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Globalization in trade routes, markets and products using modern technologies drive the spread of invasive alien species (IAS), whose introduction does, or is likely to, cause economic or environmental harm or direct harm to human health through intentional or accidental introductions (Meyerson & Mooney, 2007). As such, biological invasion has become a global problem, subjecting many countries to huge losses in their economy through negative impacts on crops, animal and human health, and the loss of biodiversity. A loss of over US$ 40 billion annually in the United States of America is accounted for by crop losses and in controlling of invasive species (Pimentel, Lach, Zuniga, & Morrison, 2000). Likewise, an estimated total of US$ 29.47 billion annual agricultural damage in Southeast Asia were attributed to non-indigenous weeds, pests and pathogens (Nghiem et al., 2013). In some European countries, 10-20% of patients are accounted for pollen allergy caused by common ragweed leading to hay fever, rhinoconjunctivitis and asthma-like symptoms (Scalera, Genovesi, Essl, & Rabitsch, 2012).
Some of the detrimental effects on biodiversity include competitive exclusion of native species through modification of habitat and facilitating subsequent invasions (Simberloff & Von Holle, 1999). Such processes lead to disruptions of ecosystem services such as disturbing water flow regime by increasing evaporation rates, reducing stream flow and dilution capacity in case of South Africa (Chamier, Schachtschneider, Le Maitre, Ashton, & Van Wilgen, 2012; D'Antonio & Vitousek, 1992). Ageratina adenophora, Ageratum conyzoides, Chromolaena odorata, Lantana camara, Mikania micrantha and Parthenium hysterophus were long recognized as IAS in South and Southeast Asia (Pallewatta, Reaser, & Gutierrez, 2003) and recently as global IAS by the Centre for Agriculture and Bioscience International (2018). In China, some of these species were found fast spreading, from lowlands to mountains causing extirpation of native local plants, death of animals (by A. adenophora) resulting in serious economic losses to agriculture, forestry and livestock and severe damage of ecology (Wan et al., 2010; Yan, Zhenyu, Gregg, & Dianmo, 2001). In India some of the species such as P. hysterophorus, were found to be noxious, allergenic and poisonous to humans and animals, while others were found to reduce crop yield, cause loss of native biodiversity and prevention of forest regeneration (Singh, 2005).
Impacts of invasion would mean a worse scenario in the case of Bhutan, given its inhospitable geographical terrain (Ohsawa, 1987, 1991), holding only about 3% as arable land (Gaden, Choden, & Tshomo, 2015) with high vulnerability to climate change (International Center for Integrated Mountain Development, 2009). Although the Bhutan quarantine act clearly states that Bhutan shall prevent introduction of pests and control the widespread of those already present, since mid-960s, the import of invasive plants either as ornamental or alternative food has become popular. Between 1997 and 2000 some 60 plant species were introduced (Weiss et al., 2004). As such, Weiss et al. (2004) recommend that a risk assessment of introduced plants for their invasion potential is essential.