Population dynamics and feeding ecology of the black Rhino (diceros bicornis) in Ngorongoro conservation area, Tanzania

Abstract

Africa was once known for abundant populations of the now Critically Endangered Black rhino (Diceros bicornis). However, large-scale poaching in many parts of this continent during 1970- 1980 led to a 95% decline in the rhino numbers. Like in other countries that host remnant populations of the Black rhino, Tanzania’s Black rhinos are now largely restricted to protected areas such as the Ngorongoro Conservation Area (NCA). This study sought to map the spatial distribution of the Black rhino population in the Ngorongoro Crater (NC), assess the temporal trends in selected population attributes (size, growth rate, sex ratio and age-structure) over the past 15 years and evaluate food selection by the Black rhino. Black rhino population trends and spatial distribution were assessed using records obtained from NCA and ranger posts. Ecological surveys were conducted at Black rhino foraging sites to assess food availability and diet selection during wet and dry seasons. There was a steady growth in population of Black rhino from the initial 14 in 2000 to 44 in 2014. The highest growth rate (1.3 individuals /year) occurred in 2005, while the lowest growth rates (0 individuals) occurred in year 2006, 2011 and 2013. The sex ratios were skewed in favor of females (2:1) and approximately constant for the entire 15-year period. Spatial distribution patterns varied across seasons, with rhinos occupying the crater floor during the wet season and crater walls and rims during the dry season. In addition, the spatial distribution patterns were influenced by anthropogenic activities; rhinos avoided areas proximate to busy roads, lodges and livestock. Grasses and forbs occurred in nearly equal proportions at the Black rhino feeding sites, constituting 50.4% and 49.6%, respectively. During the wet season, rhinos mostly selected Commelina banagalensis, Amaranthus hybridus, Gutenbergia cordifolia, Justicia betonica and Lippia ukambensis while in dry season; Hibiscus aponeurus, Justicia betonica and A. xanthophloea. There were significant differences in forage items utilized in each season. A. hybridus (p=0.001) were more utilized in wet than dry season as well as C. bengalensis (p=0.008), E. arabicum (p=0.001), A. longiscupsis (p=0.019) and G. cordifolia (p=0.010). These results recommend that human activities such as livestock grazing, tourism infrastructure etc. should be minimized in crater to avoid disruption of rhino movement and habitat selection patterns. As well, it is vital to maintain the monitoring regime, and possibly improve the ratio of ranger to rhinos or area of patrol to enhance effective monitoring and management.