Phenotypic variations alter the ecological impact of invasive alien species: Lessons from Parthenium hysterophorus

Invasive plant species constantly adjust their behavior with ecological shifts by virtue of phenotypic plasticity and/or local adaptations. Changes in the phenotype of an invasive species may also trigger variations in its community level impacts, which is an acceptable, yet unexplored aspect of invasion biology. Our study attempts to fill important knowledge gaps on the basic behavior and ecological interactions of invasive species. Parthenium hysterophorus, a widely distributed invasive alien species of tropical and sub-tropical regions, was evaluated for variations in its morpho-functional traits and ecological performance at a common spatial and temporal scale. Field surveys were conducted in Chandigarh, India, in five sites identified as invaded with P. hysterophorus. Individuals of P. hysterophorus randomly sampled from these sites, showed from trait analyses that the population is differentiated into two morphotypes, P_A and P_B. Morphotype P_B exhibits traits comparable to the shrub life-form in terms of woody stem (with higher stem circumference [+32.26%], stem specific density [+128.57%], twig dry matter content [+25.15%]); profuse branching (+46.38%); larger canopy (+91.16%); and better reproductive output (+190.29%) compared to P_A. P_A, on the other hand, reflected herbaceous characteristics with greater leaf area (+67.58%) and higher content of chlorophyll (+21.92%) compared to P_B. Based on these morphotypes, the plots were divided into three invasion categories: areas invaded by P_A [IP_A], areas invaded by P_B [IP_B] and uninvaded areas [UI]. Ecological indices and soil chemical properties were compared across IP_A, IP_B and UI. Shannon's index (p < 0.001), evenness index (p = 0.008), and richness index (p < 0.001) were significantly lower in IP_B compared to IP_A. UI areas were found to have higher soil pH, phenolics, organic matter, and concentrations of N, P and K, compared to IP_A and IP_B, but lower Ca and Mg. Results suggest that phenotypic variations within population of P. hysterophorus regulate its ecological impact on associated vegetation. Conservation managers would benefit from studying its invasion patterns and identifying the morphotype with higher ecological impact to prioritize management efforts. Monitoring these behavioral and ecological patterns in P. hysterophorus over the long-term may also help in anticipating challenges to preventive measures.