The effect of equid bark stripping on Boscia albitrunca populations

MSc (Enviromental Sciences), North-West University, Potchefstroom Campus

Boscia albitrunca (Shepherd's Tree) is a protected tree providing important ecosystem services and functions within its distribution area. Populations of this species are under increasing herbivory pressure due to its rather unique function of providing evergreen foliage in drought-prone African savannas. Not only is B. albitrunca a preferred browse species to native wild herbivores, but also favoured by domesticated livestock, specifically equids, such as horses and donkeys with a preference for its nutritious bark when other forage resources are limited. The primary aim of this study was therefore to critically assess the effects of bark-stripping on B. albitrunca populations by three different equid species in the Mopane-Sand River area of the Limpopo Province. During the dry seasons of 2012-2014 it was observed that large numbers of B. albitrunca individuals were subjected to severe bark-stripping by free-ranging donkeys (Equus asinus africanus) that were kept in fenced-in areas or on communal rangelands. To a lesser extent, free-ranging horses (Equus caballus) also bark-stripped B. albitrunca. Burchell‘s Zebra (Equus quagga burchelli) was included in the study to discover what impact, if any, this species had on B. albitrunca trees. Based on the observed bark-stripping practices of donkeys and horses, it was hypothesized that: (i) the population structure and condition of B. albitrunca populations would vary significantly across land-use types that are exposed to different intensities of equid browsing and (ii) populations of B. albitrunca in areas exposed to donkey browsing would be unstable and characterised by severely damaged individuals. Boscia albitrunca populations were examined along 30 transects in five different land-use types which consisted of: (1) control areas that were exclusively exposed to local game species, (2) areas that hosted free-ranging donkeys with local game species, (3) enclosed camps in which donkeys were kept with local game species, (4) enclosed camps in which horses and local game species were kept and (5) enclosed areas in which zebras were kept with local game species. Population structure was critically evaluated through results obtained by measures of tree population densities, size-class distributions, proportions of single- to multi-trunked trees and population trends. Further analyses included tree height, diameter at breast height, lowest reachable foliage and abundance measures to quantify the effects of equid foraging type and intensity on the overall population structure and stability. An ‗Overall Population‘ Index was developed to present an overview of B. albitrunca population structure and stability for each land-use type. This study revealed significant variations in the population structure and stability of B. albitrunca populations across land-use types. Zebras, followed by donkeys, had the highest impact on B. albitrunca population structure and stability. The highest densities of B. albitrunca trees were recorded in the areas that were exposed to donkeys. Regeneration was healthier in the areas in which donkeys were fenced-in compared to areas hosting horses and free-ranging donkeys. However, the moderately steep positive regression slope displayed by the zebra-areas suggested B. albitrunca population declines under this particular land-use system. Approximately 15% of the sampled B. albitrunca individuals exhibited very poor tree condition, although mortality was not significant. Assessment of the damage effects on the various size classes across the land-use types revealed that B. albitrunca individuals are most susceptible to bark-stripping damage by donkeys and exhibit the least robust overall tree condition. However, larger trees (>45 cm diameter) were less affected. The highest impact on B. albitrunca population health was caused by bark-stripping by enclosed donkeys , followed, respectively, by free-ranging donkeys, horses and zebras. Despite the severity of bark damage imposed by donkeys and to a lesser degree by horses, these effects seem to have little effect on the overall population stability of B. albitrunca in the Mopane-Sand River area of the Limpopo Province, South Africa.

Masters