Keywords

1 Biological Invasions: Concept, History, and Current Perspective

Human-mediated intentional or accidental migration of exotic species beyond their native geographical range leads to a well-known phenomenon, biological invasion. Expansion of international trade, transport, and tourism led to the breakdown of biogeographic barriers and enhanced the cross-border movement of non-indigenous species (Meyerson and Mooney 2007; Hulme 2009; Capinha et al. 2015; Bertelsmeier et al. 2017). A small proportion of these migrated species, competent enough to endure the biotic and abiotic challenges presented by the novel habitat and capable of causing apparent ecological and economic impacts are defined as invasive species (Richardson et al. 2000; Canning-Clode 2015; Kaur et al. 2019; Shackleton et al. 2019). Being a second leading cause (after habitat fragmentation) of global biodiversity loss (Wilcove et al. 1998; Bellard et al. 2016) and the major cause of species extinction in island ecosystems (Brockie et al. 1988; Tershy et al. 2002), the biological invasion has emerged as a gruelling challenge for the conservation managers.

Naturalists have observed the phenomenon of invasion since ages and invasive species were described by several nineteenth- and twentieth-century scientists, e.g. Charles Darwin, Alphonse De Candolle, Joseph Hooker, Charles Lyell, Frank Egler, Herbert Baker, Marston Bates, and Carl Huffaker (Richardson and Pyšek 2008; Richardson 2011). However, the precise concept of biological invasion was introduced by British ecologist, Charles S. Elton in 1958 in his book, The Ecology of Invasions by Animals and Plants (Elton 1958). He, therefore, is considered the unofficial Father of Invasion Ecology and his book is now accepted as a landmark in the field of invasion science (Davis et al. 2001). However, it was in 1982, during the general assembly of the Scientific Committee on Problems of the Environment (SCOPE), that a project named SCOPE Programme on the Ecology of Biological Invasions was initiated. This step provided momentum to the notion of biological invasion, resulting in a series of publications and regional/global synthesis associated with the concept (Simberloff 2011). Later, with the participation of the International Union for Conservation of Nature (IUCN) and the Centre for Agriculture and Bioscience International (CABI), Global Invasive Species Programme (GISP) was developed in 1997 (GISP 2020). This program addressed the factors driving the phenomenon of biological invasion, suggested prevention/management strategies, and developed a database for information exchange among researchers and conservation managers (Richardson 2011; GISP 2020).

At present, the growing attention towards this global problem can be estimated from (1) increased rate of publications/books on biological invasion; (2) scientific journals exclusively dedicated to tackling problems related with the issue (e.g. Biological Invasions, Aquatic Invasions, NeoBiota, Bio-Invasions Records); (3) conferences aiming to bring together the invasion biologists on a common platform (e.g. International Conference on Marine Bio-invasions; NeoBiota—European Conference on Biological Invasions), and (4) International Research Programmes such as GISP (1997), NEOBIOTA (1999), DAISIE (2005), INVASIVES (2013), GloNAF (2015), etc. addressing this issue on a global level (Canning-Clode 2015). Further, protocols such as Invasive Species Environmental Impact Assessment (ISEIA) and Environmental Impact Classification for Alien Taxa (EICAT) allow the classification of alien and invasive species under different risk categories (Vanderhoeven et al. 2017).

Furthermore, with the development of advanced techniques, molecular approaches, and DNA tools, a better understanding of the origin, evolution, and consequences of biological invasions is being captured (Ward et al. 2008; Darling et al. 2017). Attempts have also been made to forecast the spatio-temporal distribution of non-indigenous species in future climate change scenarios. Such studies can make reliable and robust predictions about population dynamics, potential outcomes, and preventive measures of the invasive species (Gallien et al. 2010). Nevertheless, invasion science has now become an independent sub-discipline of ecology (Davis et al. 2001). It has not only embraced a full spectrum of interdisciplinary fields, e.g. sociology, economics, and risk assessment but has also attracted socio-ecological collaborations amongst researchers, government bodies, non-government organizations, conservationists, landscape managers, and stakeholders (Canning-Clode 2015; Vaz et al. 2017).

In this chapter, we addressed the concept of plant invasion and the challenges associated with it. Beginning with the course of establishment of an alien plant into a new geographic range and the attributes which could facilitate its successful invasion in a non-native environment, this discussion aimed at enhancing the understanding of the phenomenon of plant invasion. Later on, the current status of invasive plant species at a regional and global scales is presented along with their consequences, which highlights the issues we are dealing with at present. Finally, we concluded the chapter by focusing on the potential risks that we are needed to be prepared for in near future.

2 Process of Plant Invasion

The framework of the invasion process and the associated terminologies are explained by a number of biologists (Williamson and Fitter 1996; Richardson et al. 2000; Blackburn et al. 2011). However, in a botanical context, the Richardson framework fits most appropriately (Blackburn et al. 2011). Here, an overview of the invasion process is provided in a generalized manner, taking insights from the model proposed by Richardson et al. (2000).

  • The first stage of the invasion process requires the transportation of a plant or its propagule across the major geographical barrier(s) (inter-continental or intra-continental or both) through any agency (mostly humans, but there can be other factors such as wind, water, etc.). The species can be called “alien,” “exotic,” “non-native,” “non-indigenous,” and “introduced” (terminologies are interchangeably used by the researchers) at this step of the invasion process (Fig. 19.1).

  • Upon introduction, the first and foremost challenge faced by an alien species is the novel environment (consisting of biotic/abiotic components) of the introduced habitat that a species needs to be acclimatized to for its survival (Fig. 19.1).

  • Thereafter, a species needs to overcome any barrier(s) guarding the long-term and consistent production of offspring (either by vegetative or generative means). A species can be considered either “casual” or “naturalized” at this stage of the invasion process. The casuals are defined as the introduced species that can successfully survive and occasionally reproduce; however, they are incapable of producing self-replacing populations, and therefore, rely on repeated introductions for their existence within the non-native boundaries. On the contrary, naturalized plants are competent enough to reproduce on their own, freely, and for several generations (with or without human intervention) (Fig. 19.1).

  • Finally, the naturalized species that produce offspring by generative means in hefty numbers and surmount the local/regional dispersal barriers, thereby spreading at considerable distances from parent plants, are called “invasive” (Fig. 19.1).

Fig. 19.1
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A general scheme of the process of plant invasion (after Williamson and Fitter 1996; Richardson et al. 2000)

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In addition to the given stages of invasion process, Richardson et al. (2000) further added post-dispersal environmental barriers (disturbed habitats and natural/semi-natural habitats) to include the resistance posed by various factors during disturbances and process of succession. The authors also pointed out that the process is reversible, and any ecological shift or fluctuation may augment the spread of an alien species or result in its total extinction.

The statistical rule proposed by Williamson and Fitter (1996) is also explicitly and implicitly adopted by invasion biologists regardless of the identity of the taxon. The rule (popularly known as “Ten’s rule”) states that only one-tenth of the species (i.e. 10% of the total species) survive at every step of the invasion process (Fig. 19.1). Nevertheless, ecologists also have a contradictory viewpoint in this regard that the rule undermines the negative impacts posed by alien species that are still in the process of invasion (Jarić and Cvijanović 2012).

3 Determinants of Successful Invasion

The introduction-naturalization-invasion continuum depends upon interactions amongst the introduced species, the invaded habitat, and the chance/timing of introduction (Pyšek and Richardson 2008; Moravcová et al. 2015). Researchers argue that apart from these factors, the species of the invaded ecosystem and their interactions with the introduced species also regulate the trend of invasion (Szabó et al. 2019). Efforts are being made to provide the best explanation of the mechanisms underlying the invasion process. Several hypotheses have been proposed in this context, a few of which even reflect contradictory opinions (Enders et al. 2018). It has also been accepted that multiple factors govern the phenomenon of invasion and success of an invasive species and does not rely on any single theory/concept (Gurevitch et al. 2011). Common hypotheses/theories proposed so far in the context of plant invasion are listed in Table 19.1.

Table 19.1 Common hypotheses/theories proposed in context of plant invasion
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The dominance of an invasive species can be explained by one or more of these hypotheses. However, there could be many more factors that have not been included in these assumptions and yet have a strong influence on the invasion facet of a species. Also, there are aspects that have been postulated but deserve more consideration, understanding, and pragmatic evidence.

4 Data on Invasive Alien Plants

Invasive alien plants are present in every part of the world; however, the numbers vary from region to region (Inderjit et al. 2018). The highest numbers of naturalized alien plants are documented from North America (~6000) and Europe (~4000); whereas, the lowest numbers are reported from Antarctica (~160), followed by temperate Asia (~2200) and tropical Asia (~2000 species) (van Kleunen et al. 2015). However, considering the fact that the majority of the global biodiversity is yet to be explored, it is expected that the actual number of the alien or invasive species are far more different than our current speculations (Jarić et al. 2019). This is particularly true for the emerging economies, where there is a lack of research opportunities and facilities.

4.1 International Statistics

Nearly one-sixth of the world’s geographical area, including 16% of the global biodiversity hotspots, is predicted to be susceptible to invasion (Early et al. 2016). A global study by van Kleunen et al. (2015) anticipated that a total of 13,168 vascular plant species (approximately 3.9% of the world’s total extant flora) have naturalized in 843 continental and island regions. The authors also stated that North America held the maximum naturalized flora, whereas Pacific Islands showed the maximum rate of accumulation of alien species (van Kleunen et al. 2015). Another report on naturalized alien flora of the world corroborated this study stating that California, North America, is the world’s richest region in terms of naturalized alien flora with 1753 alien plant species (Pyšek et al. 2017).

The majority of the world’s worst invasive plants belong to a relatively few families (Asteraceae, Poaceae) and genera (Acacia, Mimosa, Cyperus) (Mack et al. 2000). A recent study also confirmed the maximum contribution of Asteraceae (1343 species) to the global naturalized alien flora, followed by Poaceae (1267 species) and Fabaceae (1189 species) (Pyšek et al. 2017). On the contrary, this new report suggests Solanum (112 species), Euphorbia (108 species) and Carex (106 species) to be the most representative genera of the world’s naturalized alien plant community (Pyšek et al. 2017). It has also been ascertained that horticulture and nursery trade are the main pathways for intentional plant introductions, whereas ignorant possessions and transportation are accounted for the maximum unintentional introductions (Turbelin et al. 2017). GISD (2020) lists 100 worst invasive species of the world, of which 36 are invasive plant species (Table 19.2).

Table 19.2 Worst invasive plant species of the world (source: CABI 2020; GISD 2020; GRIN 2020; Plant List 2020)
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4.2 National Statistics

Estimations about the share of alien or invasive species in the Indian vegetation over the last ten years have varied to a great extent. In an earlier report, Reddy (2008) described 173 species consisting of 117 genera and 44 families to be invasive in India. Later on, another study revealed that exotic species constitute 8.5% (1599 species) of the net extant vascular flora of the country with 14% (225 species) being invasive (Khuroo et al. 2012). Of late, a total of 471 naturalized alien species were reported in India, representing 2.6% of the total flora of India (Inderjit et al. 2018). This report also stated that lower altitudinal regions lying in tropical/subtropical areas have greater number of alien flora with the maximum figures being recorded from Tamil Nadu (332 species) and the minimum being recorded from the Lakshadweep Islands (17 species) (Inderjit et al. 2018).

A survey of the Indian Himalayan Region showed the presence of 571 alien species, of which 21% (96 species) were invasive (Khuroo et al. 2007). Another study described a total of 190 invasive alien species from the Indian Himalayas representing 112 genera and 47 families (Sekar 2012). On the other hand, a study of the Srinagar city revealed a higher percentage of alien species (58%) rather than natives (48%) in the local vegetation (Mehraj et al. 2018). It has also been observed that the richness of alien species plunged rapidly above an altitude of 2000 m asl (Khuroo et al. 2011). In an attempt to identify the invasion hotspots in India using the approach of Ecological Niche Modelling, it was predicted that nearly 49% of the total geographical area and 19 of the total 47 eco-regions of the country are susceptible to invasion with biodiversity hotspots and coastal areas being the most sensitive regions (Adhikari et al. 2015).

Similar to the data reported at the global scale, the most representative families of naturalized Indian flora are Asteraceae, Fabaceae, and Poaceae (Khuroo et al. 2012; Inderjit et al. 2018). However, the Indian Himalayan region is also dominated by Solanaceae, Convolvulaceae, and Brassicaceae (Khuroo et al. 2007; Sekar 2012). As per the previous findings of Khuroo et al. (2012), the three most species-rich genera were Eucalyptus, Ipomoea, and Senna; whereas according to the recent data, Solanum, Ipomoea, and Euphorbia are the dominating genera of the alien flora of India (Inderjit et al. 2018). The most obnoxious alien invasive species of the country include Parthenium hysterophorus, Ageratum conyzoides L., Lantana camara L., Chromolaena odorata (L.) R.M.King & H.Rob., Ageratina adenophora (Spreng.) R.M.King & H.Rob., Leucaena leucocephala (Lam.) de Wit, Prosopis juliflora (Sw.) DC. and Mikania micrantha Kunth among the terrestrial exotics and Eichhornia crassipes (Mart.) Solms and Pistia stratiotes L. among the aquatic exotics (Sharma and Raghubanshi 2012). Some of the alien plants, notably, Tagetes minuta L., Anthemis cotula L., Sapium sebiferum (L.) Roxb. and Broussonetia papyrifera (L.) L’Hér. ex Vent. are in the process of establishment and hold the potential to become invasive in future (Kohli et al. 2012). Biogeographically, the majority of the alien flora of the country is native of the USA (Khuroo et al. 2012; Sekar 2012; Inderjit et al. 2018). A list of troublesome invasive alien plant species of India is presented in Table 19.3.

Table 19.3 A list of major invasive alien plant species of India (source: CABI 2020; GRIN 2020; NBA-MoEF & CC 2020; Plant List 2020)
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5 Ecological and Socio-economic Implications of Plant Invasion

The establishment of invasive plants not only poses a threat to the ecosystem processes and natural biodiversity but also affect important socio-economic assets (Lazzaro et al. 2020). Exact estimates of the damage imposed by invasive plants on the invaded habitat are difficult to gauge; however, monetary losses via disruption of ecosystem services and socio-economic provisions, and imposition of management efforts may be determined. Furthermore, invasion dynamics are rapidly changing over time due to globalization (Meyerson and Mooney 2007). Thus, it is even more complicated to predict the ecological and economic costs of invasion in the future scenario.

5.1 Ecological Impacts

The impact of invasive plant species on community structure (via an effect on plant communities and higher trophic levels) and ecosystem processes (via interference in natural biotic/abiotic interactions, soil chemistry, nutrient cycling, hydrology, fire regimes, and other microclimatic conditions) is quite evident (Mack et al. 2000; Levine et al. 2003). Invasive alien plants have altered the ecological landscapes, degraded the ecosystem services, threatened the existence of native species, and triggered the homogenization of the world’s biota, both in terrestrial and aquatic ecosystems (Vilà et al. 2011). In the forest ecosystems, certain additional threats are experienced such as the risk of hybridization, the transmission of diseases, and interference with forest regeneration (Langmaier and Lapin 2020). However, these impacts are strongly context-dependent and vary depending upon the characteristics of invasive species and invaded habitat (Pyšek et al. 2012). A diagrammatic representation provided in Fig. 19.2 describes the multitude of ecological impacts inflicted by invasive alien species on an invaded landscape.

Fig. 19.2
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Diagrammatic representation of the ecological impact of invasive alien plants

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Statistical figures representing the extent to which invasive plant species pose a threat to the native biodiversity are severely lacking; however, some regional examinations provide interesting insights. Researchers argue that alien plants are more likely to cause displacement and community change rather than species extinctions. However, allying with natural/anthropogenic disturbances, these declined the population of 410 of the 602 plant species and 19 of the 68 bird species in the USA (Gurevitch and Padilla 2004). Threatened by the continuous spread of alien plants, nearly 166 and 113 indigenous plant species of New South Wales are listed as Endangered and Vulnerable, respectively, which together represent 49% (279 of 565) of the indigenous flora of the region (Coutts-Smith and Downey 2006). A recent study suggested that alien invasive species (here, both plant and animal invasions were taken into consideration) are responsible for 27% of the Extinct (EX)/Extinct in wild (EW) plant taxa (as per IUCN Red List 2015), all of which were island endemic species (Bellard et al. 2016). Downey and Richardson (2016) suggested that interventions of alien flora may not directly lead to extinction, but is evidently responsible for altering the extinction trajectory of species, and conservation managers should take into account the issue of alien invasions before further extinctions happen.

5.2 Socio-economic Impacts

Invasive plants pose a significant threat to agriculture, forestry, fisheries, and other human enterprises (Bhowmik 2005). Many invasive plants are noxious weeds of important food and cash crop species such as P. hysterophorus, A. conyzoides, Echinochloa crus-galli (L.) P.Beauv., Striga hermonthica (Delile) Benth., Datura ferox L., Pennisetum spp., Amaranthus spp., Chromolaena spp., Cyperus spp., and Digitaria spp., etc. Invasive weeds are usually more adaptable, capable of generating a large propagule pressure, tolerant to different biotic/abiotic stresses, and extremely competitive for resources (Bhowmik 2005). As a result, they compete aggressively with the crop species and cause substantial yield loss.

Similarly, invasive plant species may also lead to enormous economic losses by jeopardizing ecosystem services (Szabó et al. 2019). The habitats drifted towards invasion and were found to lose the native species, which used to provide basic food, fodder, fuel, and medicinal services to the locals (Kohli et al. 2006). Invasion of rangelands by exotic plant species reduces the availability of grasses and forbs for the livestock which has affected the practice of animal husbandry (O’Connor and van Wilgen 2020). Several other provisions substantial to human life, such as water resources, pollination services, wildlife-based tourism, and recreational activities are directly or indirectly influenced by the spread of exotic plant species (O’Connor and van Wilgen 2020). In addition, various human health hazards could also be a possible outcome of plant invasion. Some of the invasive plants have direct implications on human health (allergies, skin diseases, respiratory problems, etc.), while others influence indirectly via transmission of pests that cause diseases in humans (Allan et al. 2010).

Thereafter, the management of invasive species attracts huge finances that sometimes may not even fit in the budget of countries with low economies. The United States inhabits nearly 5000 invasive plants incurring annual monetary losses of up to $35 billion (Pimentel et al. 2005). A report from South Africa stated that an amount of nearly $38 million was spent to control alien plants in the protected areas of the Cape Floristic Region and $11–$175 million will be required in the future to address the issue (van Wilgen et al. 2016). Data on the expenditure required to control invasive plants are largely unavailable and close estimations are nearly impossible to draw. However, considering the current situation and future environmental challenges, it can be safely predicted that these figures are going to be raised exponentially to keep invasive plants in check in the near future.

6 Potential Risks and Future Challenges Associated with Invasive Alien Plants

Invasion dynamics are shifting at a much faster pace because of various natural and anthropogenic factors, namely, climate warming, enhanced nitrogen deposition, increased carbon dioxide concentrations, deforestation, habitat fragmentation, changes in land use pattern, population explosion, and rapid economic development (Hobbs 2000; Lin et al. 2007; Meyerson and Mooney 2007; He et al. 2011; Carboni et al. 2018). Consequently, the risks and challenges associated with the invasive alien plants are also multiplying. From the increased aggressiveness of the established invasive species to the constant emergence of new invasive species, the issues in invasion science are getting gruelling and worrisome.

The leading challenge in front of conservation ecologists, both from present and future perspective is the management of invasive plant species. Although the management strategies depend largely on the characteristics of invasive species and the invaded habitat, yet there are certain key points applicable in general. It has been well established that multiple factors govern the invasion success of an alien species and understanding these factors is a pre-requisite for designing any management program (Pyšek et al. 2012; Szabó et al. 2019). Thus, there is a strong need to bridge the knowledge gaps that persist in the understanding of invasion mechanisms. Apart from that, the choice of a strategy should take into consideration the long-term implications, involvement of a wide range of participants (from researchers to government and non-governmental organizations to local people), a suitable and balanced budgeting, and an assured consistency of efforts. Integrated weed management, a strategy that uses a combination of different control methods in an appropriate fashion, is the best approach to monitoring and regulating any invasive plant. Researchers also suggest utilizing the invasive species for ecological and economic purposes instead of the native flora (Huang et al. 2014; Carson et al. 2018). This is a relatively new and better alternative that can attract the involvement of diverse groups, industries, and the general public. Awareness among local people is another important issue that should be duly considered. Although most people do not oppose the management of invasive species, such actions are, however, not perceived as a high priority and sometimes may even be opposed (if an invasive species is providing ecosystem services or is of ornamental value) (Potgieter et al. 2019). Greater environmental awareness among people, particularly the youngsters, is of utmost importance, as this will not only aid the implementation of control strategies but will also ensure the success of the program.

Another crucial challenge is to identify the naturalized species which hold the potential to become invasive in the near future. Such species may already be in their lag phase and preparing to turn invasive or may get triggered by climate warming, seasonal shifts, or any other disturbances in the ecosystem to become invasive. In this case, identification of alien flora and constant monitoring of the species, which are either close relatives of the established invasive species or characteristically identical to them, might be helpful. Species that have a history of invasion in other parts of the world should also be targeted. At the same time, it is important to identify the geographic ranges and habitats which are more susceptible to invasion, so that appropriate preventive measures can be undertaken for their protection. Prior knowledge of the traits of most successful invasive species can also have long-term implications such as understanding of universal invasive attributes, prediction of prospective niches for an invasive species, and identification of potential invaders (Gallagher et al. 2015).

Last but not the least, it is crucial to establish strong quarantine measures to restrict the unintentional introduction of alien plants from one geographic region to another. Food and Agriculture Organization (FAO) introduced an intergovernmental treaty International Plant Protection Convention (IPPC) signed by 180 member countries in 1951, with an objective to “protect world’s plant resources from the spread and introduction of pests and promote safe trade” (FAO 2020). The treaty is governed by certain guidelines to prevent the entry, establishment, and spread of exotic plant pests (including weeds). Although the plant quarantine measures are followed by most of the member countries, they are not stringent enough to completely restrain the unintentional transport of plant/plant parts/seeds. On the other hand, the intentional introduction should only be allowed when absolutely necessary, and the species should undergo a well-established risk assessment protocol. Policymaking should involve both government officials and researchers so that the risk assessment system should be scientifically sound and unambiguous. Only the combined efforts in research and policymaking, and strict actions at legislative, technical, and administrative levels can facilitate the containment of potentially invasive plant species.