Long-term impacts of goat browsing on bush-clump dynamics in a semi-arid subtropical savanna

Abstract

The effects of 16 years of continuous browsing by goats in a South African savanna at stocking rates intended for bush control were compared with plots unbrowsed for the same period of time. Differences in bush-clump density, structure and species composition were recorded. Bush-clump density did not differ between browsed and unbrowsed plots. Within individual bush-clumps, browsing was shown to impact more on structure than species composition, with smaller, shorter bush-clumps, containing fewer species but much greater stem-densities. Although species presence/absence was little affected by browsing, many species showed differences in abundance, growth and location within browsed and unbrowsed bush-clumps. Species reduced in abundance in browsed plots included Cussonia spicata, Ehretia rigida, Grewia occidentalis, Jasminum angulare and Senecio linifolius. Several species showed reduced growth in browsed plots, particularly those located at bush-clump edges. The relatively unpreferred Aloe ferox was a notable exception. Although browsing had little effect on the composition of the main clump founding species, emergents or late arrivals, there were twice as many single plants in browsed plots and emergence of several species was restricted to the middle of bush-clumps. Comparison of our findings with aerial photographic evidence and other literature suggest that browsing alone is unlikely to significantly reduce scrub cover, although it can clearly control further expansion. Combinations of fire and browsing, rather than one factor alone, are considered likely to act fastest and most effectively to significantly reduce or remove scrub cover altogether.

Can heavy grazing on communal land elevate plant species richness levels in the Grassland Biome of South Africa?

“…However, species such as P. globosa which is widespread in the Nama-Karoo and the more arid parts of the Grassland Biome, increased significantly and by a much greater magnitude on HU. The heavy impact of goat grazing on A. karroo agrees with earlier observations in the Eastern Cape (Hester et al 2006). A. karroo is particularly resilient to browsing by goats.…”

Bush clump succession in grassland in the Kei Road region of the Eastern Cape, South Africa

“…Its dominance within a clump interior is attributed to continuing clump expansion. Although a palatable shrub species, it is tolerant of defoliation (Hester et al 2006). It was expected that other shrub species would also be best positioned on a clump's periphery in order to gain adequate irradiance, but only one species showed a non-random preference for the periphery (Table 3).…”

“…This should promote forestscrub species, which are well recorded to be fire intolerant (Westfall et al 1983, Titshall et al 2000. Individuals growing within the interior of a clump are protected from browsing (Hester et al 2006). This is expected to benefit mainly palatable browse species such as Cussonia spicata.…”

“…Traditional management approaches of mechanical or chemical clearing also require subsequent use of fire and browsing to ensure long-term effectiveness (Trollope et al 1989). Even browsing on its own may contain clump expansion (Watson 1999, Hester et al 2006. Preventing seedling regeneration of A. karroo would appear difficult (Story 1952, O'Connor 1995 unless fire is used regularly.…”

Defoliation depletes the carbohydrate reserves of resprouting Acacia saplings in an African savanna

“…However, although A. karroo is tolerant of frequent fires, it is susceptible to browsing, both by native browsers such as giraffe (Bond and Loffell 2001) and smaller antelope (Staver et al 2009) and by domesticated browsers (Scogings and Macanda 2005). Thus, it has been suggested that frequent fires in combination with browsing might control bush encroachment and thickening by A. karroo more effectively than fire alone (Trollope 1984;Hester et al 2006). More generally, recent models of African savanna have confirmed that the combination of browsing and fire are more effective at reducing tree density than either browsing or fire alone (Holdo et al 2009;Staver et al 2009).…”