Recently some leading newspapers of Bangladesh shared research findings that the habitats of Royal Bengal Tiger will disappear from the Sundarbans mangrove forest by the year 2070. This research was conducted by researchers from Bangladesh, New Zealand, and Australia, and recently published in the ‘Science of the Total Environment’ scientific journal. The researchers used the simulation modelling approach for this study.
Simulation modelling embodies a set of mathematical equations. These mathematical equations are based on a set of assumptions which are developed from quantitative understanding. For example, if we want to know the growth of an animal’s body based on the amount of food it consumes, we can first collect data (food quantity and body weight) for example of 50 animals. Then, we can build a mathematical model based on the data of those 50 animals, and by using this model we can predict the body weight of other animals by their food consumption.
A mathematical model needs to be biologically meaningful and statistically robust. What I meant by biologically meaningful: let’s assume that we want to know whether the geographic distribution of cattle of the Dhaka district is affected by the temperature. For that, we do a survey on the number of cattle available across different regions of Dhaka and we record the temperature of each region. We know that we will have fewer cattle in city areas (Gulshan, Banani, etc) relative to rural areas, and the temperature in cities will be higher because of the pollution. Now, if we build a model based on this data we will find that an increase in temperature decreased the number of cattle in the Dhaka.
Is this result biologically sound? Can we say that our finding is correct? No, the cattle distribution is not controlled by the temperature, but by the availability of food and shelter, and the availability of food and shelter is lower in Dhaka city area than in the rural areas of the district.
The researchers used micro-climatic variables such as temperature and precipitation to characterise the habitat of the Royal Bengal Tiger. This is inappropriate. The abundance of the tiger in the Sundarbans is not controlled by the temperature or by the precipitation. We do not have any experimental or field-level knowledge to explain the relationship between the temperature and precipitation of the climate and the tiger’s abundance in the Sundarbans. If we travel from north to south or east to west in the Sundarbans, we will not find significant differences in the temperature or precipitation. How could we predict that the tiger abundance will decrease with increasing temperature when we do not know the relationship between temperature and the habitat of the tiger population? The variables listed in table 1 of the paper are all micro-climatic variables, and no quantitative understanding exists on how these variables are associated with the habitat of the Bengal Tiger. The findings are overly speculative and misleading.
The second point for modelling is that a model needs to be statistically robust. For this, the researchers either need to evaluate the model performance by looking at the percentage of the error that the model explains or need to validate the model outcomes by collecting data from the field. The researchers did not perform any of those two approaches and did not provided any discussion of them in the paper.
The other approach that the researchers used is the prediction of habitat availability by the sea level rise. It seems that this assumption is built from understanding that the Sundarbans will sink with the expected sea level rise. This is another overly speculative assumption. It is important to acknowledge and discuss the mechanisms by which land forms in the Sundarbans mangrove forest. It is from sedimentation followed by delta formation. The Sundarbans is located at the mouth of several rivers. It is an area of high sedimentation, and it is likely that the mangrove soil surface will rise due to increased sedimentation with the rise of sea-level. However, this is another area where we still lack quantitative knowledge.
The Sundarbans mangrove is pride of the Bangladesh and we need to be critical and reasonable prior to making any broad speculations regarding its future.