effects on community
Orbits of grazing intensity and vegetation degradation around artificial features of the landscape such as waterholes and human settlements have been well-documented in the semi-arid rangelands of Australia (Andrew & Lange 1986; Pickup & Chewings 1994; James, Landsberg & Morton 1999), North America (Fusco et al. 1995) and Africa (Tolsma, Ernst & Verwey 1987; Perkins & Thomas 1993; Van Rooyen et al. 1994; Moleele & Perkins 1998; Turner 1998b). These grazing orbits provide useful systems in which to study the responses of vegetation and other variables to a range of grazing intensities (Andrew 1988).
In this study, we used the grazing gradient created around a stockpost to examine the impacts of heavy grazing on the population biology of two leaf-succulent species and an unpalatable non-succulent shrub. The presence of a grazing gradient was first established by analysis of soil parameters and grazing intensity at increasing distances from the stockpost.
Soil analysis revealed no underlying gradient of soil properties present. Previous work has demonstrated that organic matter, nitrogen and calcium are the soil parameters correlated with the abundance of leaf-succulent shrubs in the Paulshoek environment (Carrick 2001). These parameters, however, did not vary significantly with distance from the stockpost, further evidence that soil alone cannot explain differences in species composition and population dynamics along the gradient.
Indicators of grazing intensity, conversely, showed strong patterns of change with distance from the stockpost. Grazing intensity was found to be much higher in the area closer to the stockpost than in the area further from the stockpost. Thus the differences in succulent shrub population dynamics observed across the eight focal sites in this study provide a basis for understanding the impacts of different grazing intensities on species composition, percentage cover and shrub mortality, reproduction and establishment.
Grazing orbits around fixed points have been described as exhibiting three distinct zones of response (Andrew 1988). The first zone, in the immediate vicinity of the central point, is a ‘sacrifice zone’ of high disturbance intensity and is typically characterized by bare soil or ephemeral cover of annual plants (James, Landsberg & Morton 1999). The second zone is one of intermediate and progressively decreasing influence of grazing (Andrew 1988), characterized by a shift in dominance from unpalatable to palatable perennial plants with distance from the centre (James, Landsberg & Morton 1999). The third zone lies beyond the range of grazing activity. In the Paulshoek communal area, this last zone is rarely present, as livestock are kept at high densities throughout the area (Todd & Hoffman 1999). Daily grazing routes for the stockpost used in this study showed that the animals’ grazing orbit is bound by the presence of neighbouring stockposts and by the boundary of the communal area (Combrinck 2003). Thus the impacts of grazing on vegetation reported here do not include the third zone of the orbit model, and even the second zone may be somewhat truncated.
The zone of sacrifice observed at this stockpost was less than 50 m in radius. The grazing gradient from 50 m to 2 km exhibited a significant decrease in abundance of G. africana and an increase in abundance of the palatable R. robusta and C. denticulata. Galenia africana and R. robusta were the most abundant species and contributed more than half of the canopy cover of all perennial shrubs at sites close and far from the stockpost, respectively. The switch in dominance from a palatable leaf-succulent to an unpalatable woody shrub under heavy grazing follows the established pattern for the Paulshoek area (Todd & Hoffman 1999) and for the Succulent Karoo in general (Milton & Dean 1995).
In addition to changes in palatable shrub cover, the second zone of a typical grazing orbit exhibits an increase in species richness with distance from the centre of the sphere (James, Landsberg & Morton 1999). Our results, however, show no change in perennial species richness over the gradient. This may be a consequence of the scale of the grazing gradient studied; in a review of the impacts of grazing around water points on the ecologically similar chenopod shrublands of Australia, James, Landsberg & Morton (1999) report no consistent change in species richness over grazing gradients of 2–3 km. Moreover, other studies have shown that the relationship between grazing intensity and plant species richness in arid systems can vary greatly in space (Landsberg et al. 2002) and time (Oba, Vetaas & Stenseth 2001).
effects on population biology
In addition to the zones of grazing impact on community composition, zones of grazing impact on population dynamics can be found within a grazing orbit. Hunt (2001) describes three zones for the palatable shrub Atriplex vesicaria in Australia's chenopod shrubland. In the first zone, close to the water point, adult shrubs are heavily grazed and eventually die due to the combined effects of grazing and drought. In the second zone, as well as in the first zone, recruitment of shrub seedlings is generally suppressed, whereas in the third zone grazing has little impact on population processes.
We found similar zones of grazing impact for the palatable shrubs R. robusta and C. denticulata, and inverted zones for the unpalatable shrub G. africana. Adult shrub abundance for R. robusta and C. denticulata increased exponentially with distance from the stockpost, closely following the exponential decrease in grazing intensity. The patterns of abundance indicate that C. denticulata is more sensitive to the effects of heavy grazing than R. robusta; R. robusta was present at all sites, whereas live adults of C. denticulata were not found at the two focal sites closest to the stockpost.
Patterns of relative adult mortality suggest that grazing causes high adult shrub mortality only where grazing intensity is very high. Similar studies of shrub population dynamics in Australian rangelands have also shown that adult shrub mortality due to grazing only limits population persistence in areas subject to very heavy grazing (Watson, Westoby & Holm 1997; Hunt 2001).
Recruitment within the two leaf-succulent species studied was reduced in a wider zone around the stockpost than the zone of high adult shrub mortality. Ruschia robusta suffered reduced fruit production and post-rain seedling emergence at all sites less than 800 m from the stockpost, with especially low fruiting success at sites less than 400 m from the stockpost, but had high fruit production at the less heavily grazed sites. Fruit production was more severely impacted by grazing in C. denticulata, which produced very few fruits even at the least heavily grazed sites. We observed adult plants of this species on the adjacent lightly grazed privately owned farm and in permanent grazing exclosures within the communal area to produce numerous fruits per shrub during the same reproductive season. The general failure of this species to set seed in the heavily grazed communal area was confirmed by the absence of any seedlings at any sites in the post-rain seedling emergence survey. Thus, for C. denticulata, the zone of recruitment suppression around the stockpost extends for at least 2 km.
The pronounced effects of grazing on the number of fruits and seedlings of R. robusta and C. denticulata indicate that recruitment of these species is seed-limited even where adult plants can be found in relative abundance. The greater sensitivity of C. denticulata to grazing in terms of reproductive output may be explained by the fact that this species produces few large, showy flowers, whereas R. robusta produces many small flowers. Furthermore, flowers of C. denticulata appear to be actively sought-out as forage by livestock. Thus trade-offs in flower number and size, as well as floral palatability and nutrition, may play important roles in determining the impact of grazing on reproductive success.
Despite these differences, however, both species are significantly impacted in terms of seed production where grazing pressure is high. Similar trends have been observed in the Succulent Karoo for palatable perennial shrubs (Milton & Dean 1990; Milton 1994a; Todd 2000) and some leaf-succulent shrubs (Stokes 1994), and in Australian shrublands (Andrew & Lange 1986).
Seed production has been identified as one of the processes most sensitive to herbivory among long-lived perennial plants (Fenner 1985; O’Connor & Pickett 1992). Continued low levels of reproductive output under a heavy grazing regime can have negative ramifications for population persistence (Hunt 2001). Given that stockposts in the communal areas of Namaqualand are typically located 1–3 km apart, a significant reduction in seed set of leaf-succulent species in spheres 800 m to 2 km from the stockposts could pose a serious threat to the persistence of these shrubs in the landscape. This is especially the case for species such as C. denticulata that produce very few fruits even at the least heavily grazed sites. As succulent shrubs of the Mesembryanthemaceae lack long-distance dispersal mechanisms (Esler 1999; Parolin 2001) and do not persist in soil seed banks (Esler 1999), recolonization of an area where local extinction has occurred is highly unlikely.
Once germinated, however, seedlings of succulent shrubs may not be greatly impacted by heavy grazing. Survival of R. robusta and C. denticulata seedlings in this study did not vary with grazing intensity. Seedling mortality was much more often due to abiotic causes than to grazing or trampling. Studies of woody perennial seedling mortality under varying grazing intensities, both in the Succulent Karoo (Milton 1994a, 1995) and in Australia's chenopod shrubland (Lange, Coleman & Cowley 1992), have also found seedling mortality to be independent of grazing, but this result has not been documented previously for succulent shrubs of the Karoo. The primary cause of seedling mortality in semi-arid shrublands is generally thought to be drought-stress (Milton 1995; Watson, Westoby & Holm 1997) and competition from neighbouring adult plants (Milton 1994a,b, 1995). Succulent shrubs, although palatable, are usually not the most preferred livestock forage, and the cost of searching out inconspicuous seedlings of these species probably outweighs the benefit in terms of palatability (Lange, Coleman & Cowley 1992).
Seedlings of many Karoo shrubs have been shown to exhibit preferences in terms of microsites for germination and recruitment (Yeaton & Esler 1990; Esler 1999). In semi-arid shrublands, seedlings growing under the canopy of adult plants are thought to benefit from thermal stress alleviation (McAuliffe 1984; Franco & Nobel 1989) or protection from herbivory (Turner, Alcorn & Olin 1969; Todd 2000), benefits that may outweigh the effects of competition from the proximate adult plant under certain conditions. Thus heavy grazing, by changing species composition and reducing total plant cover, may negatively affect seedling establishment by reducing the availability of favourable microsites.
In this study, however, seedlings were not found to be microsite-limited. Seedlings of R. robusta and C. denticulata survived equally well in the open and under the canopy of R. robusta, demonstrating that adult plants do not provide essential protection from herbivory. Moreover, seedling mortality for both species was much more often caused by abiotic factors in both microsites.
While R. robusta and C. denticulata abundance, mortality and reproduction exhibited strong patterns of grazing influence around the stockpost, the unpalatable shrub G. africana was less responsive to the changes in grazing intensity. The abundance of adult and seedling G. africana shrubs decreased linearly with distance from the stockpost, in contrast to the exponential increase in abundance among the succulent shrubs. Relative mortality of G. africana did not change significantly over the gradient and was low at all sites. In the post-rain germination survey, seedlings of this species were present in high abundance at sites close to the stockpost and, although abundance dropped off at sites further than 400 m from the stockpost, seedlings were present even at the least heavily grazed sites. This contrasts strongly with the trends for R. robusta and C. denticulata, for which no seedlings were found at the most heavily grazed sites.
In general, G. africana populations at the least heavily grazed sites did not show strong signs of decline, whereas population processes within both species of leaf-succulent species were strongly impacted at the more heavily grazed sites. The difference in response between G. africana and the palatable leaf-succulents provides insight into the process by which vegetation degradation around a stockpost (or comparable central point of grazing intensity) occurs.
With reduced recruitment, populations of palatable shrubs are unable to replace themselves and eventually become locally extinct when adult shrubs die off in areas of highest grazing intensity. Without a local source of seed, recolonization by these species is highly unlikely. In contrast, the unpalatable G. africana survives and recruits successfully even in less heavily grazed areas, and, because it appears to disperse seeds more widely than the Mesembryanthemaceae, it can quickly colonize an area where palatable shrub populations are in decline. With continued heavy grazing, the radius of unpalatable forage around a stockpost increases, shifting the focus of grazing further and further away from the stockpost, thus creating an expanding centre of degradation in the landscape that is limited only by the distance over which animals travel (Jeltsch et al. 1997).
The stockpost system can have significant impacts on the vegetation of the surrounding areas (H. Hendricks, unpublished data). The presence of a stockpost, however, will have the greatest effect on shrub populations and communities if the stockpost remains stationary. In this study we found that marked zonation has developed around a stockpost that has been stationary over a period of 15 years. Although caution should be exercised in generalizing results based on only one stockpost, our informal observations at other semi-permanent stockposts in the Namaqualand communal areas suggest that they exhibit similar zonation. Continued and concentrated grazing impacts on plant population dynamics could ultimately lead to the local extinction of grazing-sensitive succulent species in an increasing area around a stockpost. Stockposts that relocate regularly, on the other hand, are not likely to suppress recruitment and cause adult shrub mortality in distinct zones. Frequent movement (every 1–3 years) of stockposts would allow different patches of the landscape to set seed in different years. We therefore recommend that herders be encouraged to relocate their stockposts at frequent intervals.
The results of this study support the theory that reduced seed set is the main mechanism responsible for the decrease in abundance of Karoo shrubs in areas subject to heavy livestock grazing (Milton 1995; Todd 2000; Carrick 2001), although further research is needed to confirm this conclusion for the guild of leaf-succulents in general. In light of the apparent sensitivity of fruit production to grazing, it is advisable that grazing pressure be reduced or removed from particular areas periodically. Succulent shrub populations in areas that are grazed heavily and continuously may never have adequate opportunity to replace themselves. The alleviation of grazing pressure, especially preceding and during the months of peak flower and fruit maturation and during high rainfall years when fruit production is most likely to be successful, would allow seed set to occur and thus increase the chances of successful recruitment. Further research is needed to determine whether shrubs that have been heavily grazed can set seed successfully in the first growing season after the removal of grazing pressure, or whether rest periods of several years are necessary. In general, however, it is of great importance that grazing pressure be relieved periodically, rather than concentrated around fixed points, in order to ensure the persistence of Namaqualand's unique succulent shrub species.