Monday, December 8, 2014

The Habitat Loss Club: Rise in Sea Level Threatens to Wipe Out Only Mangrove-Dwelling Tiger Population in the World

What is the most significant shared characteristic between polar bears and tigers? If you’re thinking of massive canines, try again. The answer, unfortunately, is rapid habitat loss. Just as melting ice caps are robbing polar bears of their land, anthropogenic factors have directly and indirectly been fragmenting tiger habitats in all their ranges. In the past couple of decades, the Indian and Bangladeshi governments – both countries containing a significant proportion of the remaining tigers in the world – have taken strict action to constrain direct anthropogenic influences on tiger population. Anti-poaching laws, the establishment of tiger reserves, and relocation of villages to minimize human-tiger conflict are some of the actions taken by the Indian government under what was called Project Tiger.

While such actions have helped stabilize tiger populations in the subcontinent, they have only addressed direct forms of anthropogenic threats to tiger populations. Indirect forms of anthropogenic influences such as habitat loss through climate change, although slower, pose just as big of a threat. In their 2010 article, “Sea level rise and tigers: predicted impacts to Bangladesh’s Sundarbans mangroves”, Loucks examine the potential rise in sea levels caused by climate change and its effects on the Bangladeshi portion of the Sundarbans mangrove forest, home of the only mangrove-dwelling tiger population in the world. The threat of sea level rises is especially big in the Sundarbans since it is extremely low-lying. Unlike previous sea level studies that have used large meter-scale measurements of sea level rises, Loucks and his colleagues, therefore, use a new sub-meter digital elevation model (DEM) which provides an extremely precise elevation model of the mangrove forest.  They create their continuous DEM from 80,584 GPS elevation points measured in mm above sea level. Subsequently, using 4 mm year-1 as a conservative estimate (based on the previous literature) of annual sea level rise and the year 2000 as the baseline, they predict the percentage of the Sundarbans that will go under at eight chosen sea level rise values (0, 4, 8, 12, 16, 20, 24, and 28 cm).

Their results, visualized in Figure 2, predict a 96% loss in tiger habitat by the time a 28 cm rise in sea level occurs. Assuming that current sea level rise rates are maintained, that would give us only 50-90 years. Furthermore, using average tiger home range areas, they estimate the breeding population to decline down to less than 20 individuals by the time of this 28cm sea level rise. What’s most upsetting, however, is the fact that Loucks and his team’s estimation is based on average sea level rises (which can be exceeded) and the assumption that no forces other than sea level rise are at work to threaten tiger populations in the Sundarbans (which is not true). So really, this is just a conservative estimate!

Loucks et al. (2010) demonstrates the very real threat that slow-acting, macro-level anthropogenic factors like climate change pose to natural ecosystems. It is especially worrisome since mangrove ecosystems are themselves one of the most biologically productive ecosystems in the world, having environmental and economic implications. The ecosystem services they provide include protection from cyclones, food and building supplies, fisheries, and carbon cycling in addition to hosting a wide range of biodiversity. Since the fates of tigers and mangroves are so intrinsically tied to each other and both so important, as Loucks et al. point out, I think investigations should be made into ways in which tiger conservation and mangrove conservation/restoration efforts can be linked, not just in the Sundarbans, but globally. 

Work(s) Cited:

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


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