Decreased ungulate density in Bardiya National Park, West Nepal, and the implications for increasing tiger populations. A comment on Thapa et al. (2015)

Globally, tiger (Panthera tigris) populations are decreasing and at risk of extinction; the population of breeding adult tigers in protected areas has declined by more than 20% in the past 14 years to a global baseline of approximately 3200 free-ranging individuals in 2009 (Goodrich et al. 2015). Poaching, habitat loss and prey depletion are the major threats to sustain genetically viable tiger populations in the wild, leading to the status of ‘endangered’ (A2bcd+C1 ver 3.1) on the International Union for Conservation of Nature (IUCN) Red List (Goodrich et al. 2015). In 2010, 13 tiger range countries set the goal to double the current tiger populations by 2022 (GTI 2010). Nepal is one of these countries and aims for a tiger population of 250 individuals by 2022. Recently, a debate has started on the question of whether or not it is possible to realise this goal. Aryal et al. (2015) calculated the carrying capacity for tigers of Nepalese protected areas. Based on the surface area of the current protected areas in Nepal, they conclude that the doubling of tiger numbers is too ambitious. In a reply to Aryal et al. (2015), Thapa et al. (2015) concluded that over 250 tigers can be supported by the currently available tiger habitats in Nepal, based on the following arguments: (1) tigers do not have mutually exclusive home ranges and are territorial; (2) territories of male and female tigers overlap, and thus, tiger carrying capacity should be calculated only on the basis of female territory size; (3) 18% of tigers (e.g. sub-adults) are transient and without a territory, resulting in potentially higher densities than used in the calculations of Aryal et al. (2015). Thapa et al. (2015) also implicitly assume that tigers in Nepal have not yet reached carrying capacity after a period of heavy poaching, and conclude that an average tiger territory size of 20 km would be reasonable. They compare predicted maximum densities (Karanth et al. 2004) and actual densities measured in Chitwan National Park (Carbone et al. 2001) to show that, in the presence of abundant prey, tiger densities can be as high as 16 individuals per 100 km in highly productive alluvial grasslands. However, not all protected areas in Nepal consist of productive alluvial grassland, and the average density over different habitat types of 1 tiger per 20 km observed in Chitwan is unlikely to be representative for all protected areas in Nepal. In the largest protected tiger habitat of Nepal, Bardiya National Park (BNP) sal forest outside the floodplain makes up 70% of the surface area of the park. Here, we argue that (1) the sal forest is abandoned by most prey animals during the warm part of the dry season; (2) this seasonal variation in habitat quality reduces the carrying capacity as calculated by Aryal et al. (2015) and Thapa et al. (2015); (3) tiger numbers in BNP have been more or less stable over the last couple of years, indicating that the Ethology Ecology & Evolution, 2017 Vol. 29, No. 3, 304–309, http://dx.doi. org/10.1080/03949370.2016. 1145147