The Shivalik hills in Northwestern Himalayan range of India have a rich floral diversity. Unfortunately during the last two decades there has been drastic reduction in the diversity of the natural vegetation. The available niches have been occupied by invasive exotic species that were either introduced or have entered accidently. It has resulted from a numbers of factors including increased inter and intra-continental links, import-export and climate change. These exotic species possess certain traits that provide them competitive advantage over the natives and thus aid in their fast spread in the alien environment. Even some of the plants introduced for beneficial purposes have acquired weedy habit. It has greatly altered the structure of the natural ecosystems and caused a dramatic shift in the diversity and dynamics of native flora. The situation has further aggravated due to lack of awareness, insufficient information on the species and its dimensions of the spread besides wide ecological amplitude. It was observed that the diversity, evenness and richness of the native species were drastically reduced in the forest invaded by the exotics.
Key words: Northwestern Himalayan, Shivalik hills, invasive, diversity.
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