No abstract

The study described changes in floristic and vegetation structure in relation to livestock grazing intensity in a conservation area in the Succulent Karoo, South Africa. Grazing by goats and sheep is allowed in the Richtersveld National Park ͑a contractual National Park͒ which is also an area of high floristic richness and endemism. We used goat faecal pellet density, degree of trampling and percentage bare-ground at distances from the stock posts as surrogates for a gradient in grazing pressure. A stock post is the place where farmers keep, in most cases in an enclosure called a 'kraal', their animals at night and to which they return every evening after the day's herding. Twenty-seven stock posts were located in the Richtersveld National Park; nine stock posts on flats, footslopes and mountain each. We measured plant species richness and diversity, and mean percentage cover of the various plant growth forms ͑including the number of species falling into each growth form category͒ in each of the five 10 m ϫ 10 m plots ͑each 200 m apart͒ demarcated along a transect of one kilometre length from the centre of each stock post. The results showed that distance from the stock post does reflect grazing intensity use because densities in faecal pellets rapidly declined with increasing distances away from the stock post for all habitats studied. Faecal density was positively correlated with stocking density. Plant species richness and diversity was at a minimum near stock posts. Plants able to endure the effects of heavy grazing occurred near stock posts where declines in palatable plant species, assumingly sensitive to heavy grazing and trampling, were recorded. Grazing increased vegetation patchiness up to 800 m from the stock post for all the habitats. The degree to which this change in species composition occurred did not depend on stocking densities, suggesting that both grazing and landscape variability were responsible for vegetation changes in rangelands of that area of the Succulent Karoo biome.

Large-mammal herbivore populations are subject to the interaction of internal density-dependent processes and external environmental stochasticity. We disentangle these processes by linking consumer population dynamics, in a highly stochastic environment, to the availability of their key forage resource via effects on body condition and subsequent fecundity and mortality rates. Body condition and demographic rate data were obtained by monitoring 500 tagged female goats in the Richtersveld National Park, South Africa, over a three-year period. Identifying the key resource and pathway to density dependence for a population allows environmental stochasticity to be partitioned into that which has strong feedbacks to population stability, and that which does not. Our data reveal a density- dependent seasonal decline in goat body condition in response to concomitant density-dependent depletion of the dry-season forage resource. The loss in body condition reduced density-dependent pregnancy rates, litter sizes, and pre-weaning survival. Survival was lowest following the most severe dry season and for juveniles. Adult survival in the late-dry season depended on body condition in the mid-dry season. Population growth was determined by the length of the dry season and the population size in the previous year. The RNP goat population is thereby dynamically coupled primarily to its dry-season forage resource. Extreme environmental variability thus does not decouple consumer resource dynamics, in contrast to the views of nonequilibrium protagonists.

No abstract

With probably fewer than 3000 individuals alive in the biodiversity hotspot of the Succulent Karoo in southern Africa, populations of the endemic, Giant Quiver Tree, Aloe pillansii, are thought to be declining and thus threatened with extinction. Using repeat photography and field data we investigated the long-term changes in one population of A. pillansii at its type locality, the roughly 100 ha Cornell's Kop in the Richtersveld, South Africa. There are currently 75 individuals alive at this site. Of these, 44% are <1 m in height (seedlings), 4% are 1 -3 m (juveniles) and 52% are >3 m (adults). An analysis of 14 repeat photographs shows that since 1937 an average of 1.4% of the plants >3 m in height has died annually. At this rate all the remaining 39 plants on Cornell's Kop in this size class will be dead in 71 years. The relative paucity of plants in the 1 -3 m size classes could be explained by several factors including plant theft, animal damage and unfavourable recruitment conditions during the first 80 years of the 20th century. Annual growth rates decrease as plants age. Individuals <1 m in height grow at 42.5 mm yr )1 while plants 1 -3 m and those >3 m grow at 31.0 and 16.4 mm yr )1 respectively. At 8 m, the tallest plant on Cornell's Kop could be as old as 382 years and thus to maintain itself at this site, A. pillansii would only need to recruit relatively infrequently. The relatively high proportion of seedlings suggests that conditions have recently been favourable for recruitment at this site. Seedling ages, estimated from their heights, indicate that over 50% of the plants <1 m in height germinated 5 -10 years ago. This is consistent with local rainfall records which show that rainfall was consistently above the long-term annual average of 75 mm during this period. However, the loss of six seedlings from the population in the last 5 years, probably due to grazing or theft, suggests that without intervention this species will not survive on Cornell's Kop.