Comparative study of virgin and reclaimed islands of Sundarban mangrove ecosystem through network analysis

摘要

Sundarban mangrove estuarine ecosystem is one of the best-known detritus-based ecosystems of the world. Mangroves are very specialized ecosystems found at the interface between land and sea. Litterfall of mangroves supplies the detritus, nutrients and regulates the productivity of adjacent Hooghly-Brahmaputra estuarine complex and act as an important nursery ground for many commercially important shell and fin fishes. Presently the mangrove ecosystem of Sundarban is under serious threat for different anthropogenic activities. Many islands of this ecosystem are either totally reclaimed, or partially reclaimed, but about 30% of the islands are still virgin. Two islands are selected, one is from virgin forest and another is from reclaimed part for comparative study of benthic food webs of these two islands by using network analysis for possible human impacts. Selected island with virgin forest is almost free from human interference however the island with reclaimed forest fully disturbed due to almost all sorts of anthropogenic stresses. The results demonstrate a dramatic difference between these two islands. Virgin ecosystem is dominantly controlled by detritus, supplied from the litterfall of mangroves. Unlike the most benthic system the bottom community of reclaimed island receives a large contribution from the phytoplankton populations. Detritivory and herbivory ratio is markedly varied in these two systems, about 1:12 in virgin ecosystem and almost 1:1 in reclaimed system. The number of pathways of recycle can be identified much higher in undisturbed system in comparison with that of the reclaimed. Finn cycling is also very low in disturbed part. Litterfall comprises only 16% in reclaimed island where as in virgin island it is about 70%. Pathway redundancy is rather high in disturbed system, indicating the surviving system is probably highly resilient to further perturbation, as one might expect for highly impacted system. However, in virgin forest the ascendancy value is much higher than the redundancy, showing the system is healthy and almost free from any anthropogenic stress.