Guest Essay by Kip Hansen

 

The Big Cats — lions, tigers, panthers, cougars, leopards —  have always fascinated and frightened mankind.  Tales of tigers and lions — once they have gotten a taste for human flesh — terrorizing villages as they search for another human to eat abound along with tales of the famous hunters hired to kill the man-eaters.  Val Kilmer and Michael Douglas starred in a 1996 movie “The Ghost and the Darkness” which fictionalizes the real story of the Tsavo Man-Eaters — two lions that brought the building of the Kenya-Uganda Railway to a jarring halt as workers became too frightened to work as the lions took man after man from their tents at night,  dragging them off and devouring them.

A female Bengal tiger named Thak man-eater was finally shot and killed in November of 1938 by the world-famous “great white hunter” Jim Corbett who, although colonel in the British Indian Army, was often asked by the government to track down and kill man-eating tigers and panthers that roamed the provinces of  Uttar Pradesh and Uttarakhand, which are in northern India up against the Himalayas.  Overall, big-game hunter and author Jim Corbett, hunted down 33 man-eaters responsible for well over 1,000 deaths. Nobel-prize winner Rudyard Kipling wrote books that featured man-eating tigers and panthers his stories, like The Jungle Book.

Even today, tigers killing people “can lead to intolerance of Tigers by neighbouring communities” which “presents an ongoing challenge to managers to build local support for Tiger conservation and can lead to high rates of retaliatory killing of Tigers. In some areas there have been many human deaths – for example, about 40 people were killed by Tigers in the Sundarbans mangrove forest of Bangladesh and India 2000-2010 (Barlow et al. 2013).” [ source IUCN Redlist ]

It is of the tigers of the Sundarbans mangrove forest of Bangladesh and India that I write today.   It is estimated that 440 tigers live in the Sundarbans Mangrove forests of southwestern Bangladesh and west into India.

The Sundarbans “is a mangrove area in the delta formed by the confluence of Ganges, Brahmaputra and Meghna Rivers in the Bay of Bengal. It spans from the Hooghly River in India‘s state of West Bengal to the Baleswar River in Bangladesh. It comprises closed and open mangrove forests, agriculturally used land, mudflats and barren land, and is intersected by multiple tidal streams and channels. Four protected areas in the Sundarbans are enlisted as UNESCO World Heritage Sites, viz Sundarbans National Park, Sundarbans West, Sundarbans South and Sundarbans East Wildlife Sanctuaries.  ^…. The Sundarbans mudflats (Banerjee, 1998) are found at the estuary and on the deltaic islands where low velocity of river and tidal current occurs. The flats are exposed in low tides and submerged in high tides, thus being changed morphologically even in one tidal cycle. The tides are so large that approximately one third of the land disappears and reappears every day.  The interior parts of the mudflats serve as a perfect home for mangroves.” [ Wiki ].  Much of Bangladesh is similarly affected by its overall nature as a massive delta.  A little closer up they look like this:

The Sundarbans mangrove forest covers an area of about 10,000 km² (3,900 sq mi).  Despite mostly being protected lands and nature reserves, the Sundarbans has a population of over 4 million but much of it is mostly free of permanent human habitation. While the math works out to 400 humans per km², the human population crowds into villages and cities leaving most of the forest unoccupied permanently.  People go into the forest to harvest wild foods and wood, both for building and firewood for cooking.  The people of Bangladesh and this part of India are often profoundly poor thus there is quite a bit of poaching in the protected areas.  It is when people go into the forests to collect wild foods, fire wood, or to poach that some of them fall prey to tigers.

Tigers have been in the news recently:

CNN — Bengal tigers could vanish from one of their final strongholds

Washington Post —  More tigers now live in cages than in the wild.

NY Times — Bengal Tigers May Not Survive Climate Change

The CNN and the NY Times reports are based on a study done by Sharif A. Mukul and a group of Bangladeshi and Australian scientists. Published in Science of the Total Environment, the papers is titled “Combined effects of climate change and sea-level rise project dramatic habitat loss of the globally endangered Bengal tiger in the Bangladesh Sundarbans”.

This paper makes the following claim:

“Our model predicts that due to the combined effect of climate change and sea-level rise, there will be no suitable Bengal tiger habitat remaining in the Sundarbans by 2070.”  “Our study clearly indicates a rapid decline in the Bengal tiger population and suitable tiger habitats in the Sundarbans area by 2050, and a complete loss of this species by 2070.”

And recommends:

“Enhancing terrestrial protected area coverage, regular monitoring, law enforcement, awareness-building among local residents [are] among the key strategies needed to ensure long-term survival and conservation of the Bengal tiger in the Bangladesh Sundarbans.”

The authors boldly predict that there will be no suitable habitat for tigers in the Sundarbans by 2070 — stating blandly that the tigers will go locally extinct.  Given that prediction, their recommendations for remediation are unlikely to result in long-term survival of the tigers in this ecological niche.   Readers will note that the logic involved in the finding and recommendation leave something to be desired.

The study and its recommendations have to be viewed against the backdrop of history:  Bengal tigers have inhabited the Sundarbans for at least 16,000 years — surviving droughts, failed monsoons, horrific cyclones (one in recent history that killed 300,000 to 500,000 Bangladeshi humans), and continuous hunting of tigers by  the local human population.

So how did they come to this prediction of extinction?  Easy — they used a model.

The model is called Maxent  — a machine-learning technique called maximum entropy modeling.   It is an open source product originally developed at the American Museum of Natural History (AMNH).  It runs in Java on your Windows computer and is available free here.  The Maxent homepage (same link) offers links to the latest “raw ipcc data, as dowloaded from the IPCC Data Distribution Centre.  Also the annual and monthly variables used for the study, extracted from the raw ipcc data and converted into world-wide coverages in .asc format.”

Basically, for non-modelers, the users input the locations (GPS coordinates) of where a species is currently found.  Then, from the Maxent page: “Species models are determined from a set of environmental or climate layers (or “coverages”) for a set of grid cells in a landscape, together with a set of sample locations where the species has been observed.  The model expresses the suitability of each grid cell as a function of the environmental variables at that grid cell.  A high value of the function at a particular grid cell indicates that the grid cell is predicted to have suitable conditions for that species.”

An immediate problem with this approach in this instance — tigers in the Sundarbans — is that the Sundarbans have very little climatic variation from one point to another — there is almost zero change in elevation, zero differences in temperature ranges, etc.   Secondly, Bengal tigers previously occupied almost all of the Indian continent as suitable habitat and have been reduced to their redoubt in the Sundarbans not through climatic changes but through the pressures of burgeoning human population.

The study authors point to Catherine H. Graham and Robert J. Hijmans’ 2006 paper as an example for Maxent use.  Unfortunately, our tiger authors also decided that the bio- and eco- system “parameters” used by Graham and Hijmans in their study were “biologically more meaningful to define the eco-physiological tolerances of most species”.  This is unfortunate as they limited the parameters to  the nine bioclimatic variables used in Graham (2006).  Why is this an error?  Graham was modeling “herpetological fauna of California” — herpts (reptiles and amphibians) usually have narrow ecological niches and California has almost every type of eco-region — deserts below sea level with extremely high temperatures, the High Sierra which includes the highest point in the 48 contiguous states (Mt. Whitney), coastal redwood rainforests, well, the list goes on.  Graham and Hijmans selected a limited set of bioclimatic variables that would best describe the limiting features for snakes, lizards, frogs, toads, newts, turtles, etc., variables that have very wide ranges in California.  Here’s the list, used in the tiger study because it was used in Graham (2006):

A reasonable set of choices for cold-blooded herpts, but somehow probably not the limiting factors for tigers.  Further, it is difficult to believe that these variables actually vary that much over the small area represented in the Sundarbans mangrove forests of Bangladesh and India.

A great deal of the land surface in the Sundarbans is tidal in nature — submerged at high tide and exposed at low tide.  Very little is more than a meter or so above the highest tides.  This means that sea level rise (SLR) — the kind that RCP6.0 and RCP8.5 project for 2050 and 2070 — will have a very large impact on this delta area.  If those kinds of SLR do come to pass, the Sundarbans will be reduced in size by 50% or so — crowding the tigers and their natural prey into a smaller area.

But this paper states in its conclusion “We have found that climate change always had a greater effect than that of only sea level rise alone on suitable Bengal tiger habitats.”  How so?  They don’t say — that is, they offer no biologically plausible explanations for their conclusion.  They simply say, in effect, “because the model says so (after 500 iterations).” Remember, the model has been established with the bioclimatic parameters for reptiles and amphibians.

Let’s try something a little more real.  Where do tigers live today?

It is clear that tigers historically (the tan background color) have lived all throughout Asia.  Today, they live wherever they are not being hunted and killed — either in retribution for preying on the livestock of humans or preying on the humans themselves or for the never ending Chinese Folk Medicine market, which demands tiger parts of all sorts.  Tigers have very few restrictions for habitat as long as there is adequate prey.

 

Bottom Line:

1. The Bengal Tiger is threatened — just not by climate change. Sea Level Rise may eventually (100-300 years) push the tigers of the Sundarbans to higher land to the north, where human-animal conflict will ramp up — to the detriment of the tigers.  The real threats for these tigers stem from human-animal conflict (tigers attacking and eating people) leading to retribution tiger killings and from human-animal competition for the same prey — deer and other forest animals.
   

2. The tiger study by Mukul et al. is flawed and has arrived at a scientifically unsupportable, biologically implausible conclusion.
   

3. The IPCC bioclimatic projections under RCP8.5 have been found to be extremely unlikely to impossible and should not have been used to project the future survival of Sundarbans tigers.
   

4. Tigers will continue to live and thrive wherever their human neighbors cease killing them and their natural prey, and where enough of their natural habitat is allowed to remain unharmed by human development.
   

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Author’s Comment Policy:

Readers interested in this topic should read the Mukul et al. paper in its entirety to see just how this study failed. [If you are unable to access a full text copy, email me — my first name at i4 dot net].   It is a good example of several scientific errors that are rampant in science today — and not just CliSci.  The authors depend on a previous study, Graham and Hijamns (2006) which they don’t seem to have understood yet they follow its lead in using Maxent and IPCC bioclimatic parameters inappropriate for their Sundarbans tigers study. They don’t seem to have considered the biological plausibility of their findings —  what future bioclimatic changes would harm the tigers?  How would it harm them? Also,  I very much doubt that Maxent can be used in any sensible manner over such a small and homogenous geophysical area.

 

Let us hope that the Conservation agencies of India and Bangladesh don’t read this paper and decide that they should do nothing for the Sundarbans tigers because they are all going to be extinct in 50 years.

 

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