IUCN Guidelines (IUCN 2006) define population as the number of mature individuals, defined as “individuals known, estimated or inferred to be capable of reproduction.” While in general this refers to all reproductive-age adults in the population, the Guidelines also “stress that the intention of the definition of mature individuals is to allow the estimate of the number of mature individuals to take account of all the factors that may make a taxon more vulnerable than otherwise might be expected.” Two factors which increase the tiger's vulnerability to extinction are their low densities (relative to other mammals, including their prey species) and relatively low recruitment rates (where few animals raise offspring which survive to join the breeding population) (Smith and McDougal 1991, Kerley et al. 2003). Low densities means that relatively large areas are required for conservation of viable populations; it has long been recognized that many protected areas are too small to conserve viable tiger populations (Nowell and Jackson 1996, Dinerstein et al. 1997, Sanderson et al. 2006). Low recruitment rates also require larger populations and larger areas to conserve viable populations, as well as mortality reduction in non-protected areas to maintain population size through connectivity (Carroll and Miquelle 2006). High mortality rates can be offset by an abundant prey base (Karanth et al. 2006), but prey base depletion was considered a leading threat to tigers across much of their range (Sanderson et al. 2006). The IUCN Guidelines advise that “mature individuals that will never produce new recruits should not be counted.” Low recruitment rates indicate that fewer adults than would be expected produce new recruits. Defining population size as the total estimated number of reproductive age adults in the taxon would also not take into account that many occur in subpopulations which are too small or too threatened for long-term viability. Instead, the number of mature individuals is defined as equivalent to the estimated effective population size.

Effective population size (Ne) is an estimator of the genetic size of the population, and is generally considered representative of the proportion of the total adult population (N) which reproduces itself through offspring which themselves survive and reproduce. Ne is usually smaller than N, as has been documented for the tiger. The effective population size of tigers in Nepal’s Chitwan National Park was equivalent to just 40% of the actual adult population (Smith and McDougal 1991). Therefore, the number of viable mature Sumatran tigers is projected to be 40% of the total estimated population, in the range of 176 - 271 (based on the detailed figures given above), with no subpopulation having an effective population size larger than 50, following the precautionary principle in selecting the lower bound subpopulation sizes for Kerinci Seblat, Gunung Leuser and Bukit Tigapuluh.

The Sumatran tiger is declining due to high rates of habitat loss (3.2-5.9%/yr; Achard et al. 2002, FWI/GFW 2001, Uryu et al. 2007) and fragmentation, which also occur, to a lesser extent, inside protected areas (Gaveau et al. 2007, Kinnaird et al. 2003, Linkie et al. 2003, 2004, 2006). There are high levels of human-tiger conflict (Nyhus and Tilson 2004, Browne and Martyr 2007), as well as illegal trade in tiger parts (Nowell 2000, Nowell 2007). From 1998-2002 at least 51 tigers per year were killed, with 76% for purposes of trade and 15% out of human-tiger conflict (Shepherd and Magnus 2004). Ng and Nemora (2007) found the parts of at least 23 tigers for sale in market surveys around the island.