Savannas are globally important ecosystems of great significance to human economies. In these biomes, which are characterized by the co-dominance of trees and grasses, woody cover is a chief determinant of ecosystem properties. The availability of resources (water, nutrients) and disturbance regimes (fire, herbivory) are thought to be important in regulating woody cover, but perceptions differ on which of these are the primary drivers of savanna structure. Here we show, using data from 854 sites across Africa, that maximum woody cover in savannas receiving a mean annual precipitation (MAP) of less than approximately 650 mm is constrained by, and increases linearly with, MAP. These arid and semi-arid savannas may be considered 'stable' systems in which water constrains woody cover and permits grasses to coexist, while fire, herbivory and soil properties interact to reduce woody cover below the MAP-controlled upper bound. Above a MAP of approximately 650 mm, savannas are 'unstable' systems in which MAP is sufficient for woody canopy closure, and disturbances (fire, herbivory) are required for the coexistence of trees and grass. These results provide insights into the nature of African savannas and suggest that future changes in precipitation may considerably affect their distribution and dynamics.

Achieving maximum productivity in remnant populations of black rhinoceros is crucial to the persistence of this species. It was, therefore, investigated whether the black rhino population of Pilanesberg National Park had become regulated by resource limitation 22 years after introduction in 1979. Inter-calving intervals (which are not restricted to yearly time increments, due to asynchronous reproduction) decreased with an increase in rainfall, while the percentage of male calves born increased with increasing rainfall. The percentage of reproductive cows achieving maternal success increased with increasing density until 0.085 rhinos/km 2 , after which it decreased. This positive relationship at low densities is largely due to changes in the female age structure and the adult female/male ratio. The age at first calving tended to increase with increasing density, while mortality was not related to rainfall or density. It is concluded that the Pilanesberg black rhino population is showing the first signs of density dependence. It is proposed that black rhino conservators should monitor the percentage of cows achieving maternal success to detect early indications of density dependent resource limitation and use this as a criteria for decisions regarding metapopulation management.

Plant responses to herbivory vary depending on herbivory type, yet the comparative effects of defoliation (e.g. by insects) and pruning (e.g. by large mammals) on a single tree species are poorly documented. We investigated this in the Northern Province of South Africa by comparing the regrowth of Colophospermum mopane trees previously defoliated by caterpillars or pruned by elephants, the two main browsers of C. mopane foliage. Shoots were up to 160% and 125% longer after natural (elephant) and simulated pruning and leaves ∼25% longer in regrowth after natural pruning (n = 13-15 trees per treatment). Shoot density and chemical defences in leaves (tannin:protein ratio and total polyphenolic concentration) were, however, no different from control trees. Simulated defoliation resulted in statistically insignificant changes to regrowth in terms of leaf and shoot size (both slightly decreased) and shoot density (slightly increased). Natural (caterpillar) defoliation, however, resulted in regrowth with significantly decreased shoot and leaf size (about 50% and 20% of control lengths, respectively), as well as decreased leaf chemical defence. Shoot and leaf length were longer on trees flushing for the first time after pruning and late-season defoliation had a greater negative impact than mid-season defoliation. Despite the differences in regrowth characteristics after pruning and defoliation, mopane plants showed no apparent trade-off in investment between tolerance and resistance after either herbivory type, as neither regrowth nor chemical defence occurred at the expense of the other.

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The target of the 2002 IUCN Action Plan for the Cape mountain zebra Equus zebra zebra was for a population of 2,500 animals. We assessed the validity of this goal by reviewing the conservation status of the subspecies and the prospects of achieving larger populations. All subpopulations were identified and data on each were collected by means of a questionnaire survey. The total extant Cape mountain zebra population was found to consist of at least 2,790 individuals in 52 subpopulations. The target number of 2,500 has therefore been exceeded and this success is largely attributable to the private sector, as there are at present double the number of privately owned subpopulations (35) compared to formally protected subpopulations (17) and the percentage of the population on privately owned land rose from 14% in 1998 to 32% in 2009. The security of the subspecies is still of concern, however, as the growing proportion of the metapopulation on private land is more vulnerable to threats associated with small populations and management actions. The total existing area available to the Cape mountain zebra is > 935,191 ha and it could potentially support a considerably larger population. We conclude that the IUCN target is substantially below the potential for recovery of the Cape mountain zebra and we recommend this target be revised in the light of these findings. More comprehensive conservation strategies to address current and potential future threats are also needed.