Distribution pattern and ecological determinants of an invasive plant Parthenium hysterophorus L., in Malakand division of Pakistan

After habitat loss, drastic effects of biological invasion caused by alien invasive species to the native biodiversity have been acknowledged worldwide by the agriculturists, ecologists, and governments as it is more overwhelming than pollution, harvest and disease combined. One such example of biological invasion is P. hysterophorus which is considered to be one of the topmost noxious weeds threatening ecosystems diversity in the world, particularly in Pakistan due to its invasive success and generalist nature. Yet no studies have explored the pattern and determinants of plant biodiversity in the Parthenium dominated landscapes. Here we explored the current distribution pattern of this species with relation to environmental variables in the Malakand division of northern Pakistan. Twenty-three sites dominated by P. hysterophorus were identified through a participatory mapping process combined with traditional ecological knowledge and quantitative inventories were made. In total, we found 62 plant species in 55 genera and 32 families, predominately from Asteraceae, Poaceae, and Solanaceae. Ward’s agglomerative clustering procedure showed three well-differentiated, and ecologically meaningful but overlapping vegetation communities. P.hysterophorus was found to be the leading species with an average Importance value ranging from 33.28% to 65.59%. Based on our criteria, surprisingly we found significantly less to be fully(30%) or partially invaded(21%) than the less invaded(47%) plots, indicating the gradual invasive success of the species across the landscape. Canonical Correspondence Analysis(CCA-ordination) has explained 36.9% of the cumulative variance by the first three axes and identified elevation(r=-0.526), slope(r = 0.438) and percent sand(r=-0.474) to be the major environmental factors affecting the distribution of P. hysterophorus and associated vegetation. These results suggested that elevation and percent sand negatively affect Parthenium distribution whereas slope has a positive relationship thereby promoting its populations in the region. The generalist nature of this invasive species is likely to have an overall adverse negative effect on the ecosystem functioning and cannot be ignored. Therefore, quick actions should be taken for the management of this noxious weed to avoid economic consequences and potential threats to fragile ecosystems such as that in Malakand division.