Assistant Professor University of the Punjab Lahore-54590 Pakistan Lahore, Punjab, Pakistan
Two of the key criteria for considering a plant species invasive in an ecosystem are: i) no-native (alien) and ii) causing or likely to cause economic, environmental and/or health harms to the human. The ordeal of ‘invasive’ for a certain plant become questionable over time as the latter criterion evanescent; especially, when environmental equity value of invading plant surpasses its identified or potential ecological harms. Hydrophytes are called invasive aquatic weeds when they establish in a built or natural freshwater body to an extent that their growth supersede productivity of the actual water body. In this study, the cons of two hydrophytes Phragmites australis L. and Typha latifolia L. in accidentally built urban wetland ecosystems were compared with their environmental equity value in order to reassess ‘invasive’ status of both hydrophytes. The cons of the invading hydrophytes were determined based on their contribution in altering habitat structure, biodiversity, productivity, nutrient cycling, and hydroperiod alteration. The environmental equity value of the invading hydrophytes was assessed based on counteracting anthropogenic eutrophication, fixation of carbon from the atmosphere into the harvested biomass of hydrophytes, and its application as feedstock for deriving biochar rendering further C-sequestration.
The invasion of both of the hydrophytes in the accidentally built urban wetland ecosystem didn’t induce significant alteration in the habitat structure. The overall shift in the biodiversity composition of the wetland was negligible over two years of study. Invasion of the hydrophytes had induced richness of rhizospheric microbial diversity assessed though total microbial community DNA. Annual productivity of the urban wetland ecosystem significantly increased after invasion of the hydrophytes than wetland ecosystems kept intact of any invasive hydrophytes. There was 5 times more removal of NPK from wetland water after invasion. This perhaps was because of the mineralization and conversion of organic N and P into mineralized or bioavailable form due to establishment of rhizospheric microbial community of hydrophytes. There was net positive value of C sequestered into the soil via biochar derived from the harvested biomass of both of the invasive hydrophytes. Multiple pros associated with invading hydrophytes in the urban wetland ecosystem were significantly higher than their limited cons in the form of negligible shift in pre-invasion biodiversity structure of wetland ecosystem. It is concluded that the ‘invasive’ status of hydrophytes establishing in the accidentally developed urban wetland ecosystem could be reconsidered.