The term "Invasive Alien Species" (IAS) refers to any non-native species introduced via humanmediated pathways that pose a threat to native ecosystems. This phenomenon has increased in the last few decades due to rapid globalization and the Industrial Revolution, leading to the widespread dispersion of IAS across the globe. This global spread has had profound implications for biodiversity, local economy, ecosystem services, and even human health. IAS are now widely recognized as a leading cause of biodiversity loss and the extinction of native and endemic species in regions they invade. Alarmingly, these invaders threaten critically endangered birds, terrestrial mammals, and reptiles globally. Of particular concern is the fact that 28 per cent of these IAS are located in ecologically significant island ecosystems. For instance, the introduction of species like mosquitofish and feral cats has led to the extinction of several native species in Australia. The impact of IAS is further exacerbated by human-driven climate change and deforestation, which create favourable conditions for their proliferation. The impacts of IAS vary widely, depending on their niche and their local interactions with biotic and abiotic factors. These invaders can directly or indirectly replace natural vegetation and reduce native faunal diversity, thereby altering ecosystems and their services.

In India, a country with high freshwater biodiversity and endemism, aquatic ecosystems are home to 12 recognized invasive Invasive Aquatic Fishes (IAF), posing a grave threat to native species. The ornamental fish trade and profit-driven aquaculture business have been a significant contributor to the introduction of IAF in India. Recent decades have witnessed an increase in introductions and wider distribution of these invasive alien fishes. Moreover, global climate change could exacerbate biological invasions in freshwater ecosystems, leading to further spread. IAF have not only impacted aquatic ecosystems but have also been reported within protected areas in India, where they compete for natural resources and harm native biodiversity. These invasions have inflicted substantial financial losses on the national economy, with estimated costs ranging from ₹ 8.3 trillion to 11.9 trillion INR from 1960 to 2020.

The proposed river-linking project in India is an ambitious endeavour aimed at ensuring year-round water availability for irrigation, drinking, flood mitigation, and hydropower generation. However, such large-scale hydrological connectivity projects can inadvertently facilitate the spread of IAF across aquatic systems, where they were previously absent.

To address these issues, comprehensive studies assessing the distribution of IAF in India are urgently needed, along with measures to mitigate their impact. Species Distribution Modeling (SDM), particularly using the MaxEnt approach, has proven to be a valuable tool in predicting the potential distribution of invasive alien species. It aids in assessing invasion risk, understanding climate change impacts, and planning conservation and management strategies.

Understanding the distribution of IAS is vital for their ecology and for effective management and conservation planning. Predictive models, such as Species Distribution Modelling (SDM) or Ecological Niche Modelling (ENM), have been used to map the geographic ranges of IAS. Among these models, SDMs use species occurrence data and environmental variables (e.g., temperature, precipitation) to predict a species distribution. Our results shows that in India, there is a concerning trend of introducing alien freshwater fishes both for biocontrol purposes and to satisfy the demands of the aquaculture and ornamental fish trade. This influx of invasive alien fishes poses a significant and immediate threat to the native freshwater biodiversity of India.

Our research has unveiled the present and potential future distribution patterns of invasive alien fishes within India's freshwater ecosystems. By employing MaxEnt analysis, we have scrutinized the potential impact of various climate scenarios, specifically represented by RCP 2.6 and 8.5, on the proliferation of IAF across the nation. The study has notably spotlighted Gambusia spp., which have exhibited a dominant presence throughout the country. Additionally, our findings indicate that Pterygoplichthys spp. are likely to expand their range in the face of future climatic scenarios. Furthermore, we have identified and catalogued the river basins that are particularly vulnerable to IAF. In our investigation, we have conducted an extensive examination of the proposed river-linking projects and their potential implications. These projects hold the capacity to connect invasion hotspots with areas that house endemic and threatened fish species in India. This in-depth analysis underscores the pressing need for strategic conservation and management initiatives.

Furthermore, our examination of India's proposed river-linking projects has revealed a potential risk of homogenization, which could jeopardize the existence of native and endemic freshwater species. To mitigate the adverse effects and limit the future expansion of IAF, it is imperative to refrain from further introductions and carefully reconsider connecting water bodies. To facilitate effective management, continuous monitoring of these IAF is crucial, and this can be accomplished using advanced molecular tools such as environmental DNA (eDNA) and engaging citizen science approaches.

Exploring the introduction pathways of IAS is crucial for understanding the rate, reason, and impact of their invasions. This knowledge informs our understanding of their adaptive traits and could potentially help in their management. Exploring haplotypic diversity aids in understanding invasion pathways, species origin, the number of introduction events, and propagule pressure. Genetic tools have been employed to trace invasion routes. Genetic diversity in IAS is linked to their invasion success in the introduced area and often serves as a risk assessment tool. IAS typically undergo multiple bottleneck and founder effects, resulting in genetic drift upon introduction. The initial introduction event often with fewer propogules has lower genetic variation compared to the original population. However, IAS can succeed by adapting to limited genetic resources, known as the "genetic invasion paradox." Multiple introductions can lead to enhanced genetic diversity and the introduction of novel genetic combinations capable of overcoming the bottleneck effect. Such processes contribute to the success of IAS.

Among the IAF in India Gambusia affinis and Gambusia holbrooki, commonly known as the Western mosquitofish and Eastern mosquitofish, were introduced to India in 1928 from Italy for mosquito control. These two species Gambusia spp. which were introduced worldwide, are considered among the top 100 worst IAS. However, studies assessing the impact of mosquitofish on Indian water bodies have been limited, even though they have been established in Indian water bodies for nearly a century. Their adaptability, rapid growth, and aggressive behavior have allowed them to proliferate, leading to adverse impacts on native biodiversity and ecosystems. However, accurate species identification, especially between G. affinis and G. holbrooki, remains a challenge in India. In Australia, Europe, Argentina, and India, instances of misclassification have raised concerns. Correctly identifying Gambusia species is crucial for effective management and scientific research. To address this challenge, molecular methods have emerged as a reliable approach for accurate species identification, ensuring that control measures and research are based on precise data.

Multiple introductions of IAF allow them to acquire the genetic variation needed to adapt to new niches. Assessing the genetic variability is essential to estimate admixture between IAF populations. Genetic studies reveal population clustering, as demonstrated by the introduced population of Gambusia holbrooki in Australia, which exhibited high structuring despite low genetic diversity. Insights from genetic studies enhance our understanding of IAF and guide efforts to mitigate their impact. However, it is crucial to employ scientific rigour to understand the ecology and assess their genetic variability of IAF to manage them effectively.

Our findings confirm the presence of both G. holbrooki and G. affinis in India. The has also unveiled the existence of numerous haplotypes of G. holbrooki, implying multiple introduction events within the country. In contrast, G. affinis appears to be predominantly confined to the northeastern regions of India, characterized by a single haplotype. our results shed light on the potential impact of genetic drift and founder events in shaping the genetic diversity and population differentiation within invasive G. holbrooki populations in India. It is likely that multiple introductions have played a role in the species' invasion in the region. The remarkable success of G. holbrooki in India can be attributed to its inherent traits, including rapid breeding, high dispersal capacity, and human-mediated translocation.

The widespread distribution of these fishes across various elevations during our sample collection underscores their adaptability and extensive range.

Nevertheless, despite the genetic constraints observed, it remains imperative to implement ongoing monitoring and management strategies to mitigate the adverse effects of introduced species on native ecosystems. Furthermore, while the observed haplotypes align well with the global studies, the precise history of G. holbrooki introduction to India remains unclear.

In our study, the use of mtDNA and microsatellite markers provides valuable insights, although we acknowledge that SNP markers, known to offer higher genetic variation in European G. holbrooki populations, may provide a more detailed understanding. Future genetic investigations employing SNP markers, specifically developed for G. holbrooki, hold promise for unravelling genetic patterns associated with the species' introduction and expansion. This information is instrumental in comprehending the processes underpinning the successful worldwide invasion of G. holbrooki and will inform strategic management initiatives.

This thesis is one of the few studies addressing the biological invasion of G. holbrooki and G. affinis in India. The objectives include mapping the overall distribution of twelve selected IAF in India, exploring their introduction, establishment, their impacts and suggesting management strategies. This study employs Species Distribution Modelling (SDM) to understand the current and future distribution of selected IAF in India, taking into account the potential impact of the proposed riverlinking project. The thesis also recommends developing effective management plans for managing IAF in India, with a specific focus on G. affinis and G. holbrooki. Further, the study also aims to unravel their introduction history in India using genetic tools. This objective seeks to resolve the century-old confusion regarding which Gambusia species were introduced from Italy and which species is widespread in India today. Lastly, microsatellite markers are employed to understand the extent of genetic diversity gained due to multiple introductions of G. holbrooki in India and to test the invasion ecology hypothesis.

IAS pose significant challenges globally, and each nation must grapple with managing these introduced species. The success of management efforts varies depending on the capacity and willingness to address invasions on a global scale. India's freshwater ecosystems face numerous threats, and addressing IAS management is a crucial step toward preserving their integrity.