Justification:
Most Bengal tigers are found in India, which has recently revamped its national tiger census methodology to be more scientific, extrapolating site-specific densities derived from camera trap and sign surveys using GIS. Based on the results, the national tiger population of India is estimated at 1,411 (range 1,165-1,657) adult/sub-adult tigers (Jhala et al. 2008). The previous census result of 3,642 in 2001-2002 cannot be directly compared, as a different and somewhat discredited methodology was employed (attempting to identify individual tigers on the basis of unique pugmarks, or tracks). Nonetheless, a substantial decline in India's tiger population is suggested.
Other Bengal tiger population estimates include Bangladesh 200 (Khan 2004) - 419 (MoEF, 2004); Nepal 350-370 (GTF 2007), and Bhutan 67-81 (Sangay and Wangchuk, 2005). The total subspecies population is estimated at fewer than 2,500 tigers (1,782-2,527).
In terms of conserving the wild tiger's genetic biodiversity, population biologists prefer to work with a number that approximates the actual breeding population, the number of animals which raise offspring to reproductive adulthood. This is the concept of effective population size, the size of an ideal population (in which every animal reproduces itself) which maintains the same genetic diversity as the real population, and is equivalent to the number of breeding animals per generation. The tiger's breeding population has been estimated at just 40% of the actual adult population, based on long-term demographic studies in Nepal's Chitwan National Park (Smith and McDougal, 1991).
None of the Tiger Conservation Landscapes within the Bengal's tiger range is large enough to support an effective population size of 250.
Sanderson et al. (2006) built upon previous work by Dinerstein et al. (1997) to map priority landscapes for tigers (Tiger Conservation Landscapes). The tiger's extent of occupied area is estimated at less than 1,184,911 km² (Sanderson et al., 2006), a 41% decline from the area estimated by Dinerstein et al. (1997). India suffered the most range contraction. While part of the difference is due to improved data after a decade of intensive tiger conservation efforts, and improved datasets and techniques, biologists consider the primary cause to be declines due to poaching and habitat loss (Dinerstein et al. 2007). Range decline is considered a strong indicator of population decline (Dinerstein et al, 2007). Because an average of 55% of Tiger Conservation Landscapes consist of non-tiger habitat (Sanderson et al., 2006), the declines in population and area of occupancy are greater than the 41% estimated, and thus likely indicate a 50% or greater reduction. A similar reduction could be expected over the next three tiger generations (20-30 years) unless conservation effort become more effective.
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Decreasing
