What do we really know about alien plant invasion? A review of the invasion mechanism of one of the world's worst weeds.

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CONCLUSION: This review provides an insight into alien plant invasion taking into account the invasion mechanism of parthenium weed ( Parthenium hysterophorus L.). A multi-lateral understanding of the invasion biology of this weed has pragmatic implications for weed ecology and management. Biological invasions are one of the major drivers of restructuring and malfunctioning of ecosystems. Invasive plant species not only change the dynamics of species composition and biodiversity but also hinder the system productivity and efficiency in invaded regions. Parthenium weed, a well-known noxious invasive species, has invaded diverse climatic and biogeographic regions in more than 40 countries across five continents. Efforts are under way to minimize the parthenium weed-induced environmental, agricultural, social, and economic impacts. However, insufficient information regarding its invasion mechanism and interference with ecosystem stability is available. It is hard to devise effective management strategies without understanding the invasion process. Here, we reviewed the mechanism of parthenium weed invasion. Our main conclusions are: (1) morphological advantages, unique reproductive biology, competitive ability, escape from natural enemies in non-native regions, and a C3/C4 photosynthesis are all likely to be involved in parthenium weed invasiveness. (2) Tolerance to abiotic stresses and ability to grow in wide range of edaphic conditions are thought to be additional invasion tools on a physiological front. (3) An allelopathic potential of parthenium weed against crop, weed and pasture species, with multiple modes of allelochemical expression, may also be responsible for its invasion success. Moreover, the release of novel allelochemicals in non-native environments might have a pivotal role in parthenium weed invasion. (4) Genetic diversity found among different populations and biotypes of parthenium weed, based on geographic, edaphic, climatic, and ecological ranges, might also be a strong contributor towards its invasion success. (5) Rising temperatures and atmospheric carbon dioxide (CO2) concentrations and changing rainfall patterns, all within the present day climate change prediction range are favorable for parthenium weed growth, its reproductive output, and therefore its future spread and infestation. (6) Parthenium weed invasion in South Asia depicts the relative and overlapping contribution of all the above-mentioned mechanisms. Such an understanding of the core phenomena regulating the invasion biology has pragmatic implications for its management. A better understanding of the interaction of physiological processes, ecological functions, and genetic makeup within a range of environments may help to devise appropriate management strategies for parthenium weed.