Confronting the Green Threat: An Updated Inventory of India’s Alien Plant Species

doi:10.21203/rs.3.rs-8246702/v1

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Confronting the Green Threat: An Updated Inventory of India’s Alien Plant Species

https://doi.org/10.21203/rs.3.rs-8246702/v1

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Alien plant species increasingly shape India’s ecosystems, yet national-scale assessments integrating all invasion stages remain limited. We compiled a comprehensive checklist of India’s alien vascular flora, documenting 1,022 wild alien species and 1,725 species including cultivated aliens, standardized using APG IV taxonomy and validated through global databases. These species consist of 164 families and 491 genera, with Asteraceae, Poaceae, and Fabaceae emerging as the most dominant contributors across invasion stages. Herbs constitute the largest fraction of the alien flora, while trees, shrubs, and climbers also contribute substantially, together showing strong growth-form biases in invasion trajectories. Most alien species originated from Tropical America, Africa, and Europe, with Tropical America contributing disproportionately to India’s invasive pool. Ecoregional assessments revealed South and North India as invasion hotspots, whereas Central, Northeast, and Island regions supported more distinct alien assemblages. Jaccard similarity analysis demonstrated a clear invasion continuum: casual species showed low regional similarity, naturalized species formed moderate clusters among adjacent regions, and invasive species exhibited high compositional convergence across distant ecoregions, indicating broad ecological tolerance and strong human-mediated connectivity. Chi-square tests confirmed highly significant associations (p < 0.001) between invasion status and growth form, native region, and ecoregion, underscoring the non-random distribution of alien taxa in India. This study provides the fully standardized, taxonomically updated, and ecoregion-resolved national synthesis of India’s alien flora. By integrating invasion stages, species traits, biogeographic origins, and spatial patterns, it establishes a robust foundation for risk assessment, prioritization, and evidence-based invasive plant management across India’s diverse landscapes.

Invasive species

Naturalized species

Biogeographic origins

Ecoregions

Jaccard similarity

India

In the Anthropocene epoch, the problem of plant invasions has emerged as a significant challenge, particularly for developing countries. These invasions, a direct byproduct of globalization, have intensified with the expansion of international trade, transportation, and climate change (Gioria et al., 2023; Pyšek et al., 2020). India, as a megadiverse nation, is increasingly vulnerable to this issue, with mounting evidences pointing to invasive alien species (IAS) as a growing threat to ecosystems (Tilker et al., 2019; Jehangir et al., 2024; Zenni et al., 2024; Hulme et al., 2023). The global proliferation of IAS continues unabated, showing no signs of saturation, as highlighted by recent studies (Seebens et al., 2021).

IAS, whether introduced intentionally or accidentally, are the manifestations of anthropogenic environmental change. These species disrupt biodiversity conservation, impair ecosystem functioning, and degrade the quality of ecosystem services in the regions they invade (Richardson and Pyšek, 2006; Vilà and Hulme, 2017; Bacher et al., 2017; IPBES, 2019; Roy et al., 2023). Numerous studies have consistently emphasized the adverse effects of plant invasions, which rank among the primary causes of habitat loss for native species (Bartz and Kowarik, 2019; Richardson et al., 2022; Sohrabi et al., 2023).

Addressing this pressing issue requires immediate action to safeguard current and future environmental health and ensure sustainable habitat management (Sankaran et al., 2023). However, the efforts of governments and policymakers are often hindered by insufficient knowledge about the identity, distribution, and pathways of introduction of invasive species. With the enormous number of invasive species present, the critical challenge lies in determining which species pose the highest risk of naturalization or have already established themselves.

Risk assessment and prioritization tools have been developed to tackle these challenges. These tools are extensively used to identify and rank invasive species based on their ecological and economic impacts (Roy et al., 2018; Kumschick et al., 2020, 2022). Focusing resources and interventions on species with the greatest invasive potential is crucial for successful management strategies, such as early detection and rapid response (EDRR), and long-term monitoring (Foxcroft et al., 2023). By isolating high-risk species, stakeholders can concentrate their efforts on addressing the most urgent threats.

In the context of the current Decade on Ecosystem Restoration, policymakers and researchers are prioritizing the expansion of tree cover to restore degraded landscapes and enhance ecosystem resilience worldwide. Tree-planting initiatives are considered as a proven strategy to mitigate climate change impacts, improve biodiversity, and restore productivity of wastelands. Ambitious global restoration programs aim to rehabilitate millions of hectares of degraded land by planting billions of trees. The Bonn Challenge, a global effort, seeks to restore 150 million hectares of deforested and degraded land by 2020 and 350 million hectares by 2030 through various initiatives (César et al., 2021).

However, some restoration approaches, such as the use of alien or exotic species in commercial plantations, have sparked debate due to their uncertain ecological benefits (Lewis et al., 2019). While these efforts may achieve short-term economic goals, they can accidentally exacerbate issues related to plant invasions, further complicating biodiversity conservation and ecosystem stability. Careful planning, species selection, and long-term monitoring are essential to ensure that restoration programs yield meaningful and sustainable outcomes, rather than introducing new ecological challenges.

Exotic species have proven to be valuable contributors to restoration programs due to their enhanced ability to produce fuelwood and timber, provide fodder for livestock, and deliver significant economic benefits to farmers. For instance, leguminous species capable of nitrogen fixation improve soil fertility while offering regulatory services such as erosion control, cultural services like aesthetic enhancement, and supporting services such as habitat creation for local fauna (Castro-Díez et al., 2019; Brancalion et al., 2020; Ewel and Putz, 2004). Despite these benefits, certain exotic species may transition into invasive alien species (IAS), severely undermining biodiversity and disrupting ecosystem functioning (Ewel and Putz, 2004; Castro-Díez et al., 2019; Richardson, 1998; Kaushik et al., 2022). A notable example is the widespread use of nitrogen-fixing exotic trees, such as Leucaena leucocephala (Lam.) de Wit and Gliricidia sepium (Jacq.) Kunth ex Walp. in India, Acacia mangium Willd. and Acacia auriculiformis (A. Cunn.) ex Benth in North and South America and Africa, and Leucaena leucocephala (Lam.) de Wit in Hawaii and Taiwan. These species were initially introduced to enhance soil quality, agricultural productivity, and mitigate economic losses from fertilizer use.

However, many of these species have become invasive in natural and restored ecosystems, spreading aggressively and outcompeting native vegetation. Their unchecked proliferation now poses significant challenges to biodiversity conservation and ecosystem stability in these regions (Richardson and Rejmánek, 2011; Heringer et al., 2019; Kaushik et al., 2024; Sharma et al., 2022; Chiou et al., 2013; Idol, 2019). This duality of benefits and risks highlights the need for a cautious and well-informed approach to the use of exotic species in restoration initiatives, emphasizing long-term monitoring and ecological safeguards to minimize unintended consequences.

Numerous studies have highlighted the critical role of secondary forests in mitigating biodiversity loss and combating climate change, particularly when managed effectively (Heinrich et al., 2021; Oliveira et al., 2021; Matos et al., 2020; Petersson et al., 2022). Invasive tree species, often referred to as "ecosystem engineers" or "ecosystem transformers," possess remarkable adaptability, enabling them to thrive in extreme conditions or nutrient-deficient soils (Kaushik et al., 2024). These species can drastically reshape their colonized environments, influencing ecosystem processes at a fundamental level (Sanders and Frago, 2024; Richardson et al., 2000). Due to their high production capacity and rapid carbon sequestration, invasive trees are frequently prioritized in restoration and plantation initiatives aimed at climate change mitigation (Matos et al., 2023; Nunes et al., 2021; Richardson, 1998). However, their proliferation poses significant risks to the ecological integrity of secondary forests. By outcompeting native species, they restrict natural regeneration, homogenize forest communities, and disrupt species diversity and composition (Kaushik et al., 2024; Le Maitre et al., 2011; Hussain et al., 2021; Wells et al., 2023). Therefore, providing policymakers with an updated checklist of invasive species is indispensable, as it facilitates timely identification and management of these species during the early stages of invasion. This proactive approach offers a powerful tool to conserve native biodiversity and sustain the ecological benefits of secondary forests.

While significant efforts have been made to compile invasive species lists and acknowledge their value in shaping policies and guiding management practices, these lists inevitably become outdated over time. The dynamic nature of invasive species, characterized by their rapid proliferation and geographic expansion, necessitates the regular publication of updated inventories. Our objectives in this study are: (1) To determine the progression of alien plant species within the invasion continuum, categorizing them as casual, naturalized, or invasive, based on the widely accepted framework by Richardson et al. (2000). (2) To examine the taxonomic composition of these species and identify the plant families that exhibit the highest representation among casual, naturalized, and invasive alien plants in India. (3) To analyse the distribution patterns of invasive alien flora by documenting the date of their first record, assessing their regional occurrences, and evaluating compositional similarities across different regions of the country.

This comprehensive approach aims to provide an up-to-date and actionable understanding of invasive alien plants in India, serving as a critical resource for policymakers, ecologists, and conservationists working to mitigate the impacts of biological invasions.

India, the seventh-largest country globally and the second-largest in Asia, spans an area of 3,287,240 square kilometers, with a coastline exceeding 7,500 kilometers. It is the most populous country in the world, with an average population density of 325 individuals per square kilometre. The population is predominantly rural (72%), with 28% residing in urban areas. Administratively, the nation comprises 28 states and 7 union territories. Geographically, India extends north of the equator, between latitudes 6°45′N to 37°6′N and longitudes 68°7′E to 97°25′E, showing diverse landscapes from the upland plains of the south and the vast Ganges river plains to the arid deserts of the west and the Himalayan ranges in the north. India experiences a tropical to subtropical climate, influenced by its vast geographic and topographic diversity. The country witnesses three distinct seasons: a hot summer (late April to late June), a rainy monsoon (late June to mid-September), and a mild winter (mid-November to late March). India's varied ecological landscapes support various habitats, including forests, grasslands, wetlands, coastal regions, marine ecosystems, and deserts, which collectively harbor rich biodiversity. Recognized as one of the 12 Vavilovian centers of origin and diversification of cultivated plants, the country is often referred to as the "Hindustan Centre of Origin of Crop Plants" (Vavilov, 1951). India features 12 distinct biogeographical provinces and 5 biomes (Udvardy, 1975), earning its place among the world's 17 megadiverse countries. It ranks third in Asia and eleventh globally, sharing four global biodiversity hotspots: the Himalayas, the Western Ghats, the Indo-Burma region, and Sundaland (Sharma and Singh, 2000; Mittermeier et al., 2005).

India has emerged as the world's largest democracy and is currently one of the fastest-growing economies. This unique combination of ecological wealth and socio-economic dynamism positions India as a critical player in global biodiversity conservation and sustainable development efforts.

2.1 Risk associated with Plant invasions

India has experienced an increase in the proliferation of invasive alien plant species in recent decades. National and sub-national studies focusing on diversity, distribution, ecological and socio-economic impacts, management strategies, and species distribution modelling have underscored the pressing and multifaceted challenges posed by biological invasions in the region (Shrestha et al., 2022). Despite advancements in economic development, India deals with critical issues such as overpopulation and widespread environmental degradation. Over the past few decades, the country has lost more than 50% of its forests, polluted over 70% of its waterbodies, and transformed vast expanses of grasslands into agricultural fields and urban settlements (Sharma and Singh, 2000). Alarmingly, approximately 7.7% of India’s vascular plant flora is now threatened with extinction (Rao et al., 2003).

The risks associated with biological invasions are further exacerbated by rapid land-use transformations and the escalating impacts of climate change (Mungi et al., 2020). Large-scale infrastructure projects, including highways, railways, and hydroelectric dams, have driven extensive deforestation, the displacement of indigenous communities, and the pollution of aquatic ecosystems (Khalid et al., 2022). Such developments significantly alter natural habitats, creating pathways for invasive species establishment and spread. Concurrently, ongoing climate change is expected to break climatic barriers, further facilitating the spread of invasive alien species. For instance, Lantana camara has already invaded approximately 44% of India’s forested areas, raising serious ecological alarms (Mungi et al., 2020) followed by others (Fig. 1). The economic consequences of these invasions are equally alarming. Recent studies have reported substantial economic costs associated with the damage and losses inflicted by invasive species in India. However, these assessments reveal significant knowledge gaps, as they often exclude management costs and fail to account for the economic impact of many invasive plant species. Addressing these deficiencies is crucial to devise effective control and mitigation strategies.

In light of these challenges, the development of an updated and comprehensive inventory of alien species, with focus on invasive alien species (IAS), represents a crucial step towards informed management and long-term conservation efforts in India. This initiative will provide a robust foundation for addressing the ecological, economic, and societal risks associated with plant invasions, paving the way for sustainable habitat restoration and biodiversity conservation.

2.2 Data Sources

Selection and categorization of alien species:

To compile a comprehensive inventory of alien plant species in India, we conducted an extensive review of scientific literature published between 1995 and 2024. The taxonomic validity of each species was verified using the Plant List and three key nationwide checklists: (1) the Alien Flora of India (1,599 species) (Khuroo et al., 2012), (2) the Naturalized Alien Flora of the Indian States (499 species) (Inderjit et al., 2018), and (3) the Global Register of Introduced and Invasive Species -India, Version 1.3 (2,082 species) (Sankaran et al., 2020). Species names were standardized using the taxonstand package (Cayuela et al., 2017) in R v3.5.1 (R Core Team, 2018), and family classifications were updated according to the Angiosperm Phylogeny Group IV guidelines (APG IV, 2016). The dataset was meticulously curated by eliminating duplicate, synonymous, unresolved, and misspelled species entries. Infraspecific taxa and artificially hybridized species were excluded. In line with international standards for alien-flora studies (Pyšek et al. 2004), cultivated alien species were not included in any ecological or statistical analyses. These species were recorded separately because they do not form self-sustaining populations in the wild. Our final dataset contained 1,725 alien plant species in total, of which 1,022 species were recorded in the wild (casual, naturalized, or invasive). Only these 1,022 wild species were used for all analyses, including family composition, growth-form patterns, distribution across ecoregions, introduction pathways, and similarity assessments. Cultivated aliens were reported only as a separate numerical category.

Comparison with previous national checklists:

Since our study represents an updated account of the alien flora of India, we systematically compared our dataset with previously published national-level compilations (Khuroo et al., 2012; Inderjit et al., 2018; Sankaran et al., 2020). A summary table was prepared to highlight differences among these studies in terms of scope, methodological criteria, and total species richness (Table 1). This comparative assessment allowed us to identify new additions, taxonomic corrections, and changes resulting from updated sources such as GRIIS and GloNAF. These comparisons justify the need for an updated checklist and provide a clear baseline for future work.

Table 1
Comparison of previous major alien flora datasets with the present study
Aspect	Pysek et al., (2004) Framework	Khuroo et al., (2012) Alien Flora of India	Inderjit et al., (2018) Naturalized alien Flora of India	GRIIS (Sankaran et al., 2020)	ILORA (Pant et al., 2021)	Present study (2024)
Scope	Definitions and conceptual hierarchy	First National checklist of alien flora of India	State-level naturalized flora of India	National list of invasive alien species	Digital alien flora repository	Updated national checklist with ecoregional analysis
Geographic scope	Global	India (National level)	India (State level)	India (national level)	India (National level)	India (National + Ecoregional)
Species count	Not applicable	1599 alien species	499 naturalized species	2082 invasive species	2082 alien species	1022 wild alien species, 1725 including cultivated
Focus stage	All (conceptual)	Alien	Naturalized	Invasive	Alien	All stages casual, naturalized, invasive, cultivated, cryptogenic
Definition of Alien	Explicit, conceptual	Yes	Yes	Yes	Yes	Followed Pysek et al., (2004)
Definition of Naturalized	Explicit, conceptual	Implied, not strictly Pysek	Strict criteria	GRIIS includes only invasive not naturalized	Yes (Standardized)	Fully assigned with Pysek et al., (2004)
Definition of Invasive	Clear global standard	Used loosely	Not the focus	Primary focus	Yes	Adopted from GRIIS, standardized
Classification used	Casual/Naturalized/ Invasive	Alien categories broad	Naturalized only	Invasive only	Full taxonomy and invasion status	Casual, naturalized, invasive, cultivated, cryptogenic
Taxonomic standardization	Not required	Partial	Partial	Depends on GRIIS dataset	Fully standardized	APG IV + taxonstand, standardized
Origin assignment	Not applicable	Basic	limited	Provided but not verified	Based on POWO, WCSP	Multi-database validation: POWO + GRIN + CABI + literature
Invasion status assignment	Conceptual only	Not systematic	Systematic for naturalized	Systematic for invasive	Systematic	Two-step validation: status + nativity + literature conflict resolution
Distribution Data source	Not applicable	Literature	State lists	GRIIS metadata	GBIF + Literature	GBIF + Literature + region wise ecoregional classification.
Temporal information	Not applicable	Absent	Absent	Absent	Provides some	Earliest record (residence time) compiled
Analytical methods	Conceptual	None	None	None	Basic stats	Jaccard similarity + chi-square tests + ecoregional clustering
Novel contribution	Standard global definitions	First national alien inventory	First naturalized alien flora	First national invasive list	First digital repository	First Integrated, validated, fully standardized national dataset with analytical insights

A two-step verification process was employed to classify species based on their origin status (native vs. alien) and invasion status (degree of naturalization) in India, following Pyšek et al. (2004).

Determining Origin Status:

The origin status of each species was cross-referenced with three global databases, Plants of the World Online (POWO), CABI Invasive Species Compendium (CABI-ISC), and the Germplasm Resource Information Network (GRIN). For cases of discrepancies among these databases, nativity was resolved through peer-reviewed articles sourced from Google Scholar. Species identified as native were excluded from further analysis, while species with unclear nativity were classified as cryptogenic (Essl et al., 2018).

Determining Invasion Status:

From the pool of alien and cryptogenic species, invasive alien species (IAS) were identified using the Global Register of Introduced and Invasive Species (GRIIS). Naturalized alien species (NAS) were categorized based on the Naturalized Alien Flora checklist and the Global Naturalized Alien Flora (GloNAF) database (van Kleunen et al., 2019). Casual alien species were identified following Khuroo et al. (2012). Cryptogenic species with unresolved invasion status were kept unchanged.

2.3 Data Curation

For each species, taxonomic details, including class, order, and family, were meticulously documented alongside their common English names. Information regarding the time of introduction was extracted from literature reports and global databases, with the residence status in India determined by referencing the earliest recorded year in the curated dataset. Native and naturalized range data were sourced from global databases adhering to the guidelines of the Taxonomic Databases for Plant Sciences (TDWG) (Brummit et al., 2021).

To determine species distribution across Indian states and union territories, occurrence records were retrieved from the Global Biodiversity Information Facility (GBIF). The country is subdivided into seven ecoregions for analytical purposes: North India (NI): Jammu & Kashmir, Ladakh, Himachal Pradesh, Haryana, Punjab, Uttar Pradesh, and Delhi. South India (SI): Andhra Pradesh, Telangana, Karnataka, Kerala, Tamil Nadu, Puducherry. West India (WI): Rajasthan, Gujarat, Maharashtra, Goa. East India (EI): Odisha, West Bengal, Bihar, Jharkhand. Central India (CI): Madhya Pradesh, Chhattisgarh. Northeast India (NEI): Arunachal Pradesh, Assam, Manipur, Mizoram, Meghalaya, Nagaland, Tripura. Islands (I): Dadra & Nagar Haveli and Daman & Diu, Lakshadweep, Andaman & Nicobar Islands.

Species occurrence data were compiled from peer-reviewed literature and scientific publications, ensuring high data reliability and traceability. The dataset was assessed for quality based on parameters such as reliability, usability, and accessibility. Individual records were sourced from frequently updated and detailed databases that provide traceable sources, enhancing their credibility. To ensure technical validation, a rigorous multistep approach was adopted. This included iterative and random data checks conducted by multiple team members to minimize curator bias, detect and rectify omission errors, and enhance data accuracy. The curated dataset, subjected to quality controls, provides a reliable and consistent foundation for comprehensive analyses and inclusion in the final checklist. Locations of invasive alien species distributed in the different regions of the country were marked (Fig. 2).

2.4 Statistical Analysis

To examine compositional patterns of alien flora across Indian ecoregions, we employed the Jaccard similarity index, which quantifies the proportion of shared species between region pairs relative to their combined species pool. This approach allowed assessment of overlaps in species composition among casual, naturalized, and invasive alien species across the seven ecoregions.

A Chi-square test of independence was conducted to evaluate the association between invasion status categories (casual, naturalized, invasive, cultivated) and plant attributes such as growth form, native region, and ecoregional distribution. Test statistics were computed in Microsoft Excel, with p-values obtained using the CHISQ.DIST. RT function and interpreted at α = 0.05. These complementary approaches provided insights into the ecological and biogeographic patterns shaping alien plant establishment across India.

Taxonomic Composition

The alien flora of India comprises 1,022 wild alien species, reflecting substantial taxonomic diversity. Based on invasion status, these species are categorized as casual (449 species, 43.93%), naturalized (380 species, 37.18%), and invasive (193 species, 18.88%). A comprehensive species-level dataset including taxonomy, invasion status, geographic origin, and ecoregional occurrence is presented in Supplementary Tables (S1-S5).

These species belong to 164 families and 491 genera, highlighting their broad phylogenetic representation. Tree species form a major component of the alien flora (35.42%). Among alien trees, Fabaceae (16.85%) and Myrtaceae (15.46%) are the dominant families (Fig. 3a). Casual alien trees are mainly contributed by Myrtaceae (16.79%), whereas Fabaceae strongly dominates both invasive (64.28%) and naturalized trees (19.56%). Geographically, alien tree species display clear biogeographic patterns. Most of the casual and invasive alien trees originate from Australia (18.01% and 41.66%, respectively), while naturalized alien trees are primarily from South America (17.30%), followed by North America (16.34%) (Fig. 4a).

Historical evidence shows that alien trees began to arrive in India as early as the 1500s, with introductions increasing steadily over time. These dynamics, influenced by trade, colonization, and agricultural expansion, are summarized in Fig. 5, illustrating shifting temporal contributions of different source regions.

Contribution of Shrubs and Herbs to the Alien Flora of India

Shrubs contribute 10.46% of India’s alien flora. Among them, 65.42% are naturalized aliens, and 34.57% are invasive aliens; no casual shrubs were recorded. Fabaceae (26.16%) is the leading family among alien shrubs, contributing 16.90% of naturalized and 43.24% of invasive shrubs. Solanaceae follows, contributing 22.53% of naturalized and 18.91% of invasive shrubs (Fig. 3b).

South America is the largest source of alien shrubs (32.72%), with 72.22% naturalized and 27.77% invasive. Tropical America being the second major contributor (25.45%), however provides the highest proportion of invasive shrubs (46.42%) (Fig. 4b). Alien shrubs began arriving in India as early as the 1500s, with a marked increase between 1750 and 1850, coinciding with historical land-use changes and botanical introductions (Fig. 5).

Herbs represent the largest share of India’s alien flora (50.58%). These include casual (35.78%), naturalized (38.87%), and invasive herb species (25.33%) (Fig. 3c). Asteraceae (19.72%) is the dominant family, followed by Poaceae (14.70%). Asteraceae contributes nearly 38.23% of naturalized and 39.21% of invasive herbs, while Poaceae contributes the highest proportion (36.36%) of casual alien herbs (Fig. 3c).

Europe is the primary source of alien herbs (36.55%), followed by Temperate Asia (33.65%). Europe also contributes the largest proportions of casual (38.62%) and naturalized herbs (51.85%), while South America contributes the majority of invasive herbs (43.82%) (Fig. 4c). Herb introductions increased sharply between 1760 and 1860 (Fig. 5), aligned with agricultural expansion and colonial exploration.

Contribution of Climbers and Regional Patterns

Climbers account for 3.52% of India’s alien flora. Of these, 22.22% are casual, 47.22% are naturalized, and 30.55% are invasive climbers. Convolvulaceae (27.77%) is the most dominant family, followed by Passifloraceae (13.88%) and Apocynaceae (11.11%) (Fig. 3d). Passifloraceae contributes most naturalized climbers (29.41%), while Convolvulaceae contributes 70% of invasive climbers. Casual climbers are predominantly from Apocynaceae (50%).

Tropical America is the leading source region for alien climbers (72.22%), contributing the largest share of casual (19.23%), naturalized (46.15%), and invasive climbers (34.61%) (Fig. 4d). The introduction of alien climbers can be traced back to the 1500s, with moderate growth between 1760 and 1830 (Fig. 5).

Dominant Families

Across all growth forms, Asteraceae is the dominant family (108 species), followed by Poaceae (77 species) and Fabaceae (65 species). These species-rich cosmopolitan families are known for their ecological versatility, effective dispersal mechanisms, and competitive reproductive strategies, enabling their establishment across diverse Indian ecosystems. Tree-dominated families such as Fabaceae (61 tree species) and Myrtaceae (57 tree species) further contribute prominently, while Solanaceae (41 species) and Amaranthaceae (32 species) are major contributors among shrubs and herbs. Further, Convolvulaceae is the highest contributor among climbers (10 species).

Ecoregional Contributions

Alien species are unevenly distributed across India’s ecoregions (Fig. 6). South India supported the highest alien species richness across all invasion stages, with 243 casual, 250 naturalized, and 150 invasive species, highlighting its role as a major introduction and establishment hotspot. North India followed closely, particularly for naturalized species (234), reflecting long-term cultivation history and high propagule pressure. East and West India showed intermediate levels of alien species richness, with 105–113 casual species and 83–107 invasive species, suggesting moderate introduction intensity and spread. Northeast India, despite having fewer introductions than the South and North, contained a relatively high number of invasive species (114), indicating ecological vulnerability and favourable conditions for invasion. Central India exhibited the lowest numbers of casual (24), naturalized (78), and invasive (57) species, reflecting comparatively lower propagule pressure and stronger ecological filters. The Island region had 63 casual, 122 naturalized, and 68 invasive species, showing that isolated but highly disturbed island ecosystems can also host substantial alien flora. Overall, the pattern demonstrates that India’s alien species richness is shaped by a combination of introduction history, habitat suitability, and region-specific disturbance regimes

Patterns of Similarity Across Ecoregions

The Jaccard similarity heatmap (Fig. 7) illustrates how similar ecoregions are in terms of their alien plant species composition across the three invasion stages casual, naturalized, and invasive. The dendrogram shows distinct clustering patterns that reflect different invasion dynamics. Casual alien species exhibit very low similarity among regions, indicating that early-stage introductions are highly localized and shaped by region-specific horticultural, agricultural, and propagule pressure pathways. In contrast, naturalized species display moderate similarity, with clear clusters linking ecologically or geographically adjacent regions, such as East-Northeast, West-Northeast, and Island-Central, suggesting regional-scale spread and emerging biogeographic affinities. Invasive species show the highest similarity values, with particularly strong overlap between North-East, North-West, and East-West pairs, indicating that invasive taxa have broad ecological tolerances and move across long-distance ecological corridors. Overall, the heatmap reveals a progression from localized introductions (casual) to regional spread (naturalized), and finally to widespread homogenization (invasive), highlighting the increasing synchrony of alien floras across India at advanced invasion stages.

Associations Between Species Status and Ecological Attributes

The chi-square tests of independence demonstrated that alien species status categories are strongly and significantly associated with key ecological and biogeographic attributes, underscoring the non-random nature of alien species establishment in India. The relationship between growth form and invasion status was highly significant (χ² = 456.70, df = 9, p < 0.001), indicating that biological traits such as being a tree, shrub, herb, or climber strongly influence the likelihood of a species progressing through different stages of invasion. This likely reflects differences in reproductive strategies, dispersal mechanisms, and ecological amplitudes associated with various growth forms. Similarly, a very strong association between native region and invasion status was observed (χ² = 179.12, df = 21, p < 0.001), suggesting that source regions differ markedly in their contribution to casual, naturalized, and invasive species. Some regions, particularly those with long histories of plant exchange or ecological conditions similar to India’s, act as dominant donors of invasive taxa. The association between ecoregional distribution and invasion status was also highly significant (χ² = 166.86, df = 18, p < 0.001), demonstrating that alien species do not spread uniformly across the country but instead show strong clustering patterns linked to regional ecological conditions, climatic compatibility, and anthropogenic influences. Together, these findings reveal that alien species’ invasion trajectories are jointly shaped by their intrinsic biological traits, their biogeographic origins, and the ecological contexts of the regions they occupy. This highlights the structured, non-random processes that govern alien species establishment, naturalization, and invasion across India.

Developing national-level inventories of alien flora has been essential for understanding invasion dynamics and guiding effective management interventions (Khuroo et al., 2012; Pyšek et al., 2017; Inderjit et al., 2018; van Kleunen et al., 2019; Pant et al., 2021). The present study contributes a fully updated, standardized, and taxonomically validated inventory of India’s alien vascular flora, incorporating all invasion stages and distinguishing wild alien species (n = 1,022) from cultivated aliens (n = 1,725). We documented 193 invasive alien species (18.88%) of the alien flora, a number slightly higher than earlier assessments (Reddy, 2008; Khuroo et al., 2012; Adhikari, 2015), primarily because we included species identified as “possibly invasive” or transitioning along the introduction–naturalization–invasion continuum (Richardson et al., 2000; Blackburn et al., 2011).

Growth form influences invasion trajectories

Our findings reveal that alien species status is strongly and significantly associated with growth form (χ² = 456.70, df = 9, p < 0.001). This aligns with trait-based invasion theory, which posits that functional traits and life-history strategies influence establishment and spread (Drenovsky et al., 2012; Daly et al., 2023). Herbaceous species, characterized by rapid reproduction, high seed output, and short generation times, dominate the invasive pool, a pattern widely documented across global floras (Lake & Leishman, 2004; Pyšek & Hulme, 2005; Sekar, 2012; Inderjit et al., 2018).

Asteraceae, the largest dicot family, is the most dominant contributor to alien herbs, with species such as Ageratina adenophora, Chromolaena odorata, Mikania micrantha, and Solidago gigantea widely distributed across states due to prolific seed production and long-distance dispersal (Witkowski & Wilson, 2001; Mandal & Joshi, 2014; Day et al., 2016). Their ecological versatility, competitive ability, and common use in ornamental horticulture further enhance their invasion success (Heywood et al., 2007; Medvecká et al., 2012).

Trees and shrubs, in contrast, predominantly enter floras through cultivation. Their appearance in casual and cultivated categories reflects horticultural pathways, yet several species transition to naturalization and invasion when ecological and climatic conditions match their niche requirements.

Biogeographic origins and invasion pathways

Native region strongly influenced invasion status (χ² = 179.12, df = 21, p < 0.001). Tropical America emerged as the dominant source of India’s invasive alien flora, a pattern consistent with earlier Indian and global studies (Weber et al., 2008; Jaryan et al., 2012; Inderjit et al., 2018; Gulzar et al., 2024). Many of India’s most aggressive invaders Lantana camara, Parthenium hysterophorus, Mikania micrantha, and Chromolaena odorata originate from this region, reflecting strong climatic matching with India’s tropical and subtropical ecosystems.

Historical processes such as colonial-era botanical exchanges and high propagule pressure further strengthened Indo-Neotropical introduction pathways (Ravi et al., 2024; Pant et al., 2021). Other source regions, particularly Africa and Europe, also contribute substantially to the alien flora, but Neotropical species disproportionately dominate the invasive pool due to pre-adaptation and climatic analogues (Daly et al., 2023).

Ecoregional patterns and environmental filtering

Alien species distribution across India is spatially structured, not uniform, as indicated by the strong association between ecoregion and invasion status (χ² = 166.86, df = 18, p < 0.001). South and North India harbour the highest richness across all invasion stages, reflecting long histories of horticultural introductions, extensive cultivation, and environmental suitability. High precipitation in South India likely facilitates invasion by fast-growing herbs and grasses, consistent with rainfall-invasion relationships documented elsewhere (Lambdon et al., 2008; Ascbacher & Cleland, 2015; Goldstein & Suding, 2014). Conversely, ecoregions such as North-Central and East-South exhibit distinct assemblages, indicating the presence of ecological filters, climatic constraints, or reduced propagule pressure.

A notable pattern emerges in Central India, where species not yet formally classified as invasive Solanum erianthum, Conocarpus erectus., Gliricidia sepium and Ageratina adenophora exhibit invasive-like behaviour due to extensive habitat disturbance (Fig. 8). Urban development, Smart City initiatives, and plantation drives create novel niches favouring these taxa. Documented allelopathy and competitive dominance in Conocarpus (Al-Shatti et al., 2014; Rehman et al., 2022), rapid colonization by Solanum erianthum, and soil-tolerance advantage of Gliricidia sepium suggest a probable future expansion into India’s invasion pool (Kaushik et al., 2024). Such cases highlight the need for proactive monitoring of “incipient invaders.”

Patterns of similarity across ecoregions

Similarity patterns derived from the Jaccard index further illuminate the spatial dynamics of plant invasions across India. Casual alien species show consistently low similarity among ecoregions, indicating that their early-stage presence is largely shaped by stochastic, region-specific introduction pathways. In contrast, naturalized species display moderate similarity and form distinct compositional clusters particularly between East-Northeast, West-Northeast, and Island–Central ecoregion pairs suggesting the emergence of biogeographic affinities and regional spread. Invasive species exhibit the strongest convergence across even distant regions, such as East–West and North-East and North-West, reflecting broad ecological tolerance, effective dispersal mechanisms, and strong human-mediated connectivity. These patterns align with global findings that highlight the formation of invasion corridors, increasing spatial homogenization, and the role of anthropogenic networks in facilitating invasive species spread (Iannone et al., 2016; Mungi et al., 2023).

Comparison with Previous Alien Flora Datasets

Our findings also enable a meaningful comparison with previous national and global alien flora datasets. Unlike earlier lists, which typically focused on only one stage of the invasion process such as alien, naturalized, or invasive species, our dataset integrates all invasion stages, including casual, naturalized, invasive, and cryptogenic taxa. This provides a more complete representation of India’s alien flora. The study further advances previous efforts by implementing comprehensive taxonomic standardization using APG IV and multiple global databases (POWO, GRIN, CABI-ISC), along with the taxonstand package. Additionally, a rigorous two-step validation of nativity and invasion status ensures higher accuracy, while the explicit separation of cultivated alien species (n = 1,725) from wild alien species (n = 1,022) fills a major gap in earlier inventories such as Khuroo et al. (2012), Inderjit et al. (2018), GRIIS, and ILORA. The integration of national-scale ecoregional analyses, coupled with trait-based and biogeographic assessments, positions our work as the first fully standardized and ecologically contextualized national synthesis of India’s alien vascular flora.

Implications for management

The results of this study also have clear implications for invasion management. The patterns observed highlight that invasion success in India is jointly shaped by species-level traits, biogeographic origins, and environmental receptivity of different regions. Identifying high-risk growth forms particularly herbs and climbers and high-risk source regions such as the Neotropics and Africa provides a scientific basis for prioritizing quarantine screening and early detection efforts. Similarly, recognizing hotspot ecoregions such as South India, Central India, and the islands can guide regional surveillance and rapid response planning. Our findings further emphasize the importance of regulating horticultural and landscaping pathways, which continue to introduce potentially invasive taxa into disturbed and urbanizing habitats. Prevention-oriented policies, alongside targeted monitoring of disturbance-prone ecosystems, will be essential to limit future invasions and reduce ecological and economic impacts.

Overall, the integration of trait profiles, native-range characteristics, and ecoregional patterns underscores the non-random nature of alien plant establishment and spread in India. By providing a robust, validated dataset and a detailed analysis of invasion dynamics, this study offers a foundation for developing evidence-based management strategies and long-term policy interventions. Embedding invasion biology principles into land-use planning, restoration programs, and national biodiversity management will be crucial for addressing current and emerging threats from alien plant species and for strengthening India’s capacity to manage biological invasions effectively.

Conflict of interest statement

The authors have no competing interest to declare that are relevant to the content of this article.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.

Author Contributions

Priya Kaushik: Writing- original draft, Methodology, Data curation, Conceptualization. Pranab Kumar Pati: Visualization, Methodology, Investigation, Data curation. Mohammed Latif Khan: Resources conceptualization. Pramod Kumar Khare: Supervision, Conceptualization

Acknowledgements

Authors are thankful to all contributors whose research papers and information have been cited in this study. This work was supported by the University Grant Commission (UGC), Government of India, through a Ph.D. fellowship awarded to PKP. One of the authors (PK) also gratefully acknowledges the Council of Scientific and Industrial Research-Human Resource Development Group (CSIR-HRDG) for providing the Research Associate fellowship and essential facilities that enabled the successful completion of this work. The authors extend their sincere gratitude to Prof. Petr Pyšek for his insightful and constructive comments, which significantly improved the clarity, rigor, and overall quality of this manuscript.

Data availability

The dataset generated during and/or analysed in the present study are available in the manuscript itself.

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Confronting the Green Threat: An Updated Inventory of India’s Alien Plant Species

Status:

Version 1

Abstract

Figures

Introduction

Materials and Methods

Selection and categorization of alien species:

Comparison with previous national checklists:

Determining Origin Status:

Determining Invasion Status:

Results

Taxonomic Composition

Contribution of Shrubs and Herbs to the Alien Flora of India

Contribution of Climbers and Regional Patterns

Dominant Families

Ecoregional Contributions

Patterns of Similarity Across Ecoregions

Associations Between Species Status and Ecological Attributes

Discussion

Growth form influences invasion trajectories

Biogeographic origins and invasion pathways

Ecoregional patterns and environmental filtering

Patterns of similarity across ecoregions

Comparison with Previous Alien Flora Datasets

Implications for management

Declarations

Funding

Author Contributions

Acknowledgements

Data availability

References

Supplementary Files

Status:

Version 1