Demography and viability of the largest population of greater one-horned rhinoceros in Nepal

Megaherbivores are characterized by slow life history traits which when coupled with human exploitation makes them vulnerable to local extinctions. An understanding of key demographic parameters assists in guiding management interventions to ensure their recovery and persistence over the longer term. We monitored 110 (30 calves, 80 young and adults) individually known greater one-horned rhinoceros (Rhinoceros unicornis) for seven years in Chitwan National Park, Nepal (2009-15). Using known fate model with staggered entry design in program MARK we estimated annual calf survival at 0.765 ± 0.026 SE and that of remaining older age groups between 0.96 and 0.985. Both genders exhibited a typical Type I survivorship curve. The population consisted of 62% adults, 13% sub-adults and 26% juveniles and calves (dependent animals). The adult sex ratio (female: male) was 1.23 ± 0.09 SE and dependent: cow ratio was 0.636 ± 0.03 SE. Age at first calving was 7.91 years ± 0.31 SE. Shorter inter-calving intervals were observed for young adults compared to old adults. Overall inter-calving interval was 41.28 months ± 2.33 SE. Chitwan rhino population grew at a maximum realized rate of r = 0.051 ± 0.005 SE. PHVA results showed that low level continuous poaching increased extinction probability compared to high but intermittent poaching episodes. An increase in annual poaching of over six females and 12 males over the current average of 5.5 rhinos per year, coupled with habitat degradation, caused by the alien invasive Mikania micrantha resulted in high extinction risks. Annually upto 13 rhinos (8 males and 5 females) from Chitwan can be used for reintroduction and supplementation of rhinoceros across their current and historic range.