). Another option could be using invasive species for biogas, firewood, and biofertilizer production, such as Pontederia crassipes (Kafle et al. 2009; Raj et al. 2018). However, it is essential to be cautious in order not to unintentionally promote the invasive species.

South Asia harbours a substantial proportion of global biodiversity, making it imperative to exert every possible effort in safeguarding it against current and potential future plant invasions. The region is part of a biodiversity hotspot area, yet the impact of invasive species is poorly understood. In this paper, we assess the most problematic invasive plant species in South Asia, their impacts, and management. There is no information about the effectiveness of management and policies adopted in South Asia. We show that South Asia still focuses on inventories and descriptive approaches, whereas the impacts of invasive species on the economy, hydrology, and human health are little explored and identified only for a few invasive species. Ecosystem impacts are also understudied; for example, how invasive plants affect ecological processes such as productivity, nutrient dynamics, and pollination have been poorly covered. Thus, by identifying the less explored research areas with regard to the most abundant and problematic invasive species in South Asia, this review contributes to bridging the data gap for global databases and identifies the priority areas for future research. There is an urgent need to quantify the impacts of all widespread and problematic species in South Asia, which is crucial for allocating resources for management. The management should prioritize invasive species with the highest environmental impacts and regions that are suffering the greatest loss.

Biological control is the most effective and sustainable way of retarding the spread of invasive species, but unfortunately, research on biological control is not adequate in South Asia. Our review suggests that research on biological agents should be increased, and community awareness is needed to make the management effective. It is important to recognize that the implementation of biocontrol measures can leverage insights from studies conducted in other regions, underlining the essential need to prioritize specific targets for effective biocontrol strategies.