Biological invasion is one of the greatest threats to biodiversity in the Anthropocene. Across multiple taxonomic groups, invasive species have caused substantial declines in native biodiversity, altered the composition of natural ecological communities, disrupted key species interactions and modified essential ecosystem processes such as nutrient cycling, fire patterns, and resource availability. Plant-pollinator interactions assist in the reproduction of most flowering plants; however, invasive plant species may disrupt these mutualisms by competing with native plants for pollinators, thereby altering native plant-pollinator networks and compromising the reproductive processes of native plants. Understanding how invasive plants (i) influence plant-pollinator interactions, (ii) adapt to novel environments through alternative reproductive strategies, and (iii) can be managed to design effective restoration of native vegetation is therefore critical for biodiversity conservation. My thesis investigates these questions in Eaglenest Wildlife Sanctuary (EWS), part of the Eastern Himalaya Global Biodiversity Hotspot. In EWS, historical low-elevation settlements were abandoned ~25 years ago, and the abandoned land is now dominated by three main invasive plant species - Ageratum conyzoides, Chromolaena odorata and Mikania micrantha.
In the first chapter, I compared primary forest (PF) with invasive-dominated habitat (IDH) to understand how invasive plant dominance alters plant-pollinator network (PPN) structure. Using quantitative bipartite network analyses across multiple seasons, I found that although invaded habitats support higher plant and pollinator richness, the overall plant species diversity was higher in PF. Invaded PPNs show higher nestedness and lower modularity, indicating a shift towards more generalized plant-pollinator networks with potentially reduced long-term interaction resilience.
In the second chapter, I investigated whether invasive plants depend on native pollinators for successful reproduction or adopt autonomous reproductive strategies. Through pollination exclusion experiments on three invasive species (A. conyzoides, C. odorata and M. micrantha), I demonstrated substantial differences in reproductive strategies among invaders. While A. conyzoides and C. odorata maintain seed production and average seed mass even under pollinator exclusion suggesting considerable reproductive autonomy, M. micrantha exhibits greater dependence on pollinator-mediated reproduction for both seed quality and quantity. These contrasting reproductive strategies between co-occurring invasive plant species might help explain how different invasive species establish and spread successfully in disturbed landscapes.
In the third chapter, I evaluate restoration strategies for IDH through a field experiment combining invasive plant removal, native seed addition, and soil seed bank assessment to evaluate regeneration potential. I compared alternative restoration treatments for their effectiveness in promoting native vegetation recovery. Complete clearing of vegetation plots combined with native seed addition resulted in the greatest establishment of the target native species (Pogostemon elsholtzioides) and supported enhanced native recruitment, highlighting the importance of active restoration for recovering invaded forest ecosystems.
Together, these chapters integrate plant-pollinator community ecology, reproductive biology and restoration ecology to provide a more comprehensive understanding of how invasive plants adapt to new habitats and alter ecological interactions. By linking invasion impacts with experimentally evaluated restoration strategies, this thesis contributes to a broader understanding of how biodiversity and ecological resilience can be conserved in one of the world's most diverse ecosystems.