Impacts Of Large Herbivores On Vegetation And Soils Around Water Points In Waterberg Plateau Park, Central Namibia

ABSTRACT

Provision of artificial water points to large herbivores leads to range deterioration and degradation, especially in semi-arid and arid ecosystems. This is a cause for concern in both protected and private lands in Namibia and elsewhere. Therefore, the objective of this study was to determine the impact of large herbivores on soils and vegetation around water points in the Waterberg Plateau Park (WPP). Five artificial water points were selected for the study. Four line-transects starting as close as possible to, and radiating away from the water points were surveyed at each water point, and five 20 m x 20 m plots at distances 0 m, 100 m, 200 m, 300 m and 500 m from the water, on each transect were used for sampling trees and soils. The nested plot design was used for sampling: 1 m x 1 m plots in which grasses and forbs were sampled, were nested within 5 m x 5 m plots used for sampling shrubs and saplings, which were nested within the 20 m x 20 m plots. All trees were identified, counted and their heights as well as basal circumferences were measured. All shrubs and saplings were identified, counted and their heights measured. Woody vegetation cover was estimated using the line-intercept method. Forbs and grasses were identified and their total cover was visually estimated. Forbs were also counted. Density was presented as individual trees, shrubs and saplings, and forbs per 400 m 2 , 25 m 2 and 1 m 2 , respectively. Basal area was calculated with the assumption that all stems were circular, using the formula: BA= c2 / (4*π ). Plant species diversity was calculated for each plot using the Shannon-Wienner diversity index. The results indicated significantly lower plant densities, cover, basal areas, diversity and richness close to water points, than further away from the water points. These were a result of high degradation by large herbivores through trampling and over-utilization of vegetation around water points. To determine basal area and height distributions, the basal area and height values at the different distances from the water points were arranged into classes. Height class and basal area class distributions generally portrayed a similar trend, with a high proportion of shrubs and saplings corresponding to a high proportion of plants with smaller basal areas. Stunting of woody vegetation through browsing and a reduction in fire frequencies and intensities through controlled burning may have contributed to the establishment and therefore the high frequency of small, short woody plants at the four distances further away from the water points. The HCA classified the vegetation plots into 10 main clusters based on species presence/absence data. The DCA indicated a complex interaction of gradients which influenced the patterns in species composition. The CCA indicated that soil phosphorus, CEC and clay accounted for significant variation in species composition. Phosphorus has been introduced into the soils by large herbivores through dung and urine, while clay content was lower due to the removal of vegetation around the water points by large herbivores. It was concluded that trampling and over-utilization of vegetation by large herbivores negatively influence vegetation structure, composition and diversity around water points in the WPP.