Sea Level Rise Expected to Flood Mangrove Forests in Bangladesh, Killing Native Tiger Populations by 2100

The Sundarbans mangrove ecosystem is recognized as a global priority for biodiversity conservation, housing the only tiger (Panthera tigris) population in the world adapted for life in mangrove forests. The mean elevation of most of the Sundarbans is less than one meter above sea level. Consequently, sea level rise (SLR) poses the single greatest climate change threat to the viability of the Sundarbans forests. Using scale-appropriate elevation data, Loucks et al. (2010) illustrate that the Sundarbans, and its biodiversity, is vulnerable to small increases in sea level.  Both tiger habitat and tiger populations will likely reach a critical threshold at SLR between 24 and 28 cm. At a 28 cm rise in sea level, the Sundarbans tiger population is unlikely to remain viable. The authors assert that a 28 cm sea level rise is likely to occur around 2070. If actions to both limit green house gas emissions and to increase resilience of the Sundarbans are not initiated soon, these tigers will become early victims of climate change-induced habitat loss.¾Michelle Schulte

Loucks, C., Barber-Meyer, S., Hossain, M.A.A., Barlow, A., Chowdhury, R.M., 2010. Sea level rise and tigers: Predicted impacts to Bangladesh’s Sundarbans mangroves. Climatic Change 98, 291­­–298.

Given that only a small portion of the land surface of the Sundarbans is over 1 m above sea level (asl), Loucks et al. estimated sea level rise in mm asl. They used 80,584 elevation points to create a continuous digital elevation model (DEM) with sub-meter accuracy.  The DEM was based on 1991 elevation data collected by FINNMAP. The authors used 4 mm year -1 as a conservative estimate of annual SLR upon which to predict potential impacts to tiger habitat. So as to account for the difference in sea level from 1991 to 2000, the authors factored in a 3.6 cm increase in sea level, equivalent to a SLR of 4 mm yr-1 for nine years. Next, for each of the time steps, the authors identified the land area that would fall below the rising elevation of the sea. This land area would be permanently underwater, and thus removed from the potential habitat layer. This analysis was repeated for each time step. Increasing sea level can cause both direct habitat loss as well as increased fragmentation resulting from new or expanded streams and channels.  To estimate tiger populations at each time step, the authors utilized a maximum dispersal distance of 5 km across water between potential tiger habitat patches. Second, they defined the minimum potential tiger habitat patch as being 10 km2.  To assess the potential range of tiger population for each time step, the authors factored in the relative tiger abundance of each patch of land, the average female tiger’s home range size, as well as the female:male ratio.
Both tiger habitat and tiger populations will likely reach a critical threshold at SLR at 24– 28 cm. At a 28 cm rise in sea level, the Sundarbans tiger population is unlikely to remain viable as the remaining habitat will have decreased to 3.8%, and the number of breeding individuals will be less than 20. Using a conservative rate of a 4 cm per decade increase, which is consistent with the 4th IPCC report on sea level rise and local tidal gauge records, the authors predict the Sundarbans will realize a 28 cm increase in sea level between 2060 and 2100. Uncertainty persists within the study because the authors only assessed tiger abundance in relation to habitat size. The biodiversity of the ecosystem relies on intricate and complex abiotic and biotic interactions. There is uncertainty regarding prey abundance and possible prey responses to SLR that could then alter tiger abundance. Furthermore, the study does not incorporate potential effects of geological processes, drainage, withdrawal of water, and sedimentation; factors which may reduce or increase the level of permanent inundation. In addition, there is likely a time lag from inundation to non-use of the area by tigers or their prey that was not accounted for.
Although there is considerable uncertainty regarding the degree of future habitat loss due to SLR, it is still imperative to act now to mitigate the potential habitat loss. Globally, action should include limits on carbon emissions to slow climate change. Locally, management activities that conserve habitat or limit threats include building dykes, developing and planting mangroves that can adapt to the rising seas and changing salinity, and limiting poaching or killing of tigers and their prey. 

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