A comparative assessment of baobab density in northern Mana Pools National Park, Zimbabwe

Abstract

Ecological effects of elephant (Loxodonta africana) have emerged a contentious issue considering their impact on woodlands in protected areas in sub-Saharan Africa (Kassa et al., 2013; Staub, Binford & Stevens, 2013). Hence, there is a need to establish how these impacts are distributed over space and time. Such spatial and temporal ecological studies could guide policy and decisions related to elephant and vegetation management in protected areas. Previous studies highlighted that the spatial distribution of baobabs in African savannah is site specific and influenced by a complex of establishment factors, such as herbivory (Mashapa et al., 2014), droughts (De Smedt et al., 2012), human activities (O’Connor & Campbell, 1986), climate change (Sanchez, Osborne & Haq, 2011) and/or soil type (Mashapa et al., 2013). Interestingly, a significant mortality of baobabs in northern Mana Pools National Park, Zimbabwe (hereafter, northern Mana Pools), was attributed to elephant damage between 1984 and 1988 (Swanepoel & Swanepoel, 1986; Swanepoel, 1993). Swanepoel (1993) recorded an annual baobab mortality rate of 7.3% and postulated that by the early 2000s, the baobab population would have halved in northern Mana Pools. Therefore, the aim of this study was to compare baobab density and the annual rate of baobab mortality in northern Mana Pools between 1984 and 2005, in relation to the baseline study by Swanepoel & Swanepoel (1986). Materials and methods Mana Pools National Park (2196 km) is located in midZambezi valley region, north of Zimbabwe, between latitudes 15°400 and 16°200S and longitudes 29°080 and 29°450 (ZPWMA, 2011). It was designated a United Nations Educational, Scientific and Cultural Organization (UNESCO) World Heritage Site in 1984 (ZPWMA, 2011). Colophospermum mopane woodland is the most extensive vegetation type in northern Mana Pools, and the climate is classified as semi-arid (ZPWMA, 2011). Data were collected following the baseline study method of Swanepoel & Swanepoel (1986) on the same belt transects to facilitate direct comparison with the previous baobab survey of 1984. In the 1984 survey, 124 baobabs were marked (Swanepoel & Swanepoel, 1986) and these were searched to reevaluate their status in April 2005. Variables that were recorded included: baobab density, basal circumference, level of elephant induced damage on baobabs and baobab plant status (dead/alive). Comparisons of baobab density (per km) between 1984 and 2005 in northern Mana Pools were carried out using Mann–Whitney U-tests in STATISTICA version 7 for Windows (StatSoft, 2004). Annual baobab mortality rate was calculated from a direct measurement in accordance to Krebs (1994); thus, direct measurement was achieved by observing how many previously marked baobab individuals had survived through time, from 1984 to 2005 in northern Mana Pools. Baobabs that were not found were assumed dead.